Fly Fishing Devon: How does a trout catch a fly?

This page explores in greater depth the ideas introduced in our article in the May 2020 edition of The Field magazine.

The article is now available for you to   read online

"The due appreciation of how a trout is rising forms the very essence of fishing, whether it be with floating fly or artificial nymph - and it is often no easy matter." (Skues 1939) [emphasis in original]

Some words of explanation. This is a long page. The contents cover topics some of which will be familiar to anglers, other to scientists working in various areas. The aim is to explore the possibility that a Tracking Heuristic controls an element of trout behaviour - the rise. I have tried to make the material accessible, acceptable & hopefully interesting to both audiences, but recognise that some of it may cause 'eyes to glaze' over!

How Does a Trout Catch a Fly?: Marinaro's "Edge of the Window Theory"

Anglers often ask "What fly will catch a trout?". This page turns that question on its head and explores how a trout recognizes and then intercepts a fly drifting towards it on, or beneath, the surface of the water. The answer gives an insight into how to design effective trout flies and why our artificial flies are sometimes ignored by feeding trout.

Trout see the world through a skylight - or circular 'window' - surrounded by mirrors. Marinaro's great insight was to recognise how trout use the position of the fly in this window to make an effective rise. This video sets the scene for a detailed exploration of a deceptively simple question "How does a trout catch a fly?".

Summary. Trout (salmo trutta) feed on insects on and below the surface. Evidence from fly-fishing literature indicates that trout use visual cues - reaching them through a circular window surrounded by a mirror - to recognise, rise and intercept insects. The significance of the mirror and edge of the window in controlling the trout's rise is described using video and photographic evidence.

The trout's rise to a dry or emerging fly can be described in ethological terms as a "modal action pattern" consisting of a sequence of behaviour(s) triggered initially by the appearance of an insect's body or footprints in the mirror, then parts of the wings in the window, and lastly the entire insect on the edge of the window. I use a   case study  approach to suggest that control of a trout's rise to capture an insect is an example of a Tracking Heuristic using a constant angle of approach.

Then the effect of 'drag' on trouts' ability to secure their prey under daytime and night-time conditions is described. The presence of drag disrupts tracking by day. In contrast, drag triggers tracking after dark, and the absence of drag disrupts tracking in darkness. A Tracking Heuristic can explain these contrasting effects of drag. Interference with tracking may offer an alternative explanation of Inspection and Refusal during the trout's rise. The role of tracking in the trout's response to a sub-surface artificial wet fly is discussed.

Signal detection using a search image may explain how trout distinguish between prey and debris in the drift. Successful artificial trout flies may include triggers that mimic those found in natural insects and facilitate recognising, and then tracking the trout's prey.

How does a trout position itself to ingest a floating insect?

A rising trout is something many of us see on a regular basis, and there is a temptation to take it for granted.

But catching a fly is no simple feat. Intercepting a moving object requires prediction of its future location.

The trout's problem is similar to ours when we try to catch a cricket ball. It looks easy until you try to do it !

And it proved just as difficult to devise a computer algorithm to predict a trout's route to intercept a fly.

This video shows the prey capture trajectories of a Dolly Varden trout predicted by a preliminary version of a computer model developed as part of the  Drift Model Project.  

"For each capture manoeuvre observed, the predicted trajectory is shown along with the predicted capture location (yellow dot), connected by a thin white line to the actual observed capture location (green dot)."

"This is one of the most mathematically and computationally complex models in our field" (Jason Neuswanger, personal communication 2020)

As you can see there appears to be a - sometimes large - discrepancy between the predicted interception point, and the actual location where the trout caught their prey. Therefore this page examines if a simpler approach; using a  gaze / tracking heuristic  might help to understand control of the trout's rise.

The trout's brain has evolved to cope with a complex situation involving:

  1. the constantly changing position of the trout relative to the fly
  2. the movement of the fly as it is transported downstream by the current

Traditionally angling writers have talked in terms of    deceiving  a trout into taking an artificial fly. I want to put a slightly different slant on the angler's task. I want to work with the trout rather than trying to deceive it. After all, fish want to eat. If we understand how a fish catches an insect we can present our fly to make it easy for the trout to consume. The angler's problem is to design and present a fly so that it can be easily caught by the trout.

The first problem we encounter is the fact that the diameter of the trout's window varies in size:
  1. the window gets smaller the closer the trout is to the water surface
  2. the window increases in size as the fish sinks deeper into the water

Our local West Country rivers are relatively shallow. Trout often lie close to the surface. This diagram shows that a trout lying one foot beneath the surface has a very small window above its head - 11 inches in radius. We are trying to present our artificial fly close to a very small area.

We often judge where to cast our fly by noting where a trout rises to take a natural fly. But there is a flaw in this approach which has been explained by Vince Marinaro (1995).

In this groundbreaking book - first published in 1976 - Vince Marinaro described how trout moved to intercept insects drifting towards them. He called this sequence the 'simple rise'.

He observed that trout drift downstream and tilt their head upwards as they position themselves to take a fly off the surface.

In other words, the position of the rise may not correspond to the trout's 'observation post'. The rise may occur some distance downstream of the holding lie. After taking the fly the trout returns upstream of where the rise is seen by the angler.

The implication of this important point was elegantly captured in a drawing by Dermot Wilson (1957).

The window and the mirror

If you have read this far, you may have formed the impression that to have any hope of catching a trout you must cast your fly into a circle with the diameter of a dinner plate, located some indeterminate distance upstream of where you saw a fish rise. Don't worry, if that were true no one would ever catch a rising trout, and - more importantly - most trout would starve to death !

It turns out that trout get 'advanced warning' of a fly long before it appears in their 'window'. Up to now we have concentrated on the trout's 'window'. But we must also consider the ' mirror' that plays an equally important role in the trout's view of the world.

Remember the description: "The trout lives, as it were, in a room with a ceiling made of   mirrors  except for a round skylight in the middle (the   window)   , through which the outside world is visible" Frost and Brown's (1967).

Trout are able to see parts of an insect or artificial fly that rest on - or puncture - this 'mirror'. The bodies of emerging flies break through the water surface. They hang beneath the mirror. The legs of duns rest on the surface. Therefore parts of an insect are visible to fish long before the insect has entered the trout's window.

John Goddard has calculated that trout are able to employ binocular vision to detect approaching insects in a band of water that is about 30 inches wide (see Roberts, 1994, p 15).

In addition, Marinaro (1995) and Hewitt (1948) as well as Clark and Goddard (1980) provided photographic evidence that trout could see the wings of approaching insects in their window.

Clark and Goddard explain that because of the refraction (bending) of light rays entering water, parts of an insect that protrude above an angle of   greater  than 10 degrees to the water surface are potentially   visible  in the window.

Therefore a trout has   two  cues that an approaching object may be edible:
  1. body parts that break through the 'mirror'
  2. wings appearing in the window

The next diagram shows what part of an insect are visible as it drifts downstream towards a waiting trout.
  1. At first only the legs are visible beneath the   mirror  ( ^ ^  in the diagram)
  2. then as the insect gets nearer, the trout can see more and more of the fly's wings in its   window  (  1   and    2   in the diagram below)
  3. finally, when the insect reaches the edge of the trout's window, all of its body can be seen (   3  in the diagram below)

The next series of diagrams examines the three points    1    2   and   3   in greater detail. The diagrams incorporate actual photographs taken by Clark and Goddard which show how the wings and body of the insect gradually   merge as they get closer and closer to the edge of trout's window. These photographs are important. They suggest a way in which the trout can guage the position of the fly whilst rising to consume it. And they contain important hints for the design of effective artificial trout flies.

Position#1 : legs and wings tips visible

Position#2 : legs and all of wings visible

Position#3 : legs, wings and body visible

A familiar example of a visual control system

It may help to understand the significance of these visual cues to a trout by comparing them to how we react to visual cues at a set of traffic lights.
We react to traffic lights automatically. We don't have to consciously think about what to do with our legs and arms to control the car's brakes and clutch.

It's just the same for trout. They automatically adjust the position and orientation of their body to meet the insect as it is carried towards them by the current.

Disagreement over the Relative Importance of the Mirror and Window

| Clarke and Goddard |

"The importance of the mirror to the trout (and so to the angler) cannot be overstated. ... Although the window has been the subject of far more discussion in angling literature, we believe that its importance pales beside that of the mirror for a trout feeding on the surface. Once the significance of this is grasped it opens up a new dimension for fly-fisher and fly-dresser alike. The mirror has been the forgotten frontier of the angler's world." Quote from the Postscript to 'The Trout and the Fly', Brian Clarke and Dr John Goddard (2005 edition).This video clip is from the TV programme  The Educated Trout  available online

It is unfortunate that Clarke and Goddard, in their influential book that quite rightly promoted the importance of the mirror in the trouts rise, felt necessary to include Vince Marinaro and E. R. Hewitt in a list of previous authors of books on fish vision that - in their opinion - contained misunderstandings, or out of date scientific knowledge (p 60, 1980 edition)

Vince Marinaro emerges from Gordon Wickstrom's (2000) analysis as someone who didn't suffer fools gladly: "There were, undeniably, those who found him acerbic, stubborn, irascible. But Marinaro appreciated the contributions of earlier English and American authors on trout vision: "All of these do not tell the whole story, but they represent, at least, a solid foundation for the neo-impressionist who must make a beginning with the accumulated knowledge of others before he can make a contribution of his own." (Marinaro, 1970, p 66).

I agree with Marinaro. Therefore it may be useful to review the work of earlier observers who explored the role played by the mirror and the window in influencing the trout's behaviour. Drawing on an accumulation of observations builds a more balanced argument than relying on Clarke and Goddard's "A New Approach".

For many years innovative and influential fly designers on both sides of the Atlantic tied dry flies with hackle resting on or puncturing the mirror.

| Edward R. Hewitt |

Hewitt's 1947 book supports Clarke and Goddards thesis. He included photographs illustrating the appearance of dry flies with extra long hackles in the mirror.
  • Hewitt wrote: "When the dry fly is floating high on the surface the fish can see only that part of the fly which punctures the surface, if the fly is outside its window. This generally means the hook or parts of the hackles. Each one of these breaks the surface and makes a miniature lens which catches the light." As we  saw above,  when the fly is some distance from the edge of the trout's window, the wings are not visible.
  • He added: "If the dry fly is moved or strikes the water outside the window it causes miniature light explosions which are very visible at long distances. It is these which warn the fish of the approach of insect food and can scarcely fail to attract its attention."
  • "If the fly is moved on the surface beyond the window it makes brilliant light flashes almost like explosions from the point of view of the fish "
  • (quotes from Hewitt, 1947 edition, p 66-7).

    These observations on their own do not mean that the effects recorded by Hewitt on photographic film have any effect on trout behaviour. But Hewitt took the next important step. He constructing dry flies that consisted of nothing more than hackles that rest on, or puncture, the surface of the water. Hewitt's    Bivisible flies  catch trout. Their success provides support for Hewitt's theory of the importance of light effects in the mirror. Simple Bivisible dry flies continue to be celebrated in America (Valla, 2013). The name Bivisible refers to the use of two diferent hackle colours to make the fly visible to the trout and the angler.

    Hewitt's approach to designing dry flies would be familiar to behavioural scientists who use simple models to discover stimuli that control animal behaviour.  Ethologists would point out that further experiemnts are required to rule out alternative explanations for the success of Bivisible dry flies. For example, it could be that hackles protruding above the hook shank represent wings on an insect, and contribute to the success of Bivisibles. Hewitt should be recognised for his significant contribution to understanding the trout's rise, and developing an   easily tied fly  that continues to be popular.

    The Bivisibles shown in this section were tied by professional fly tier Mary  Dette.  Dick (2004) provides a reassuring anecdote about Hewitt's flies. In 1948 Dr Dick, a keen young fly fisherman was invited to Mr. Everitt's home in New York. After dinner he asked, "Would you please show me some of the trout and salmon flies you have tied?" Dick reported that: "They were as poorly but sparsely tied as mine" which came as reassurance to the young angler. And teaches us a lesson, flies don't always need to be perfect, they just need to capture the essential element that will persuade a trout that a meal is imminent.

    | Dr William Baigent |

    Dr William Baigent (1862-1935) was a medical doctor in Northallerton, a market town in North Yorkshire. He is remembered for Baigent’s Brown, and his influence on the Catskill tradition of fly-tying through correspondance with American authors George La Branch and Preston Jennings. (Rob Smith, 2018) The fly pictured here is from a collection of patterns dressed by Baigent. It bears a remarkable resemblance to Hewitt's Bivisible patterns. It's not clear if Baigent was aware of Hewitt's observations of the light effects of dry-fly hackle through correspondance with these influential American flytiers.

    Baigent had struck out on his own. He did not follow the local fly-fishing culture. Born and living his life in North Yorkshire he fishing dry flies in an area of England where fishing wet flies reigned supreme. He fished dry flies at a time when the Halfordian doctrine of precise imitation was de rigueur and reigned supreme. He eschewed precise imitation, instead devoting many years to breed Old English Game Cock to supply hackle to tie his unique 'variant' style of flies including this long-hackled Baigent's Brown. "Asked what the Baigent Brown was tied to represent, Baigent replied: "It is not tied to represent any fly, it is tied to catch trout."(Roberts, 1994, p 215)

    Baigent's daughter  Sheona Lodge recalled that it "was 1925 when Baigent's Brown and Variants first appeared in Hardy's catalogue. Austin Hogan remarked that it was "Either Hardy or the good Doctor had named the series "Refractra" based on the theory that it was essential a proper light refraction result when the fly was on the water, if it was to be functional." The fly illustrated here was tied by  (Rob Smith, 2018)  and corresponds to the dressing given by Courtney Williams (1979, p 93-4). It has a prominent wing, and looks very smart; I can imagine it being bought in Hardy's shop at 61 Pall Mall in London and taken to a chalkstream without 'frightening the horses' - unlike the doctor's original tying shown above !

    I think Baigent's lasting legacy is this suggestion about why a trout takes a fly.
    In a letter to W. K. Rollo, Baigent wrote : “... the Baigent’s Brown based entirely on what the trout think themselves... It was made by finding out what combination of dry fly hackles would easily stimulate what  Pavlov  calls the trout’s “investigating reflex", ... The fly will start such ocular reflexes when properly presented which will more often than not get the other reflexes in motion, and so the fly is taken in lamb-like fashion without suspicion and fuss.” (Rollo, p 91-2, 1944 reprint). Dr. Baigent may have read in the medical literature about Pavlov's award of the 1904 Nobel Prize in Physiology or Medicine for his research on the physiology of digestion.

    Baigent sought to make flies according to the wishes of trout rather than an exact copy of the natural. "Think like a trout, these are probably the first words I remember. He tried to see the flies he tied from the trout-eye view that is why he had to breed his own Game Cocks" (quotes from his daughter  Sheona Lodge)

    Baigent removed the angler's skill, presentation and stealth, and - shunning anthropocentrism - put the trout's evolution, motivation and reflexes centre stage. His approach in more in the spirit of Datus Proper's book title "What the Trout Said" than "Deceiving Trout - The Flytier's Art". This contrasts with the anthropocentric approach of "build a better mousetrap (rod, line, fly or cast) and the world will beat a path to your door".

    Baigent's reference to Pavlov and reflexes is his signpost to a future path paved with scientific research into how stimuli  trigger  animal behaviour i.e. ethology or behavioural ecology. This link explains the differences between  reflexes, fixed and modal action patterns.

    | Vince Marinaro |

    Marinaro (1995) provided photographic evidence that trout could see the wings of approaching insects in their window. Wickstrom (2000) commented: "From his study of a trout's vision, he [ Marinaro ] believed that the dun mayfly's wing was the single most noticeable thing about the insect from the trout's point of view, and that the body was of little or no importance because it did not touch the water and so could not be seen by the trout as the fly floated toward the fish's window of vision.". Note the size of this wing on the artificial dun pictured on the cover of Marinaro's book A Modern Dry Fly Code (1970)

    Marinaro also appreciated that stimuli in the meniscus of the mirror "signals the approach of the fly to the trout" but termed this more prosaically as a light pattern (Marinaro 1995,p. 16).   Marinaro's first book A Modern Dry-Fly Code(1970) contains a detailed and appreciative discussion ( p 68-73) of Col. Harding's 'light-pattern theory' that stressed the importance - to the trout's rise - of the dun's feet on the meniscus. But Marinaro disagrees with Harding's view that the appearance of wings outside the trout's window are not normally as important as the light pattern. Marinaro stated that "In actual practice there is ample assurance that wings for floating duns are of paramount importance" (p75 op cit). Marinaro cites   Hewitt's hackle patterns to make this point. His success with this pattern led Marinaro to develop the thorax dun style with "two two short-fibred hackles turned at opposite angles angles in the manner of an X" which creates a light pattern effect in the mirror and window (p77 op cit). Clearly Clarke and Goddard's focus on the mirror had been recognised by previous authors they chose to dismiss.

    | Clarke and Goddard |

    This short tour of the literature indicates that mirror, window, wings and legs should all be considered in any analysis of the trout's rise.

    For me, Clarke and Goddard's (2005) contribution is subtle but potentially important. They suggest that hackle and wing play successive roles in triggering the trout's rise to a dry fly. Their use of the term 'trigger' introduces into British fly-fishing literature a concept discovered by the University of Oxford ethologist and Nobel Laureate in 1973 Nikolaas Tinbergen.

    Underwater shots from Ozzie Ozefovich's video Underwater World of Trout Part Three | Trout Vision

    Clarke and Goddard stressed that denting of the mirror by an insect's feet act as the ' primary trigger' for the trout's rise.

    They wrote : "It is these   star-bursts of light created by the indentations of the feet of the dun floating on the surface, that are the first trigger to the trout's pedatory mechanism."

    Then - once the fish has started to rise - the insect's wings play their role in triggering the rise. Clark and Goddard (p. 73 & 85) call this a second signal and present photographs of what they desribe as "flaring" like a gas flame when wings first appear in the trout's window.In their opinion experienced trout wait for this second signal before rising.

    Ozzie Ozefovich's underwater shots have been used in this video to show how a natural insect floating on the surface appears to a trout.

    The video illustrates three important points:
    1. Trout see the world through a skylight - or circular 'window' - surrounded by a mirror.
    2. At times only parts of an insect's wings are visible in the window.
    3. Parts of an insect that rest on, or puncture the water surface, are also visible as "star-bursts" in the mirror that surrounds the window
    Use the Full Screen icons on this clip's toolbar to get the best view of the insect in the trout's window and mirror.

    Clicking each of these still images from Ozzie Ozefovich's video reveals a larger picture, in a new tab in most browsers.

    Artificial fly filmed in a tank. Window at top of image. Mirror (blue) in lower section of image.

    Tip of wing visible in window. Body visible in mirror.

    More of wing visible in window as body gets nearer to edge of window

    Wing and body close to edge of window

    Wing and body merge at edge of window

    | Underwater view of artificial fly filmed in river |

    | Underwater view of natural fly filmed in river |

    Wing visible in window, body visible in mirror

    Body and wing merge at the edge of the window

    Star-bursts of light created by the indentations of the feet of the dun trigger the rise

    The "...trout places the fly always at the edge of the window for all purposes: viewing, inspecting and taking"(Marinaro, 1995, p. 20)

    Under normal circumstances an insect's wings do not penetrate the water surface. Consequently, because of the laws of refraction, fish cannot see any part of an insect's wing that lies below an angle of 10 degrees to the edge of their window. Therefore, wings only become visible when the insect is very close to the edge of a trout's window.

    For example, the table below shows that the wing tips of an insect with wings that are half an inch high will only be visible to a trout when the insect is just over two and a half inches from the edge of the trout's window.

    Wing height (inches) 0.5 in 0.4 in 0.3 in 0.2 in
    Distance (inches) from window at which wing tips first become visible to trout 2.8 in 2.3 in 1.7 in 1.1 in

    Clark and Goddard (1980, p. 72-4) emphasise that "It is these starbursts of light created by the indentations of the feet of the dun floating on the surface that are the first trigger to the trout's predatory mechanism." (emphasis added). The second trigger is the wings of the fly. Clark and Goddard place much greater emphasis on the first trigger. They argue that the trout has  remarkable eyesight   that enables them to respond to the first trigger even in rapid broken water.

    Marinaro's "Edge of the Window Theory": Dry flies

    Marinaro's two books describe all the external environmental elements needed to analyse the trout's rise to artificial and natural flies as an example of a modal action pattern (MAP): wings, legs, mirror and the final component - the window.

    There is accumulating video and photographic evidence - from Ozzie Ozefovich, Marinaro as well as Clark and Goddard - that when an insect reaches the edge of the trout's window a significant event takes place.

    The wings, body and legs of the insect merge together.

    Marinaro (1995) summed up his extensive observational studies of trout feeding behavour as follows:

    "It is an inescapable conclusion that the trout places the fly always at the edge of the window for all purposes: viewing, inspecting and taking"(Marinaro, 1995, p. 20)

    "Why does the trout keep the fly at the edge of the window?"
    If trout behaved in this way they would be able to judge the  exact   position of the fly. By keeping the fly in a precise position relative to its body, the trout stands a very good chance of engulfing the insect.

    I'm not for one moment suggesting that trout do mathematical calculation. But I am suggesting that the trout's behaviour has evolved in response to the physical laws which describe its everyday environment.

    A trout's ability to recognise and capture prey may require practice as it matures. The behavioural sequence may be an example of an intercalated behaviour, particularly in learning the characteristics of suitable prey items to discriminate them from non-edible items.

    We know that:
    1. the trout's window has a width of 97 degrees
    2. and that the radius of the trout's window is a precise function of the depth of the trout in the water
    3. Therefore - as the next table shows -the distance between the trout and the insect can be calculated. This distance is a precise function of the depth of the trout in the water

    Depth of trout (in ft and in) 3 ft 2 ft 1 ft 6 in
    Distance (in ft and in) between trout's eye
    and fly on edge of window
    4 ft 2 ft 8 in 1 ft 4 in 8 in

    The trout stands a very good chance of successfully ingesting the fly if they drift downstream keeping the insect on the edge of the window.

    Of course the water surface is hardly ever absolutely flat. The acquisition of this skill may involve learning, maturation and practice.

    But before proceeding it is necessary to consider published criticisms of Marinaro's Edge of the Window theory.

    A difference of opinion over the importance of the edge of the trout's window

    | Marinaro |

    Marinaro is quite clear in his opinion that trout do not use the centre of the window to inspect the fly. But there are dissenting voices, some stronger than others. Therefore I've included this section for the sake of completeness.

    Marinaro wrote: "It is an inescapable conclusion that the trout places the fly always at the edge of the window for all purposes: viewing, inspecting and taking. He does not use the clear central area of the window. ...It would be virtually impossible for a trout to take a fly if he placed it above his head in the centre of his window. ...the fly in the center of the window would probably be invisible, because of the blind spot above his head. ...Like the wing shot who must hit a moving bird with a moving gun in order to get ahead of the bird, the trout must get or keep ahead of a moving fly with his moving body else the fly escapes him. Keeping the fly at the upstream side of the window solves this problem for him."(Marinaro, 1995, p. 20)

    | Brian Clarke |

    In 1996, Brian Clarke agreed with Vince Marinaro's conclusion about the importance of the edge of the trout's window.

    In his 1996 book Trout etcetera, Clarke discusses evidence that supports his suggestion that trout lie tilted up towards the surface (Clarke 1996 p119). This orientation facilitates detecting the starbursts of light created by the indentations of the feet of the dun floating on the surface, as well as parts of an emerging insect below the surface film.

    Commenting on the two diagrams below, Clarke concluded:"I suspect the fish will, once having begun its lift to the surface, instinctively position the fly on the edge of its window as soon as it can, not only to get a better look at what is coming, but to put itself on track for an automatic intrception" Clarke (1996 p121).

    | John Roberts |

    Others express a different point of view, for example John Roberts in his book "To Rise a Trout" page 75 writes:

    "I cannot fully support Marinaro's theory that the central window area is not used for inspecting a fly. My own experience is that time and time again trout use the central zone for viewing surface food, and often do not decide to rise until the fly is directly overhead or even beyond the trout. Of course there are many times when a trout makes its mind up on the basis of what it sees of the fly on the edge of the window, where its view of the colour of surface flies is clearer than at the centre of the window, and reacts accordingly. But some selective or fastidious trout make a judgement only when the fly is in the centre of the window, where they have a more accurate view of the critical features of size, shape and silhouette."

    Robert's may be referring to trout behaviour described by Mainaro as complex and compound rises. They are  considered below.

    | Datus Proper |

    Datus Proper (1989, p 146-7) said: "No one can offer a second opinion without repeating Vince's experiments. Still, in some of his own photographs, the fly looks to me as if it is in the window (bearing in mind that the fish is deeper than it seems to be )." Proper and Marinaro were friends and fished together. Marinaro wrote in the Foreword to Proper's seminal book What the Trout Said (2nd edition 1989) "How I would have chesished a book like this in my formative years."

    | Hayes and Stazicker |

    Hayes & Stazicker (2019, p. 92) echo Proper's reservation. "We agree—it is too easy from the human perspective to look at a trout reacting to an approaching fly, and think the fly is outside the window, forgetting that the trout is in actuality significantly deeper than it looks." This prompted their wide ranging critique published in 2019.

    I was intrigued in 2019 when Clarke and Goddard's and Marinaro's analyses of the role played by the mirror and window in the trout's rise were questioned on the pages of two UK angling publications, and a book by Peter Hayes and Don Staziker.

    Peter Hayes established himself as an fly-fishing iconoclast with his first book Fly Fishing Outside the Box: Emerging Heresies. This warning in Charles Jardine's Foreword to Hayes' second book - co-authored with Don Staziker - continues in the same spirit: "This treatise will challenge you - and many of your popular held beliefs. Also, the book takes no prisoners. None. Some of our long and established - and enshrined - trout fishing beliefs have been surgically dissected and a keen scalpel blade has sliced through them to the observed reality of actual situations: and then they have been thrust under a fly fishing microscope. Few have come away unscathed." (Hayes and Stazicker,Trout and Flies - Getting Closer 2019, p 6 ). So I can't complain that I wasn't warned that I was about to get my leg-pulled ! Or was I?

    In the December 2019 edition of Trout & Salmon magazine, Hayes and Stazicker surmised that their underwater photography has exposed the fallacy that "the trout holds the fly on the edge of the window closely examining it before deciding whether or not to take it....Because of earlier writers' failure to allow for refraction, the fish is lying deeper than it seems and the fly is not on the edge of the window: it is actually within the window in a plain-sight zone where it can be examined without geometric or chromatic distortion."

    Maybe this criticism was a little harsh. In 1995 Marinaro acknowledged that "There may be some error in [his photographs of] the position of the fly because of the effects of refraction on the trout itself." He performed an experiment to check and correct for this effect and included this to illustrate the necessay correction. Most importantly "he found that the fly came to the inside edge of the window, probably sitting on the fuzzy edge [i.e. Snell's circle] of the rim ..."(Marinaro 1995, p.25).

    In their 2019 book (Trout and Flies - Getting Closer, p.88 ) Hayes and Stazicker have used the term "Plain Sight Zone" to describe this region of the trout's window first identified by Marinaro. They wrote: "The fish acquires its first clear view of a floating fly in the Plain Sight Zone just after the fly has crossed the edge of the window i.e. as soon as it is clear of the fuzzy edge." (page 90). It would be hard, as they say, to put a cigarette paper between these locations.

    Turning now to the 2019 annual journal of the Wild Trout Trust (Salmo trutta), where Hayes and Stazicker had Clarke and Goddard in their sights when they reported that "By using Ultra High Definition (UHD) underwater video and slow motion video capture we have come to doubt the assumption that the light pattern initiates the rise ..." They describe how previous authors (e.g. Clarke and Goddard) said "... that the trout detects a sparkle of light in the mirror caused by the feet and sometimes the body of the dun distorting the surface film, ..."

    Likewise in their book, Hayes & Stazicker (2019, p. 89) wrote:
    "Goddard and Clarke's assertions that trout initiate the rise on sight of the tiny indentations of the dun's feet in the mirror, well upstream and in advance of the fly appearing in the window--and that the fly stays on the edge of the mirror as the fish rises and the window shrinks-- get precious little support from the hundreds of relevant video clips out of our total of 3,500. We have not seen rises to already-floating flies initiated that early." (emphasis added).

    Hayes & Stazicker (2019, p. 101) acknowledge that parts of an insect or artificial fly that penetrate the mirror can trigger a rise. This is consistent with Clarke & Goddard's suggestion that more attention should be paid to elements that penetrate the trout's mirror.

    There is also the problem of tying an artificial hackled dry fly to successfully imitate the legs of the natural dun resting on the surface. Lawrie (1967)   wrote a book  All Fur Flies and How to Dress Them  because of the difficulty obtaining the Old English Game Cock hackles needed to suggest the legs of an insect and support the artificial.

    Therefore it would be useful to see the evidence from Hayes & Stazicker's videos of the effectiveness of this trigger: i.e. the rise being triggered by parts of the emerging natural fly penetrating the mirror when it was outwith the trout's window.

    Neuswanger (2014) and Neuswanger et al, (2014) describe the 3-D video method they used to analyse salmonid feeding behaviour using underwater video. Unfortunately Hayes & Stazicker do not describe in sufficient detail the 3-D video method they used to analyse their video recordings to evaluate their conclusions. It would be revealing to analyse Hayes & Stazicker's videos using the computer software   VidSynch developed by Dr. Neuswanger as part of his PhD dissertation.

    Hayes and Stazicker's (2019) final conclusion does not yet add to my understanding of how a trout catches a fly:
    "And we don't any more think that there is some formulaic, diagrammable, universal targeting method: it's just that they know their eyes and their nose and their bodies and their fins and it's a case of "trust in the Force". It is, after all, something they have done a million times before. Very fast turns and re-adjustments are fairly normal as the prey moves: the rise does not always follow choreography or diagrams. Your trout is a free style dancer." Quote from Peter Hayes and Don Stazicker. "Trout and Flies - Getting Closer" (2019, Page 92), Kindle Edition.

    That sounds like 'vitalism' to me " in living organisms is caused and sustained by a vital principle that is distinct from all physical and chemical forces .." (Marler, 2005)

    Unless I see a trout waving a Light Sabre, I'll trust in the suggestions made by earlier authors that the insect's legs and wings, mirror and window - rather than the Force - control the trout's rise !

    But, seriously, their endeavors did have one intriguing finding. In an interesting video from Don Stazicker  "Rise commences as fly drifts into window" the narrator identifies "deployment of the pectorals as the moment when the fish first notices the fly". That got my attention. Why? Because it is the first time I have seen an external behaviour that can be seen and recorded by a human observer defined as the start of a trout's rise. Behavioural scientists love behaviour that can be identified, recorded, and subjected to checking via intra- and inter-observer reliability.

    It seems unlikely that this behaviour is only seen when a trout begins its rise. Nevertheless it would be worthwhile checking that during a rise this behaviour is only recorded when a fly is within a trout's "Plain Sight Zone". This can be calculated because Hayes & Stazicker know the depth of the trout that provided the raw data for their failure to replicate the suggestions of previous authors that elements in the mirror trigger the rise (Clarke & Goddard, Marinaro). Therefore it should be possible to decide where (mirror or window), and what parts of an insect initiate a rise - because :
    1. the trout's behaviour (pectoral deployment) indicates the start of the rise
    2. the known depth of the trout at the start of the rise gives ...
    3. the size of the trout's window at the start of the rise, and therefore ...
    4. the position of the floating fly relative to the edge of the trout's window at the start of the rise

    Nowadays there seems to be less attention paid to using dry flies tied with hackles to represent the legs of duns resting on the surface. An increasing number of effective trout flies are designed to incorporate elements that trigger a rise by penetrating the mirror. Examples of this type of fly - including Bob Wyatt's Deer Hair Emerger (DHE) - are described in Hayes & Stazicker's book.

    What can trout see in the mirror and window? It's time to call in the experts.

    What can trout see in the mirror and window?

    Professor Threadgold (1998) has written a comprehensive description of trout vision for a fly-fishing audience. Threadgold as well as Clark & Goddard consulted   Professor Muntz for advice on fish optics.

    Threadgold explains how trout are able to receive advance warning of an approaching fly in their mirror, and closely examine artificial and natural flies in their window.

    "The optimum angle of most acute binocular vision is considered to be 40 degrees from the horizontal which would normally impinge on the mirror. However, trout tend to lie with their bodies inclined slightly upwards which would bring the most acute point of binocular vision to about 45 degrees.Since this angle is within the 48.5 degree angle of the window, the trout's sharpest vision would be to the anterior edge of the window where it would be most effective." ibid, emphasis added.

    Threadgold summarises research on the optical capabilities of trout vision in this   full-sized diagram from his book.

    Threadgold's comment on how tilting of the trout's body during the rise brings its most acute point of binocular vision within the window addresses Hayes and Stazicker's criticism of Marinaro's, perhaps over-emphasis, on the importance of the edge of the trout's window.

    Binocular vision also plays an important role before and after a fly enters the trout's window. Threadgold describes this as follows:

    "The sector of binocular vision intersects the window as a band (coloured pink in the diagram above) which progressively widens towards the anterior. For a trout lying with his eyes situated at 15.2 cm (6 in) below the surface, the band is about 5.1 cm (2 in) wide overhead widening to about 6.4 cm (2.5 in) anteriorly." ibid, emphasis added

    Clark & Goddard (p.68) asked Prof. Muntz to investigate their observation that trout often move up to 5 feet to intercept a fly or nymph. Muntz supplied a series of calculations, presented in the table below , showing the furthest distance at which the trout's eyes are capable of accurate binocular vision when they are focussed at distances ranging from 1 to 25 inches. Accurate vision when a fly is present in the trout's window is restricted to only a few inches. But when a trout is focussed on objects 2 feet or more away, accurate vision extends to infinity. Thus the trout is able to see:
  • accurately nymphs, wet flies, star-bursts of light made by the feet on duns resting on the mirror, from a considerable distance.
  • fine detail of a fly in its window
  • The trout's depth of accurate vision

    Distance (in inches) at which eye is focussed 1 2 3 4 6 10 15 20 23 25
    Furthest distance (in inches) at which accurate vision is possible 1.033 2.16 3.39 4.76 7.97 17.15 39.37 104.17 312.5 Infinity ∞

    Blind spots

    Marinaro and Threadgold considered that the trout has blind spots in-front and overhead. Others do not agree.

    More recently Schullery (2008 p79-87) has pointed out that muscles attached to the eye enable eye -movement in a similar manner to humans. This view is shared by Ozefovich. His conclusions are presented in this video clip.

    The longer version has more examples of the trout's eye movements.

    Clip from "Ozzie" Ozefovich's   Underwater World of Trout Part Three | Trout Vision

    Visual Acuity

    The term 'visual acuity' refers to the ability to distinguish between fine details. Does this matter to a fly-fisher? I think it matters a lot when we design an artificial fly to trigger a trout's rise, and withstand close inspection during the rise.

    Hayes and Stazicker make the important point that a trout's visual acuity is less than that of humans. They use these side-by-side photographs of a trout fly as seen by humans, and possibly by trout with their poorer acuity.

    "The fly on the right is as we would see it, the fly on the left is as the trout would see it if both flies were viewed from the same distance [1 metre, 3.3 feet]. 15 times less acuity than humans."(Page 69). Hayes and Stazicker's videos could provide very important insights into the visual experience of trout during the rise.

    Behavioural scientists have tried, but found it difficult, to present to the human eye a picture of what an animal perceives in its environment. "For example a common and key misinterpretation is that these portrayals describe the actual perceptual world of the animal in question, and that species with low visual acuity perceive the world as “blurry.” Due to edge enhancement and other forms of neural processing, this is likely not true. Instead, these portrayals only allow us to assess the information content of a scene, to a given viewer. Caves and Johnsen (2018)

    Dr Caves (personal communication 2020) gave this very useful explanation of how visual acuity is measured: "Acuity is often measured in units called cycles per degree (cpd), a number which represents how many pairs of black and white stripes you can distinguish in a single degree of visual angle.

    If you give a thumbs up and stick your arm out as far as it goes, your thumb nail covers about 1 degree of visual angle. So, human acuity is roughly 72 cycles/degree (you could jam 72 pairs of black and white stripes into your thumbnail and still be able to distinguish them all)..

    .. rainbow trout acuity is about 4-5 cycles/degree and  Coregonus hoyi  it's about 3 cycles/degree." ; both fish are salmonids.

    The acuity value of the lagoon triggerfish (Rhinecanthus aculeatus) is 3.4 cycles per degree, which is comparable to that of the two salmonids in her database. She confirmed that rainbow trout do have acuity about 15 times less sharp than humans.

    The images below are derived from Fig 1. in Caves et al (2017) and show the effect that distance has on the visual acuity of a fish comparable to salmonids.

    Original image

    Viewed from 2 m

    Viewed from 5 m

    Original image

    Viewed from 2 m

    Viewed from 5 m

    Original image

    Viewed from 0.5 m

    Viewed from 2 m

    It is important to appreciate that these pictures do not show us what a fish perceives i.e. in human terms sees.

    Because there may be edge enhancement and other processing by the retina and brain that sharpens or improves an image. What these pictures do show is the "raw material" available for further refinements.

    But clearly the amount of detail available for processing by a trout's visual system  increases  as the trout gets closer to a natural or artificial fly.

    Triggers, Fixed & Modal Action Patterns

    | Triggers |

    It is increasingly common for fly-fishing authors to use the term 'trigger' to describe a feature of natural and artificial flies that may elicit or guide the trout's behaviour. This is a perfectly legitimate use of the term. Ethologists also seem to use this term in preference to the older, original phrase, 'sign stimuli'. But it's important to point out that, until we have supporting evidence, these are just putative candidates for triggers. They appear to us to qualify as triggers, but may not be triggers from a trout's point of view.

    Ethologists have a way of testing this. They construct models that incorporate one feature at a time, present it to the animal, and observe the animals' behaviour.

    Flyfishers have been doing a similar thing for hundreds of years; presenting models of insects to trout and observing their behaviour. But with one important difference. Fly tyers have not always been content to present simple models each with only one trigger in a fly pattern. Most of our flies consist of a bewildering mix of features: size, shape, colour etc.

    And, of course, the best trigger we can construct may not behave in a natural way. A good example of an artificial's misbehaviour is drag caused by their attachment to a leader.

    It is important to draw a distinction between sign stimuli and 'search images'.  Search images  have a temporary existence. This distinguihes them from the relatively permanent effect of a sign stimulus.

    | Fixed versus Modal Action Patterns |

    In classical ethology sign stimuli elicit fixed action patterns (FAP). These were defined as instinctive (unlearned) rigid stereotyped behaviours that continued even if the sign stimulus was withdrawn. Strictly speaking in classical ethology a stimulus that triggers a FAP ceases to exercise further control over the behaviour (Barlow, 1996 p 142). Clearly this is not the case in a trout's rise; the natural or artificial fly controls behaviour throughout the rise. This is not unusual, Barlow (1996) gives examples of fish FAPs where the trigger modulates feeding, as well as behaviour between conspecifics.

    "Action patterns are the behavioral units of ethology; the formerly used adjective "fixed" in the term "fixed action patterns" is now often dropped because of its implication of developmental fixity and lack of individlual variability" (Shettleworth 1974, Schleid 1974).

    Barlow (1996) introduced the term modal action pattern (MAP) because close examination revealed that many animal behaviours were fluid in form, not fixed. Importantly MAP describes a sequence of behaviours that respond to changes in the environment. In my opinion, the trout's rise is an example of a modal action pattern because it varies according to environmental conditions, and meets the criteria for a MAP suggested by Barlow (1996):

  • The trout's rise is variable in its form e.g. the simple rise,  compound rise, complex rise  to a dry fly on the surface, and the variety of rise forms depending on the species, location and behaviour of the insect in the water column (Schullery, 2006 ). This table from Bulmer (2015) lists the rise forms described by Eric Taverner in his book “Trout fishing from all angles”.
  • A rise is triggered by a   search image  that varies from time-to-time according to the availability and physical properties of particular prey items.
  • The rise is a pattern of coordinated movements organised in time and space. The behaviour is recogisable to a human observer i.e. called a 'rise' by fly-fishers.
  • Variability in the form of the rise, and the role of peripheral stimuli (e.g. aspects of an artificial fly that control the rise ) remain to be determined.
  • An important feature of modal action patterns is that the same stimulus can have different effects depending on the physiological state of the animal (Domjan, 2010 p. 37). Sea trout (sea-run brown trout) are a clear example of this feature. The response of trout (salmo trutta) to natural flies depends on whether they are resident brown trout, or brown trout that have migrated to sea and returned as sea trout. Generally sea trout do not feed in daylight on their return to freshwater before spawning, and are rarely caught on artificial flies in daylight, but can be caught at night on surface lures.
  • | Appetitive and Consummatory Behavior |

    In 1917, the early  ethologist Wallace Craig (1876–1954) introduced the terms appetitive behavior and consummatory behavior. Appetitive behavior refers to an active searching process for stimuli (triggers) at the start of what is now called a modal action pattern (MAP). When these stimuli are encountered appetitive behaviour is replaced by consummatory behavior - a more stereotyped activity that completes a MAP. From our perspective appetitive behaviour refers to a trout employing a  search image to locate prey. Consummatory behavior describes the trout's rise when stimuli corresponding to prey have been detected.

    A difference between appetitive and consummatory behaviour is that appetitive behaviour is more variable and can be influenced by an animal's experience - practice, maturation and learning. (see  Beach, 1956 Model C  Figure 2 in Ball et al, 2008). In contrast, consummatory behaviour is described as stereotyped and  species-specific .

    Thus a trout's search image is probably not innate - fully formed - fixed at birth. It can alter as a result of experience. On the other hand, consummatory behaviour, the rise, is less influenced by experience.

    Ball et al. (2008) comment that "In the case of the appetitive/consummatory distinction it is not always apparent when the transition occurs."

    | Ethograms |

    An ethogram may give a clue where to locate the transition point.

    An  ethogram  is a list of an animal's behaviour(s). It consists of a brief description of behaviour(s) that enable an independent observer to recognise and record the behaviour(s). It should not include speculation on the purpose or intention(s) of the animal's actions. It can help to include drawings or video to illustrate the text in an ethogram. This is a drawing of a 'simple rise'.

    This ethogram was written by Ringler (1985) as part of his research into prey switching (between mealworms and caterpillars) by five (8.5 to 10 in.) three year old brown trout in a 'drift feeding' laboratory situation. Trout sometimes forage for food, but normally food is brought to them by the river - drift feeding. In simple terms shopping in a supermarket is foraging, dining in a conveyor belt sushi restaurant is drift feeding. Foraging is considered to be an appetitive behaviour (Alvarado et al. 2018).

    But note how Ringler's definition of Inspection differs from this description used by Neuswanger et al. (2014): "Fish made a range of motions that did not culminate in opening their mouths to capture drifting items. Motions were classified as inspections of potential prey if they began and ended with sudden changes of body orientation or if the particles of interest were clearly visible. These stringent criteria were necessary to avoid counting both brief and extended motions made for other reasons." .... "Making the assumption that fish reacted to items immediately upon detecting them, we recorded detection positions in the frame immediately preceding movement toward an item"

    The function / purpose of Inspection in  compound and complex rises is discussed below.

    On Ringler's ethogram I would classify Detection as an appetitive behaviour, and Approach - the start of the angler's 'rise' - as the first behaviour in the consummatory element of the modular action pattern.

    | Predicting the transition point |

    If the transition from appetitive to consummatory behaviour - the start of the rise - occurs when the natural or artificial fly crosses the boundary from mirror to window there are two ways of measuring this point:
  • The edge of the trout's window is a mathematical function of the depth of the trout's eye from the surface. This can be determined using the 3-D video method developed by Neuswanger (2014) and Neuswanger et al, (2014) to analyse salmonid feeding behaviour using underwater video.

  • Provided that the depth of the trout's eye is known, the  'pectoral deployment'  reported by Stazicker may provide an observational approach to identify the start of the consummatory phase in the trout's rise.

  • Alternatively an appropriate variation of these two techniques could be used to test if the transition from appetitive to consummatory behaviour - the start of the rise - occurs when the trout responds to an alteration in the mirror made by the natural or artificial fly.

    To answer the question "How does a trout catch a fly?", it may be helpful to treat the trout's rise to artificial and natural flies as a modal action pattern (MAP) with appetitive and consummatory phases.

    Interim summary

    The main message from this analysis so far is that a successful trout fly will harbour several stimuli that trigger a sequence of behaviours in what anglers call a rise, and ethologists would term a   modal action pattern.

      The artificial fly:
    1. may present a primary trigger stimulus that indents or penetrates the mirror. For example, a dubbed fur body may imitate legs penetrating the mirror. These may guide  appetitive behaviour  to reposition the trout closer to the approaching fly prior to the rise.
    2. wings on the artificial may act as secondary triggers during a rise to maintain the trout's movement towards the fly
    3. academic studies of the trout's - and other salmonids' - optical capabilities suggest possible perception of the signals - suggested in the fly-fishing literature - of an approaching fly .
    4. the amount of visual detail for a trout to 'inspect'   increases  as the trout gets closer to a natural or artificial fly.
    5. the presence of the merged image of the body and wings as it passes through the edge into the window may allow the trout to precisely judge and maintain its distance from the fly. A pronounced thorax on an artificial fly may enhance this visual trigger for the final  consummatory stages  of the rise that are controlled by a  Tracking Heuristic  - described below.
    6. a dry fly must not 'drag'. Drag will cause an unnatural disturbance in the spatial relationship between the insect and the edge of the trout's window - discussed below

    Work by academics and fly-fishing authors has laid a foundation for further study of how stimuli control a trout's rise. The next section takes up the story from the point where the rise is triggered, to consider how a trout Tracks the location of a fly in order to capture it. This is a part of the trout's rise that remains a mystery to me. How does the trout intercept its prey ? An apparently simple process, but one in which a lot can go wrong for the trout as well as the angler....

    How does a trout intercept a fly?
    The Gaze / Tracking Heuristic

    Why use the case study approach to address this question?

    In situations where quantitative data is deficient in some way (e.g. lacking, unreliable or disputed), a case study approach (Crowe et al, 2011) may provide useful insight into behaviour(s) that cannot be studied under laboratory conditions.

    I added this section to encourage interest in, critical analysis of, and research into a possible role for heuristics in trout feeding behaviour.

    Classic Drift Feeding Behavior - Arctic Grayling in Clear Creek

    Full length video   available here

    Link to  The Drift Feeding Project

    Over the last century there has been a steady stream of books reporting detailed examinations of the trout's rise to intercept natural and artificial flies on, or emerging through, the water surface.

    Our understanding of this fundamental trout behaviour has benefited from reported observations by anglers, advances in underwater videography and photography, as well as classic ethological and laboratory studies of fish.

    But one question about this predatory behaviour has not been addressed. What mechanism controls the trout's movement(s) to intercept and capture a moving natural or artificial fly?

    Heuristic case studies have been particularly successful in understanding predatory behaviour in mammals, birds and insects. Evidence in the fly-fishing literature, based on careful analysis of photographs taken of fishing rising to intercept prey, suggests that it too may be controlled by an heuristic.

    This case study investigates the possibility that the trout rise to intercept its prey is controlled by a tracking heuristic employing a constant angle of approach. This may lead to an explanation of why, under certain conditions, trout may fail to consume artificial flies.

    In addition ethological studies on the role of sign stimuli in eliciting animal behaviours may offer an insight into what features of anglers' artificial flies trigger a rise.

    " Tinbergen found, it is often the case in animals that quite crude tricks suffice, itself perhaps a reflection of animals’ greater reliance on simpler rules of thumb." (Hutchinson & Gigerenzer, 2005) i.e heuristics.

    To borrow from American fly-fishing author John Gierach:
    'Let me introduce some ideas, just some things to kick around.'
    Quote from Gierach (1989).

    What is the Gaze Heuristic? - A simple and efficient rule of thumb

    In this video mathematics philosopher Gregory Wheeler explains how humans and animals use the Gaze - or Tracking - Heuristic to intercept  moving objects without solving differential equations.

    I am grateful to   Prof Wheeler for pointing me in the direction of relevant papers on Gaze Heuristics.

    "The gaze heuristic is an interception heuristic that utilizes a single input (deviation from a constant angle of approach) repeatedly as a task is performed." Hamlin (2017)

    More about how the  co-pilot Jeffrey Skiles used the Gaze Heuristic in the Hudson River incident

    I have included the next diagram from Dr. Hamlin's 2017 paper “The Gaze Heuristic:”  because it portrays three important features involved in a Gaze or Tracking Heuristic :
    1. constant angle of approach
    2. repeated decision points
    3. future position predictions
    Incidentally the wartime situation it describes is the origin of what is now known as the 'Gaze Heuristic'.

    “... the gaze heuristic was discovered accidentally by Royal Air Force (RAF) fighter command just prior to World War II. As it was never discovered by the Luftwaffe, the technique conferred a decisive advantage upon the RAF throughout the war.” ibid.

    It was an important element in the Dowding aircraft  interception system  and continues to be used in AIM-9 Sidewinder  missiles

    Given the distance between the fighter and bomber an isosceles triangle was constructed with the distance as its base value. Point X - the height of the isosceles triangle - represents the first predicted interception point - based on the bomber's position and track towards the presumed bombing target. The values of distance and height in the isosceles triange were used to   calculate the two equal angles at the base of the triangle. This angle was used as a constant ( affectionately called the   "Tizzy Angle”   denoted as T Θ) to predict subsequent interception points as the distance between fighter and bomber changed over time. (DeGering 2018)

    A magnified view of the bomber track and fighter vector is   available here

    A magnified view of the bomber track and fighter vector when the bomber changes direction is   available here

    In both situations the fighter intercepts the bomber from the front

    Dragonflies don't do maths, but they do use a Gaze Heuristic with a constant angle of approach to pursue and intercept moving prey.

    The constant angle is labelled the "Tizzy Angle” in this diagram to maintain continuity with the explanation above.

    Diagram modified from Figure 4 in Stevens and King (2012).

    Gonzalez-Bellido et al. (2013) have shown that dragonflies are able to capture 95% of their prey using a small network of 16 specialised nerves called target-selective descending neurons.

    Nordström (2013) provides a companion paper with a 'user-friendly' description of this research.

    At first sight it may seem strange to suggest that humans, birds, and insects share a tracking mechanism to intercept moving objects. In fact, the way the brain of these different animals perform this feat is remarkably similar. "We can therefore, somewhat surprisingly, use the insect visual system to understand the coding of visual cues in our own brain. Dragonflies do not play tennis, but they are extremely efficient predators who intercept tiny prey with astonishing success rates, entirely guided by visual cues. This tells us that they must have the neural machinery in place for detecting target motion, even in complex visual surrounds." (Nordström, 2013).

    The "...gaze heuristic is the only known technique used by predators [mammals, birds, and insects ] to intercept prey..." Hamlin (2017)

    You will have noticed that fish are missing from the list - humans, birds and insects - of creatures that use a common mechanism to intercept moving targets. (Nordström, 2013 and Hamlin 2017).

    This video suggests that archer fish may use a Gaze Heuristic involving a fixed angle of approach to catch their prey: "the hunting behavior of the archer fish is composed of surfacing concomitant with rotating the body around the direction of the fish’s fixed gaze towards the target, until the snout reaches in the correct shooting position at water level." (Ben-Simon et al 2009) [emphasis added].

    The next section proposes that trout may intercept prey using the same mechanism as other animals.

    Trout behaviour and the Gaze Heuristic

    Can heuristics explain a trout's rise to a dry fly?

    Inasmuch as I'd like to, it's impossible to give a straightforward 'Yes' to this question. There is a remarkable lack of research in our area of interest - trout - compared to a wealth of study into much more important applications of heuristics: e.g. catching balls, and selecting stock market investments !.

    But Shaffer and McBeath (2002) mention that a teleost fish locks onto the motion of their target in a way that maintains optical angle constancy.

    Gerd Gigerenzer describes the Gaze Heuristic (from minute 14 to 26)

    Gerd Gigerenzer's Gaze Heuristic is specifically designed to deal with this type of problem - using a constant optical angle of approach to track a prey that changes position.

    So it seems to me a good starting point for exploration of the technique used by trout to inspect and catch a fly.

    In this section of his video Gerd Gigerenza introduces the Gaze Heuristic by describing the problem of a fielder catching a ball. Substitute a trout for the human, and a fly for the ball. This should clarify why I chose to use the Gaze Heuristic as an explanation of how a trout rises to catch a fly.

    The Gaze Heuristic predicts that the trout will be able to intercept the fly if it maintains a fixed of angle of approach when moving towards its prey.

    In the simplified situation described in this diagram maintaining a fixed angle   (  48.5 degrees  )  of approach will enable inspection and interception

    There is a significant body of research showing that by maintaining a constant angle of approach predators are able to lock on to, and then track, the movements of their prey.

    The term 'Gaze Heuristic' is unfortunate in the sense that a human gaze suggests continuous unbroken looking at an object. Perhaps a better term is 'Tracking Heuristic' as suggested by Gigerenzer and Brighton (2009). Tracking describes the trout's behaviour more accurately as: A series of repeated actions - an iterative process - to reach the final goal of ingesting a prey item.

    Setting the fixed angle of approach (the Tizzy angle) for the Tracking Heuristic

    The still images below show that the edge of the trout's window :
  • is not sharply defined at all times, and ...
  • the insect's wings, legs and body merge and separate from moment to moment at the edge of the window
  • These effects are to be expected because the surface of moving water is not absolutely flat.

    Ripples and undulation on the surface suggest that trout may have significant problems keeping an insect at the edge of their window for inspection purposes.

    But this is less of a problem than it might appear at first sight. In order to engage the Tracking Heuristic , the trout simply has to settle on a value for its fixed angle of approach. Once this value has been fixed, the Tracking Heuristic takes control of the approach process using this fixed value.

    Once fixed the  (the "Tizzy Angle” T Θ) remains constant as the predator tracks its prey.

    When does the trout fix the Tizzy Angle? Pectoral deployment is the external observable trout behaviour identified by Don Stazicker as the start of the rise.

    Three Triggers are suggested to initiate the rise and may therefore elicit pectoral deployment:
    1. Trigger #1, star-bursts of light in the mirror
    2. Trigger #2, the appearance of wing tips in the window
    3. Trigger #3, the merging of wings and body at the edge of the window

    Trigger #1 ...

    or Trigger #2 ...

    or Trigger #3

    Calculating the Tizzy angle depends on which Trigger is seen first, and used by the trout to set the fixed angle of approach.

  • Trigger #1. If the trout's rise is triggered by star-bursts of light emanating from a fly on the surface, then a reasonable value for the fixed Tizzy angle would be 40 degrees - the angle of most acute binocular vision for a trout lying horizontally  Threadgold  . Using this angle we can   calculate the distance between a trout's eyes and the star-burst created by an insect's feet. For example, this distance is 56 in ( 4.7 ft ) for a trout lying horizontally 36 in (3 ft) beneath the surface. Distance is used to calculate   Point X, the first predicted interception point. Of course the trout doesn't do the maths. Mother Nature handles that, this is an evolved mechanism.
  • During a rise the trout's body tilts upwards. Maintaining this fixed angle of approach (i.e. a Tizzy angle of 40 degrees) throughout the rise would bring the most acute point of binocular vision to about 45 degrees ( Threadgold).
    Since this angle is within the 48.5 degree angle of the window, the trout's sharpest vision would be adjacent to the anterior edge of the window.
  • Trigger #2. The distance at which wings tips, and star-bursts of light, are both visible in the trout's window is, as we have seen, a function of the height of the insect's wings. Because of this uncertainty, and the closeness of Trigger point #2 to the edge of the trout's window, it may be less likely that Trigger #2 is used to fix the constant angle of approach to the fly.

  • Trigger #3. This is the position favoured by Marinaro and  Brian Clarke (1996).With wings and body merged at the edge of the window and with a clean edge to the window - an effective fixed angle would be 48.5 degrees.
  • With an uneven edge to the window this value could vary within a range extending between more than 48.5 degrees (the leading edge of the window) to less than 90 degrees directly overhead (the position of a possible blindspot, Threadgold, p 31-2). This is within Hayes & Stazicker's "Plain Sight Zone" .

  • Incidentally, this problem - when to fix the angle of approach - is faced by any animal or mechanical tracking system. An example is provided in this diagram from Huai-Ti Lin and Leonardo's in-depth study   of dragonflies capturing prey. And it requires remarkably little "brain power". Gonzalez-Bellido, Paloma T et al. (2013) have shown that dragonflies are able to capture 95% of their prey using just 16 specialised nerves.

    During tracking, shifts in the position of prey is a problem faced by all predators. The Tracking Heuristic enables predators to track their prey by repeated checking for alterations in the prey's location at a sufficient rate to deal with changes in the position of the prey.

    Therefore - during the rise - a trout may need to alter its position several times to retain the fixed angle of approach towards the fly.

    What is the Tracking Heuristic & what does it do?

    The Tracking Heuristic repeatedly checks   (Input) the predicted position of the fly. Any deviation  (Decision)  from that position prompts correction  (Task)  of the trout's position. Tracking continues until the trout consumes the fly Outcome,  or the Tracking Heuristic in interupted.

    Tracking Heuristic: Figure 2b in Hamlin (2017)

    Checking occurs at a fixed rate set by the trout's 'visual rendering rate' . For humans it is about 10 cycles a second. For birds of prey it is much higher.

    Currently, I do not know the trout's visual rendering rate.

    Each Decision point in this model of the Tracking Heuristic is 'Write-Once-Read-Once': previous positions are not stored in a memory bank that can be referenced. Consequently - as we are about to see - if the position of the fly cannot be determined, the heuristic breaks down, with serious consequences for the angler !

    Tracking in the natural environment: Riffles - a problem for the gaze heuristic?

    It's not unusual for a natural or artificial fly to be buffeted by currents as it approaches a trout.

    The tracking heuristic is particularly well suited to deal with 'evolving decision environments' (Hamlin 2017); environments that change significantly as the task is being performed. Riffles - fast moving, relatively shallow water upstream of deeper pools - are good examples of evolving decision environments, and provide prime insect habitat in freestone rain-fed rivers.

    Peregrine falcon hunting manoeuvres (Mills et al 2018)

    The trout's problem is similar to that encounter by other predators dealing with movements made by prey. "The ability to fixate on a moving target is a common feature among most predatory animals" Gonzalez-Bellido et al (2013)

    Inevitably an insect floating downstream towards a waiting trout will be subject to lateral movements in riffles where water flows discontinuously. Trout are able to track and intercept an artificial fly in this chaotic environment. An insect or artificial fly is converted into moving prey by unpredictable alterations in its trajectory.

    Hamlin (2017)uses the example of a hawk (Kane et al, 2015) pursuing a duck to illustrate how a tracking heuristic works in this situation.

    Fig. 1d shows that the hawk is able to intercept the duck because the hawk's heuristic cycling rate (intervals between successive Inputs to the heuristic) enables it to check the position of the duck, and if necessary, reposition its own flight path to continue the constant angle of approach to intercept the duck. (see also Brighton et al 2017, Mills et al 2018).

    Dragonflies use a tracking heuristic with a constant angle of approach to intercept prey during very short (∼200–500 ms) predatory flights. "If the prey image drifts on the retina, compensatory motor signals sent to the wings adjust the dragonfly body position to bring the prey image back." Gonzalez-Bellido et al (2013).

    Sometimes a fly will pass a trout, the fish turns and chases the insect downstream to take it. In this case a 'direct pursuit' is made with constant angle of approach set to zero. It works if the pursuer can overtake the pursued.

    There is an example of downstream pursuit in this video from Don Stazicker  "Rise commences as fly drifts into window" 

    Fig. 1b shows this type of direct pursuit using the hawk and duck exemplar.

    There is an interesting balance between the preferred position of trout beside riffles that provide greater access to food per unit of time than the slower moving water in pools, and the protection offered to insects that live in moderate to fast riffles by unpredictable trajectories as they are swept downstream. Unpredictable trajectories are a recognised   anti-predator behavior (Moore and Biewener, 2015). Maybe some aquatic insects benefit from 'passive' anti-predator behaviour

    Drag: Application of the Tracking Heuristic to a practical problem

    Much has been made of the importance of avoiding drag in fly-fishing books and articles. The fly may be 'refused' if it drags on the water surface. Therefore anglers strive for a drag-free drift. Some anglers believe that drag actually scares fish.

    Marinaro (1995, p 29) states: "Many books by competent writers and fishermen contain learned discussions about drag and its effects. ... All conclude that that a dragging fly frightens the trout. I do not agree with that at all." emphasis added

    "Drag may not spook the fish; they just go "nope" and resume watching for the next bite of food" (Wyatt, 2004, p.44)

    I agree with these opinions for the following reason: "The brain systems known to be necessary for the experience of fear or other emotional experiences are not present in fishes." (Rose, 2002)

    The Tracking Heuristic offers an explanation for why a trout may - under certain circumstances - ignore an artificial fly that drags, and incidentally support for Bob's view of the insouciant trout.

    In a 'Tracking Heuristic' the trout:
    1. Checks the position of the fly
    2. Predicts the future position of the fly
    3. Moves to that position

    Discard the value predicted in Step 2. (I think of this as a 'Write-Once-Read-Once' process.)

    1. Checks the position of the fly
    2. Predicts the future position of the fly
    3. Moves to that position

    Discard the value predicted in Step 2 'Write-Once-Read-Once'

    Repeat Steps 1, 2, and 3 as necessary to reach a position where the fly can be ingested.

    Drag and the Tracking Heuristic

    I would like to suggest that the problem faced by anglers - drag interfering with a trout taking a fly - is caused by a complete breakdown in the Tracking Heuristic.

    If our artificial fly is dragged so far away from its predicted position, the trout will abort its rise, because a necessary Input has been removed, the next Tasks can not be performed. The behavioural sequence controlled by the Tracking Heuristic is literally stopped in its track because it is 'Write-Once-Read-Once': previous positions of the fly are not stored in a memory bank that can be referenced.

    "It's just not cricket ! "

    The impact on a trout of a fly that drags, is similar to the effect on a batsman of dealing with a cricket ball that has been   tampered with.

    A hare avoids capture by hiding to defeat the Tizzy angle

    I hope this will reassure anglers that drag does not necessarily scare trout.

    In my opinion there is no reason to believe that simply interrupting a Tracking Heuristic causes fear.

    If it did, animal rescue centres would be crammed with predators suffering from nervous exhaustion !

    The Skittering Caddis

    In some circumstances trout will take a fly that resembles a dragged fly. For example, holding the rod high to dibble a bob-fly may mimic the behaviour of an adult sedge that skitters across the surface.

    The trout's rise to skittering caddis may be triggered by the disturbance created - in the trout's mirror - by the insects movements, and can be deliberately mimicked by an angler. For example, the 'flutter' cast described by Solomon and Leiser in "The Caddis and the Angler" (1977).

    But the authors are careful to distinguish between drag and 'flutter': ".. a "flutter" is not a yank. The fly should move only an inch or so at a time. There should be a pause during which the fly is allowed to move naturally" ibid page 95.

    This is quite different to the drag caused when an artificial fly is dragged by the influence of moving water on fly line, leader or tippet.

    Hugh Falkus' Surface Lure: Exceptional drag that proves the rule?

    There is one time when the normal effect of drag is reversed - on dark nights. On dark nights brown trout and sea trout   (Salmo trutta)  will enthusiastically take a dragged fly, but refuse the same lure if it does not drag.

    In this photograph the fly is being dragged from left to right creating a wake (Falkus,1975. p.79)

    Are fish using a Tracking Heuristic in this situation?

    "Provided the night is dark enough, this drag, or wake caused by a floating lure that is skimmed across the surface - either by the force of the current or through action on part of the angler - can provide a deadly attraction for sea trout. And, incidentally, for brown trout too. It is clear that in order to produce this drag the lure must be kept moving over the surface of the water. If, when fishing across a current, the line is allowed to go slack, the lure will begin to drift - and so loose its wake." ..."It is the wake of the lure and not the lure itself that attracts the fish"

    Quote from Hugh Falkus "Sea Trout Fishing" (1976) Second edition, Chapter 3 (111) The Surface Lure. Witherby Ltd.

    The same technique (dragging a fly across the surface) is used to catch large brown at night in Montana. See here for more ...

    "Only an idiot would go fishing through a summer’s night for sea trout without having some floating lures in the box and giving them a good go.

    Quote from Malcolm Greenhalgh's Blog | I must tie floating lures for sea trout | 10.01.2012 more here

      The trout's behaviour towards Falkus' surface lure has several advantages as a candidate for a Case Study of the Tracking Heuristic.

    1. We have seen above that the Tracking Heuristic can account for a breakdown in interception behaviour when drag is   present.
    2. Can the Tracking Heuristic account for a breakdown in interception behaviour when drag is   absent ?
    3. Pursuit of a wake producing surface lure allows us to compare the Tracking Heuristic ( a cyclic decision making process) with the non-cyclic (normal) decision process described by Hamlin (2017, Fig 2) .

    A surface lure can be made from any material that floats and will support a single or treble hook. Colour is immaterial, and feathers can be added simply to satisfy the angler's fancy. It is reminiscent of the sign stimuli models used by early ethologists to study animal behaviour.

    The behaviour takes place in darkness. This begs the question - Do trout use vision to track the lure, or some other sensory cue ? For example,   vibrations detected through the lateral line is worth further research.

    As daylight retreats   the retina adapts. The cone cells recede and the sensitive rod cells are exposed, engaging the trout’s night vision and turning the world to shades of grey. See Rader et al (2007) for detail on the scotopic (low light) vision of trout. They concluded that "The scotopic sensitivity of brown trout and brook trout was sufficient to allow foraging during all twilight periods and under average nighttime light intensities in open and shaded reaches."

    Falkus is quite clear labout the behaviour of trout towards the Surface Lure. His observations will be familiar to sea trout anglers, and suggest to me that the fish are employing a Tracking Heuristic.

  • Brown trout, as well as sea trout, will take a Surface Lure at night.

  • the wake / drag is the trigger that attracts the fish

  • sea trout will sometimes "stalk" the lure by "following close behind for some distance" ibid
  • This is consistent with the suggestion that the fish is using a Tracking Heuristic
  • drag must be maintained. If the lure falters and the wake disappears, the "fish will have nothing more to do with it during that cast" ibid
  • We would expect this to happen if the fish is using the Tracking Heuristic: The fish is tracking the wake. If the wake is not in its predicted position, the behavioural sequence will be aborted. As we saw above when Input is removed, the Task can not be performed. The behavioural sequence controlled by the Tracking Heuristic is literally stopped in its track.

    When the Input provided by the wake of the surface lure stops, the behavioural sequence (Tasks) controlled by the Tracking Heuristic is literally stopped in its track.

    It's as if an approaching enemy aircraft -being tracked by   Grenfell’s and   Tizard’s  fighter interception heuristics - simply disappeared. There's no point wondering where it went. Select another enemy target.

    Summary: Influence of drag on the Tracking Heuristic in daylight and darkness

  • In daylight, drag can disrupt a trout's rise because drag removes the fly from where it is predicted to be by a fish using the Tracking Heuristic
  • In darkness, drag / wake is used to locate and predict the position of the fly by a fish using the Tracking Heuristic. In the absence of the wake the Tracking Heuristic can no longer be employed by the fish
  • Effect of drag on the Tracking Heuristic:
    Time of day: Drag present Drag absent
    Daytime Disrupted Enabled
    Nightime Triggered Disrupted

    Drag: A test of two psychological theories?

    Does interception of a surface lure involve a cyclic (Heuristic) or non-cyclic (Normal) decision-making process ?

    It seems to me that trouts' reaction to a surface lure provides a test of the two theories. A non-cyclic account would predict successful interception because all the Input is gathered before the approach is launched. The heuristic theory would predict the opposite - interception would terminate as soon as Input was removed.

    This may help anglers! My simplistic understanding is that the non-cyclic model shown here is basically: "We're suited-and-booted, locked -and-loaded with information, let's go and get that pesky critter!" But I could have misheard my undergrad lectures on Cognitive Psychology.

    The   cyclic ( Heuristic) model is like kids in the backseat repeatedly asking "Are we there yet?"

    Some suggestions for future research

    I have been very impressed by the enthusiasm, expertise and innovative computer software encountered during my enforced absence from local rivers due to Corvid-19 lockdown.

    The   Drift Feeding Project  uses video cameras to record in 3D fish feeding behaviour. Analysis of these videos - using   VidSync  software developed by Dr Jason Neuswanger - informs a drift-feeding model for fishery management purposes. There are acknowledged limitations to the existing model which, for example, sometimes fails to predict the path taken during prey interception. Given the richness of data already collected on this project, it might be worthwhile examining whether incorporating a tracking heuristic element into the existing model improves performance on this aspect of feeding behaviour.

    Stereo video 3-D analysis of a Dolly Varden trout drift feeding

    "Short clip from a stereo video 3-D analysis of a dolly varden from interior Alaska drift feeding in a small creek. The green pac-men represent the positions of the fish when it first reacted to an item and again when it intercepted (or rejected) the item."

    Link to  The Drift Feeding Project

    Playlist: Prey Detection Locations from The Drift Feeding Project

    "Yellow dots represent the estimated position of a possible prey item when the fish first reacted to it. All positions are shown relative to the fish's position at the time of detection. The fish is shown facing straight upstream. The 'x' direction is straight downstream and 'z' is vertical."

    Link to  The Drift Feeding Project

    Although the Drift Feeding Project is not specifically targeted towards understanding the relationship between the trout's window and prey interception, the ability to precisely measure the spatial relationship between detection position and prey location might throw light on visual acuity and prey selection.

    Dr Eleanor Caves'  AcuityView  software could be used to determine the visual information available to a fish at the prey distance when the rise is started. This could provide insight into the 'search image' used by the predator that results in ingestion, as well as rejection and expulsion (spitting out) of an item in the drift.

    An alternative view on Inspection and Refusal during the rise

    | The Conventional Interpretation: Inspection and Refusal |

    Marinaro's insight is summed up in his sentence:"It is an inescapable conclusion that the trout places the fly always at the edge of the window for all purposes: viewing,   inspecting and taking"(Marinaro, 1995, p. 20)

    In addition to a   simple rise,  Marinaro described two additional rise forms - compound and complex that were interpreted as evidence of the trout's close inspection of natural and artificial flies.

    Compound rise

    "The compound rise is essentially a simple rise wherein the trout takes more time to examine the food source. It will continue to drift downstream with the fly and may even turn a little before it either takes or refuses it." Description from Fishing Small Flies By Ed Engle

    Complex rise

    "The complex rise occurs when the trout has considerable doubt about the food source. As the trout drifts downstream with the fly, it will stay further away from it and take longer to inspect it, it will either turn and quickly pursue the fly or refuse it. Vince Marinaro says that once the trout decides to turn downstream and chase the fly, he will never refuse it" Description from Fishing Small Flies By Ed Engle

    In his description of the Compound rise Marinaro writes: "... in the compound rise the trout will stay with the insect (or artificial fly) throughout the entire drift, constantly inspecting while deciding whether to take or refuse." (Marinaro, 1995 edition, colour pages following text page 8)

    The behaviour described as inspection by Marinaro bears a striking similarity to the behaviour described by Ringler (1985) as  inspection   by mature brown treat presented with natural prey under laboratory conditions : "Drifting downstream, tail first, at a fixed distance from the prey. (This behavior was not observed during every feeding sequence)... The surface of the water from which the prey was captured was smooth."

    Marinaro and Ringler make the important point that compound and complex rises are seen in response to   natural  as well as artificial flies. And both types of fly can be refused.

    Clearly Inspection of natural (as well as artificial) flies is not always followed by trout capturing the prey. This raises the question. Why?

    | Dealing with Refusals: The conventional approach |

    I think I'm safe in saying that most - if not all - fly-fishing authors would agree with Marinaro and Engle that trout scrutinize artificial and natural flies and may refuse to eat them if they do not pass inspection. And behavioural scientists - by their use of the term 'inspection' - seem to agree as well ! Strictly speaking an   ethogram  should not include speculation on the purpose or intention(s) of the animal's actions.

    Interpretation of the term 'inspection' in this way has led fly fishers to suggest several ways of dealing with refusals.
    For example by :
  • changing the colour of the fly because: "Close-range refusals sometimes happen with the trout drifting right under the fly, literally nose-to-nose with it, while inspecting it. Those refusals are more likely a matter of color because the size and shape already passed the test at a distance." (  Jason Randall, undated)
  • dealing with micro-drag, reducing tippet size, looking for a masking hatch (Fishing the Dry Fly, FAQ3, 1995. available online)
  • using a  smaller fly
  • This part of Clarke and Goddard's TV programme  "The Educated Trout"  is an example of how dealing with 'educated trout' prompted them to go one step further and devise innovative artificial flies.

    Changing the fly or aspects of presentation are perfectly reasonable and are often a successful tactic. I don't want to discourage anyone for leaving it there and getting on with enjoying their fly fishing.

    But I want to dig deeper because only one of these solutions - a masking hatch - explains why a trout would refuse a natural fly. And as Jason Randall points out size and shape have already passed the intial inspection test.

    I think it's worth trying to understand what is being inspected, and why sometimes inspection terminates the  consummatory component  of the trout's modal action pattern to natural insects.

    | An Alternative Interpretation of Refusals: Alignment |

    I would like to offer an alternative albeit novel explanation of a trout's apparent refusal after a period of inspection.

    It's not unusual for predators to fail to catch their prey.

    During the downstream drift the trout's window is very small because the trout is close to the insect on the surface, "literally nose-to-nose with it" (Randall, undated). The trout may be using the Tracking Heuristic to adjust position so that its   Manipulation and Ingestion  behaviours (Ringler, 1985) enable consumption of the fly.

    Marinaro (1995) comments on the photograph on page 22 that the trout drifted "perhaps twenty feet, obviously inspecting the fly very carefully." This seems a very long journey to undertake to inspect the physical characteristics (size, shape, colour etc.) of an artificial fly.

    Therefore I think a trout's so-called refusal after a downstream drift beneath a  natural or artificial fly is the result of a breakdown in the trout's Tracking Heurstic, rather than rejection as a result of inspection of the fly's size, shape or colour by the trout. This is a more parsimonious explanation .

    Maybe it is time to reexamine the status of Inspection in the fly-fishing literature to better capture the function of this stage in the trout's rise. I now think in terms of Alignment rather than Inspection, and view a refusal during a rise as a failure of a trout to be able to align itself with the natural or artificial fly.

    Alignment failure to an artificial fly may result from:
  • drag
  • buffeting by   water currents
  • Alignment failure to a natural fly may result from:
  • buffeting by   water currents
  • I know that in many ways this is a radical explanation, but as I said at the start of this page: "I want to work with the trout rather than trying to deceive it. After all, fish want to eat. If we understand how a fish catches an insect we can present our fly to make it easy for the trout to consume. The angler's problem is to design and present a fly so that it can be easily caught by the trout. " I have sympathy with trout. I suspect they have a hard time getting enough to eat (see Neuswanger et al's, 2014  findings )

    | The role of suction in ingesting a fly |

    Trout ingest insects of various sizes via suction which presumably requires very precise alignment - lining up with the fly to maximise the chance of sucking it into their mouth.

    The human equivalent of in-flight refueling, or threading a needle ... in the wind.

    Paul Schullery used this image in his book 'The Rise' to explain how "The trout's suction feeding typically pulls a double-tapered column of water into its mouth along with the prey".

    The prey is the black dot in the "... center of the column at its greatest diameter..."

    We should be grateful that Paul Schullery has studied the original scientific papers and presented a very readable summary. He writes " The trout can exert an uncanny amount of control and precision in using suction.This is not just an indiscriminate vacuuming operation that sucks in what is nearby. As circumstances require, the trout can open or close its mouth at the right instant and to the right extent to tighten the focus of the suction, thus increasing the intensity and reach of the pull.

    Hayes and Stazicker (2019 p. 186) offer an additional mechanism that works in conjunction with suction: "Thus the most common way a trout eats food in or on the surface is to open its mouth under water, initiating suction, and then as its mouth breaks the surface the accelerated water and the target food item flow (accurately) in; much in the same way as water would flow into an empty glass that is submerged until its rim is just below the water surface." Their book gives access to a video illustrating this principle in action, with a "plastic tube acting the part of a trout taking a fly."

    I think, relying on water flowing into the mouth, would fail if the trout  turned downstream  to take a fly.

    | Trout can 'miss' flies |

    Sometimes we fail to hook a trout that 'rises' to our fly. Splashy unproductive rises are commom on fast flowing rivers in Devon (UK). I often fail to hook trout that show an interest in a dry fly.

    There's a wealth of advice on coping with fruitless rises: sharpen the hook, use finer/softer/longer tippet, delay/alter the direction of the strike, use a softer/longer/shorter rod, change fly or hook size, reduce drag/micro-drag etc.

    It is worth considering whether lack of success is down to the trout 'rejecting' our offering, or an outright 'miss'.

    I stopped worrying about failing to hook fish that 'miss' my fly when I saw, towards the end of this Orvis video, a trout missing a natural fly not once but three times ! as well as  Neuswanger's finding that 39 % of prey were visually inspected but not captured.

    Here is a  report from John Juracek  with a revealing photograph of a trout that missed an artificial fly four times !

    We can now leave the world of the dry fly, where refusals and misses are down to the trout, and enter the world of the wet fly where these problems probably exist unseen, but are added to by man-made problems ...

    Applying Marinaro's "Edge of the Window Theory"to wet flies, nymphs & flymphs

    "The due appreciation of how a trout is rising forms the very essence of fishing, whether it be with floating fly or artificial nymph - and it is often no easy matter. The late Colonel E. W. Harding in his invaluable volume, "The Flyfisher and the Trout's Point of View" shows how the trout lying in wait with an upward gaze below a smooth surface is enabled to watch the reflection of the approaching nymph in the mirror made by the surface beyond the window through which he can see - and how, in order to keep the reflection in view of his upward gaze he has to come to the surface to meet the actual nymph as it and its reflection come together there " (Skues 1939) [emphasis in original]

    Marinaro (1995 p20-21) provides an answer to a "vexing and nagging question" about wet flies that had been debated for decades. How does the trout decide which of the double images to take, the actual fly or its reflection in the mirror?

    Wet fly reflection in the trout's mirror

    Ozzie Ozefovich's underwater shots in this video show how a wet fly and its reflection in the mirror appear to a trout.

    Ozzie Ozefovich's video Underwater World of Trout Part Three | Trout Vision   is available online.

    The next diagrams show Marinaro's explanation of how a trout intercepts a natural or artificial fly beneath the surface.

    As the sunk fly approaches the fish sees:
    1. two images:the actual fly and its reflection in the mirror
    2. a single image when the fly crosses the edge of the window

    The trout can intercept the sub-surface fly by maintaining a constant angle of approach the  Tizzy angle, T Θ that keeps the single image of the actual fly on the edge of trout's window.

    In the example illustrated here the Tizzy angle, T Θ = 48.5 Θ

    If the fly moves outside the window's boundary, the trout will see a second reflected image of the top of the fly in the mirror .

    Wing Commander Guy Gibson's attack run as portrayed in the 1955 film The Dam Busters.

    The underlying geometric principle was used by the RAF to carry out the Dam Busters attack on German dams in 1943. To enable bombing from an altitude of 60 ft (18 m) two spotlights were mounted, one under the aircraft's nose and the other under the fuselage, so that at the correct height their light beams would converge on the surface of the water  (Operation Chastise  Wikipedia). The origins of this device is described in a lecture given by Robert Owen , the official historian of the 617 Squadron Aircrew Association, in 2012. (link in references)

    Still from BBC Dambusters Documentary

    If the aircraft deviated from the required height of 60 ft (18 m):
  • the beams would separate apart if aircraft height was greater than 60 feet
  • the beams would cross over each other if aircraft height was less than 60 feet
  • At the correct height:
  • the beams would converge when the aircraft was 60 feet above the water

  • A trout will see two images of a sub-surface fly outside its wndow. One image of the actual fly, and a second image which is the reflection of the fly in the mirror. These two images correspond to the two spotlights mounted beneath the aircrafts fuselage.

    The trout will see one image of the fly if the fly is on the edge of the window, or within the window. The single image of the fly can be used to set the  Tizzy angle, T Θ .

    The trout sees two images if the fly is moved outside the window. This can happen if the swing element causes the fly to be dragged across the current at an unnaturally fast speed. But from the angler's point of view the effects of drag on a wet fly seem to be less serious than on a dry fly. Why? The next section discusses the implications of this suggestion.

    I've suspected for some time that sub-surface insects are easier for trout to catch than dry flies floating on the surface. Why? A wet fly presents a much more prominent target consisting of two images; one is a reflection in the mirror of the fly , the other the actual fly in the water column. Due to the reduction in current speed as a function of depth, these two images approach the trout more slowly than a fly resting on the water surface.

    Wet flies have a long history in fly fishing and are enjoying a revival. For example the popularity of soft -hackled nymphs in America (McGee, 2007) that are based on  North Country Spiders

    In this video John Shaner in conversation with Paul Gaskell describes how Vince Marinaro - who codified American dry fly fishing - spent the last years of his life fishing, tying and exploring the history of these simpler flies. Shaner's collection of Marinaro's flies are shown here. Their history is discussed 46 minutes into the conversation.

    Trout find most of their food underwater rather than floating on, or emerging through, the surface film. "Numerous studies have confirmed that 70 to 90 per cent of a trout's diet is composed of insects in the underwater nymphal stages." (Hafele, 2006, p xii)

    I've always been struck by what is NOT said about a trout's reaction to wet flies. Compared to dry flies, there's less discussion in the wet-fly literature about drag as well as inspection, selectivity, and the supposed intelligence, suspicion, fastidiousness and capriciousness of educated trout.

    The lack of emphasis on the evils of drag is particularly noticeable in contemporary wet-fly literature dealing with sub-surface presentation. For example, McGee's (2007) index has no entry under drag but several references to obtaining the correct depth of drift. And American writers refer to swinging a wet fly - a technique that will drag a wet fly. Likewise the British author Roger Fogg's (1979) index does not contain an entry under 'drag'.

    This video conveys the simplicity and effectiveness of down-and-across wet fly presentation. Experienced West Country fly-fishing instructor David Pilkington (1983) writes "Beginners should start off with the downstream wet fly because it gives the indifferent caster a chance of a few fish and helps to boost confidence." In other words, let's not make this over-complicated for beginners or the trout!

    Roger Fogg (1979) in his discussion of upstream versus downstream presentation opines "It would be wrong to denigrate the downstream wet fly as merely a crude and unsporting technique as some anglers do, and indeed, the simplicity of the downstream technique is to be recommended to beginners."

    The method is often criticized precisely because of its simplicity! For example by Oliver Edwards in  this video clip  on the grounds that the swing element does not always present the artificial fly in the way that a natural insect behaves. That's true, the swing element can cause the fly to move across the current at an unnaturally fast speed. Depending on current speed, takes tend to be more frequent during the initial downstream drift, and at the end when the fly is on the dangle. As the angler gains experience they are encouraged to mend the line or cast upstream - techniques that prolong the drift element by removing or reducing the adverse effects of swing (McGee, 2007, p58-).

    Fishing a wet fly downstream is not always as simple as it appears on first sight.

    Davy Wotton used a team of wet flies in Wales before he moved to America and began guiding on the White River in Arkansas.

    This article by Dave and Emily Whitlock present  Wotton's wet fly techniques  that can be used to represent different stages of an insect's life cycle: dun, emerger and nymph.

    Paul Gaskell gives valuable advice on how to overcome drag when fishing a wet fly downstream.

    Criticism of fishing downstream with a wet fly is sometimes dressed up as unsporting - targeting small, young and therefore 'uneducated' trout. Here is an example of that attitude written in the style of an earlier age: "My main objection to wet fly fishing in the smaller rivers is that when the current carries the flies speedily the little brownies dart madly at the artificials, and do not give the bigger fish a chance of taking the lures. Again and again, with one throw, I have hooked a brace of the rascals of five or six inches in length ..." Cass (1946)

    Trout on rain-fed West Country rivers are relatively small, but older and more experienced than you might think. More on local  trout size and survival here.

    In this  video Tom Rosenbauer makes an interesting comment "We don't really know why fish take a swung wet fly". At least that gives us a clean sheet. The search for understanding is not burdened with previous explanations involving 'educated' trout that may colour our understanding of how a trout catches a dry fly. It's an advantage to start with a simple situation - inexperienced anglers catching lots of trout with basic flies.

    Why is it more difficult for beginners to catch a trout with a dry fly fished upstream than a wet fly swung downstream?

    A non-angler might wonder why we even bother to fish with dry flies cast upstream. Maybe it's the challenge influenced by fly-fishing ethics. Writing in 1938, Howard T. Walden, put it thus: "To make it difficult is to make it sporting, and to make it sporting is to make it excusable."(extract online) 

    I think the difficulty extends to the trout as well !  Because of drag it's more difficult for a trout to catch a dry fly on the surface, than a wet fly below the surface.

    The drag imposed on any artificial fly is an unique man-made phenomenon. Natural flies are not subject to drag. To state the blindingly obvious, natural flies are not attached to bits of string! 

    Surface drag is normally visible to the angler in the form of chevrons (i.e.  >>>>>>>>>  ) on the surface. So-called micro-drag is less apparent.

    The underlined words in this quote from Vince Marinaro get to the heart of the matter on drag: "What you have done is destroy his [the trout's] nice calculations and to take the fly away from his predetermined point of interception. He may make one try for the fly and likely miss, thereafter he will ignore it. You have not frightened the trout, you have disappointed him ! " (Marinaro 1995, p 30)

    I have suggested that trout intercept dry and wet flies by employing a  Tracking Heuristic  that keeps the fly on, or close to, the edge of its window.

    Failure to catch a fly will be caused by a breakdown in the Input to this tracking mechanism.

    Drag may have a profoundly disruptive effect on a trout's ability to track a dry fly because the visual stimuli (Inputs) that control tracking change dramatically when a dry fly is dragged out of the trout's window. The Inputs that are required by the  tracking heuristic  disappear. They are replaced by chevrons (i.e.  >>>>>>>>>  ) on the surface which are a clear sign that the artificial fly is experiencing drag.

    The trout is now faced with stimuli (chevrons) that were not used as Inputs by the heuristic to track the position of the prey during the rise. Consequently the Tracking Heuristic stops and the rise cannot proceed.

    The potency of drag to disrupt a trout's Tracking Heuristic bears a striking resemblance to military pilots using countermeasures to disrupt Input to an approaching missile's Tracking Heuristic. This is not surprising given the military roots of the discovery of the Tracking Heuristic, and its subsequent adoption by scientists as a way of understanding the way in which animals capture prey.

    The  Tracking Heuristic  was discovered and used by the RAF during the Battle of Britain to intercept enemy aircraft. It continues to be used in AIM-9 Sidewinder missiles.

    It's not surprising that countermeasures have been developed to disrupt missile guidance systems.

    Various forms of 'chaff' can be used to disrupt Input into the Tracking Heuristic. This video with examples of Chaff Flare Countermeasures was provided by the U.S. Department of Defense.

    Chevrons do not 'scare' trout. They simply influence the Tracking Heuristic. For example, trout respond positively to chevrons created by a surface lure. Chevrons form the wake that is used by trout tracking prey under  cover of darkness.  In that situation the chevrons are the Inputs to the Tracking Heuristic and their absence terminates tracking.

    In daylight chevrons may act as Inputs for tracking  skittering-caddis  flies. But that is a more subtle effect than currents acting on a leader to drag an artificial dry fly.

    Disruption of the Tracking Heuristic is less likely to happen if a sub-surface artificial wet fly is dragged because the trout has two distinct images that provide Input to the Tracking Heuristic and indicate the location of the prey : the actual fly and its reflection in the mirror. And these 2 stimuli - that have controlled the trout's behaviour throughout the rise - remain within the trout's visual field when the artificial fly is dragged outwith the window.

    This situation is not unusual when a trout is tracking prey subject to the vagaries of sub-surface currents. It is reasonable to suggest that trout have evolved the visual capability to cope with an environment in which their food is buffeted about by underwater currents.

    My analysis is based on the Tracking / Gaze Heuristic used by   co-pilot Jeffrey Skiles  in the Hudson River incident who explained: "It’s not so much a mathematical calculation as visual, in that when you are flying in an airplane, things that — a point that you can’t reach will actually rise in your windshield. A point that you are going to overfly will descend in your windshield.” This time the point they were trying to reach did not descend but rose. They went for the Hudson.

    Frank Sawyer's 'Induced Take', 'Leisenring's lift' and 'Hidy's subsurface swing': Same result but different methods

    Frank Sawyer (1906–1980) was a professional keeper on the Avon River, a chalk stream in Wiltshire (UK). He is recognised internationally for inventing Sawyer's Pheasant Tail Nymph, and a method of fishing it - the Induced Take. Sawyer was inflenced by Skues books on the use of nymphs cast upstream to catch wild brown trout on chalk streams

    In Sawyer's method, the nymph is cast upstream of where a trout can be seen in the water, allowed to sink and drift downstream towards the fish. It is then deliberately lifted towards the surface in a movement known as the “induced take”.

    A caveat. It is difficult to transfer Sawyer's method to catch wild brown trout in rain-fed South Devon rivers because Sawyer fished on clear chalk streams to fish that he could spot in the water. Furthermore he was fishing for stocked fish: ".. rainbows .. during the past thirty years have been put into the majority of our waters. With these, and the large browns put in from stock ponds, an entirely different concept of nymph fishing has arisen, for these fish are much easier to deceive and in many cases will accept artificials readily that in the past would have scared a wild brownie." (Sawyer, 1979, p159 )

    Sawyer's induced take has been compared to the American “Leisenring lift”. However Sawyer and Leisenring's 'lifts' differ; Leisenring relied on stopping the downstream drift and allowing the current to lift the fly in front of the trout as the line came to the end of its downstream swing. ".. the water will do all that is necessary to make a fly deadly if it is properly tied" (quote from Leisenring in Nemes 2004 p132). Here is a description of  Leisenring's method(s). In contrast Sawyer cast upstream and raised his rod tip to lift the fly towards the surface as it approached the trout from upstream. Thus in Sawyer's method the angler plays an active role, whereas in Leisenring's method the current causes the fly to move.

    Beginners fishing the 'simple' wet fly downstream are encouraged to let the fly dangle for a few seconds at the end of the cast because trout often take a wet fly 'on the dangle'. This pause will have the same effect as Sawyer and Leisenring's methods; mimicking the behaviour of an insect ascending prior to hatching.

    Pete Hidy had this to say about the 'subsurface swing' which he felt was a more effective technique than the Leisenring lift : "The subsurface swing, on the other hand, is far more versatile, taking place in the top couple of inches under the surface, in many different kinds of stream conditions. The effectiveness of the swing is that the slight underwater drag causes the fibers in the hackle to come alive. The movement of the fly resembles the swimming insect, struggling to escape into the air. This contrasts with a dead drift, which, as its name suggests, causes the fly to appear lifeless. Hidy let the  flymph  drag across the current, enhancing the illusion of life and of struggle." (Hidy, 2019)

    My conclusion is that extending Marinaro's "Edge of the Window" theory to incorporate a Tracking Heuristic
    1. accounts for the profound effects of drag on artificial flies (dry flies, emergers, surface lures) in the surface film that are NOT associated with a reflection in the trout's mirror
    2. and the lesser, absent or beneficial effect of drag on sub-surface artificial flies that ARE accompanied by a reflection in the trout's mirror (wet flies, nymphs, flymphs, streamers)

    Helping anglers deal with - and exploit - drag in daylight

    In daylight, surface drag on a dry fly is a serious problem that will eventually be encountered by all fly-fishers. "Very often the drag is so imperceptible that the drag is unnoticeable to the fisherman. ...What you have done is destroy his [the trout's] nice calculations and to take the fly away from his predetermined point of interception. He may make one try for the fly and likely miss, thereafter he will ignore it. You have not frightened the trout, you have disappointed him ! "  (Marinaro 1995, p 30)

    On the other hand,   Sylvester Nemes  concluded "... underwater drag is not as noticeable and doesn't spook trout like a dry fly dragged on the surface" (cited in McGee, 2015, p39 ).

    I hope my suggestions above go some way to reassuring anglers that drag does not necessarily scare trout. Drag interferes with a trout's ability to catch the fly they want to eat.

    I think it is helpful to distinguish between the effects of drag on dry and wet flies.

    | Drag and Wet Flies |

    W C Stewart's influential book The Practical Angler was first published in 1857. He advocated the the 'upstream' or 'upstream and across' style of wet fly fishing. He cast a short line and allowed the flies to drift naturally downstream with no tension on the line. Nowadays we would call this high sticking with a drag-free drift.

    This technique, and variations of it, are covered by Oliver Edwards in  Wet Fly Fishing on Rivers.  But Edwards recognises that the technique may not appeal to all fly-fishers: "Today very, very few river fly fishers, including regulars, fish upstream. They don't like it, for several reasons. It's repetitive, has a high work rate, and requires complete focus."

    The chalkstream angler and celebrated author   John Waller Hills  expressed another problem viz "the alchemy often practiced by wet fly fishermen, and the veiled and inexplicable cues that cause a successful wet fly angler to tighten and hook his fish." (Schwiebert 2007 p 39). "If you are to get anything of a bag, you had to strike a trout before you felt him, often without seeing the rise." Hill goes on to report that some anglers are able to do this "almost by instinct or some sixth sense". (Hills 1941)

    Paul Gaskell  puts his finger on a problem I detect with beginners: "Many anglers are quietly terrified of fishing a wet fly upstream. It has a reputation for being impossible – probably because it is rare to feel a fish grab the fly."

    I think this reputation also arose because Skues developed a style of nymph fishing suitable for clear water chalkstreams where anglers only cast in an upstream direction to fish they could see in accordance with the ethic of dry-fly fishing that was de riguer at the time. It is very unusual to find these conditions on local South Devon rivers. Skues fished a nymph dead-drift downstream. He did not impart movement to the fly. Apparently Skues understood the efficacy of moving the nymph to induce a take, but avoided writing about this technique to avoid the charge, from dry fly purists, that his nymph was little more than a wet fly in disguise (Berls 1999).

    Lay aside alchemy, instinct and your sixth sense, face this demon armed with two earthly weapons: stealth and an indicator!

    When I'm instructing it usually takes place in difficult low water summer conditions. I'm very conscious of the need for  stealth.  Therefore I encourage anglers to wade uptream because there is a possible  blindspot  behind a trout. Our local rivers are blessed with a run of sea trout in Summer. Sea trout are notoriously 'spooky' in daylight. At times they seem to be afraid of their own shadows. They may also be sensitive to wading-induced   vibrations detected through their lateral line. I find it helps to "teach anglers to walk again" by slowing right down, walking ahead of the angler, hardly raising each foot, explaining that I try to walk like a ghost, and pointing out trout that remain just a few feet ahead. But if I can get an angler close to fish (1) it shows the importance of stealth, and (2) it placates the casting upstream demon.

    Yes, it can be difficult to detect when a fish has taken a fly underwater; visual indicators may help. Originally anglers greased the leader to register the take. Nowadays there is a variety of visual aids ranging from  Roman Moser Minicon Leader Loops  to Thingmabobber strike indicators. Tom Rosenbauer gives a very good description of the uses, advantages and disadvatages of various devices as well as 'Dry Droppers' as indicators  in this video. Note:  EA byelaws  prohibit the use of any float when fishing for trout in South Devon; contraptions such as Thingmabobbers may be regarded as floats. Rick Hafele (2006) describes 11 nymph fishing tactics and provides a table showing how to employ them when targetting fish lying at various depths in rivers of different velocities.

    Many years ago I was put off casting a nymph upstream because the literature described Frank Sawyer's technique. He was casting to fish he could see in clear water chalk streams. This video - of a just-released trout -shows how difficult it is to see trout in the South Devon rivers I fish. I'm still seeing references to Sawyer in articles on 'sight-nymphing', and videos about 'sight-nymphing' in rivers where the trout are visible, but the nymph is presented below a Thingmabobbers ! It's either that or a hatch of round orange flies !

    Whatever method you adopt remember that it has to cope with two things: firstly the speed that salmonids can  spit out  something they have sucked into their mouth, and your  reaction time  to respond to indicator movement - about a quarter of a second. This is probably the reasom competition anglers devote considerable time developing, modifying, perfecting and practicing indicator methods (e.g. Euro and Tight Line Nymphing rigs). We benefit - one way or another - from these developments. For example, my indicator fly is the  Antron Caddis  designed by Craig Coltman who captained the Australian Flyfishing Team in 2012.

    On relatively shallow South Devon rivers, when trout are feeding on or just below the surface, I fish with two flies on my leader - one dry the other wet - and use the dry fly to indicate when the wet fly has been taken. This also gives you the chance of a fish taking the wet fly even if the dry fly drags. This method appeals to me because it recognises the problems faced by a fish that feeds oportunistically. A dragged dry fly creates a visual stimulus - chevrons. Chevrons replace or interfere with the prey image of a dun on the surface and disrupt the trout's  Tracking Heuristic. In contrast, a dragged wet fly presents a changing visual pattern consisting of the distance between two parts: the actual insect and its mirror image in the underside of the surface. Sylvester Nemes points out that drag may not be viewed as unusual by a trout. Maybe this is because as a trout persues sub-surface insects these two separate parts will coalesce in the window, and then appear to separate as the insect's actual position within the water column changes.

    When trout are rising I often cast upstream with a   flymph beneath a dry fly because it is reasonable to expect ascending and emerging nymphs to be present at the same time as fully emerged duns.

    There may be an advantage for the nymph of dry-fly drag in this set-up. A nymph dragged by the dry fly will be  pulled towards the surface. This is the effect produced by  Sawyer's and Leisenburg's techniques.  As Tom Rosenbauer (2008 p115) points out ".. the motion of a fly moving towards the surface, as if it were hatching, can at times be an irresistable motion to a fish."

    There's a temptation to think that fishing a nymph beneath a dry fly is a recent development. The Dry-Dropper duo method was first mentioned by William Lawrie in 1939 but described in greater detail in his book published in 1947. Keith Rollo was an early advocate :“If trout are nymphing, a nymph or wet fly could be mounted on the point, whilst a dry fly could be mounted on the dropper.”  (cited by Rob Smith)  Rollo recommended this technique for fast-running streams in Devonshire (Rollo, 1944)

    Both authors were probably influenced by Dr William Baigent's  Two Dry Fly Technique. In his book Nicholas Fitton (1992) gives a detailed description of the history, and his experience using, this almost-forgotten technique.  And of course going back even further, North Country spiders were - and continue to be - fished as a team of three flies or more, with close attention paid to the position of the top fly (Rob Smith personal communication, 2020). Recently Paul Gaskell has described using a   spider fly  tied with a white hackle on the top dropper as a sighting fly on a French leader.

    There are several popular ways of attaching the dropper to the dry fly: e.g. tying the dropper to the hook bend on the indicator fly - the New Zealand Style, or hooks with a tippet ring on the bend. In this video Stephen Cheetham describes a simple knot used by anglers fishing with a team of North Country spiders. It aligns the flies 'in-line' to eliminate droppers tangling around the leader, and avoids the dropper obstructing the hook of the indicator fly - a recognised problem with the New Zealand attachment method.

    I think worrying about drag-free drift may be a distraction when fishing a sub-surface fly. This idea may date back to Skues who introduced nymph fishing on chalk streams and ran into a bit of bother with dry fly over-enthusiasts. Skues probably knew that moving a nymph was an effective technique, but kept quiet about it because he was in enough trouble for suggesting that dry-fly fishing was a relatively minor tactic anywhere, let alone on a chalk stream. ( Berls, 1999)

    | Drag on Dry Flies |

    There are books, articles and videos describing the design and construction of leaders that facilitate drag-free drifts, as well as specialised casts that present line, leader and dry fly in ways that overcome drag.

    There is no doubt that these solutions can be effective in the right hands. But before turning to these remedies one problem needs to be addressed: Helping anglers, particularly less experienced anglers, locate and track their dry fly to spot drag.

    As a fly-fishing guide I often have anglers tell me that they have difficulty seeing their dry fly. This is especially the case if I have taken them to the faster 'popply' water at the head of a pool, where they stand a better chance of catching a trout than in the slower moving water in the body and tail of a pool.

    Their difficulty is understandable. It can be hard to pick out a small dry fly in fast broken water. How can we make it easier?

    Obviously it helps to have a fly that stands out from the background. Dry flies tied with a parachute post are a good choice, but may only solve part of the problem.

    Some anglers are still not able to find their fly on the surface of the water. Like the trout lying in shallow water, their 'window-to-search' is either very small, or like a trout lying deeper, very large.

    One guide's trick is to recruit every person's Tracking Heuristic. Help them to find the location of their fly by concentrating on the area a leader length beyond the end of the fly line. Hopefully the fly will be somewhere within a semi-circle with a maximum radius equivalent to the length of the leader.

    This 'window-to-search' will get narrower and smaller as their casting ability increases.

    The next step is to utilise their Tracking Heuristic to track the fly as it is carried downstream i.e. the human version of the trout's 'edge of the window' problem . In fast moving rippled water, a fly may occasionally disappear from view. I advise anglers to "watch that fly like a hawk'. They can't of course because a hawk's visual acuity beats ours hands-down, but the analogy helps concentration.

    Once an angler is confident in locating and tracking their artificial fly, then it's time to introduce limp tippet materials, long leaders and slack line casts to overcome drag.

    Baigent's   Two Dry Fly Technique.  can be used when trout are refusing relatively large dry flies in favour of tiny "no-see-ums" midges. Use a larger dry fly as an indicator.

    | Tenkara: Drag-Free Drift |

    Daniel Galhardo, founder of Tenkara USA, talks to author John Gierach about his interest in tenkara

    Tenkara was developed by Japanese fishermen who earned their living by catching fish in mountain streams.

    For them a drag free drift would have been critical. Tenkara rods are long (11-15 ft) with a light line of similar length attached to the end of the rod. No reel is used. The line is held clear of the river surface to eliminate drag. Tenkara avoids the drag problems associated with a bulky fly line.

    If you have any lingering doubts about the need to avoid drag when presenting a dry fly, take a look at this excellent website covering  tenkara in detail.

    Tenkara does not solve every problem and comes with its own limitations: wind can catch the thin tenkara line and move the fly away from the trout's window; the lack of a reel makes handling a large fish that would take line from a reel difficult if not impossible if you connect with a sea trout on a South Devon river. Nevertheless there is much to be learned by adapting tenkara methods for eliminating or reducing the effects of drag.

    A change in focus: Prey recognition

    I've suggested that trout employ a constant angle of approach - a Tracking Heuristic - to intercept prey items. Now we need to ask the question "How do trout recognise potential prey?". Is there a similar constant factor that can be used. The answer appears to be No. Even for humans, it's not straightforward. Nevertheless an heuristic may be involved: a Recognition Heuristic. But this heuristic is more complicated for humans and trout.

    The difficulties emerged when radar was used in World War II to identify enemy aircraft (prey) in the Dowding aircraft  interception system

    This image is a replica of a WWII bomber’s H2S radar display built by the  Imperial War Museum.  

    The radar operator was faced with the problem of recognising an enemy aircraft against background noise on the display screen.

    Signal detection theory (SDT) emerged in the 1950s as a way of understanding and dealing with this problem. It has subsequently been used to tackle the same problem in other areas involving human decision making, for example medical diagnostics.

    "In psychology, SDT is a model for a theory of how organisms make fine discriminations" Swets et al, (2001). It may aid our understanding how trout recognise prey.

    Trout live in an environment with a constant stream of inedible debris. I was struck by this entry in  John Simonson's   blog

    "This Fall while standing in the Madison River looking upstream I could see a tremendous amount of small debris and other objects drifting on and below the surface. Yet mixed in with the plethora of junk drifting downstream I could spot a struggling Baetis dun size #20, a size #14 spent Callibaetis spinner, and several tiny winged ants drifting downstream towards me just by their shape and slight movements."

    Hunting by Search Image

    The term 'search - or prey - image' was introduced to fly-fishing readers by Bob Wyatt in his first book "Trout Hunting" in 2004, and later in "What Trout Want: The Educated Trout and Other Myths" (2013).

    It was originally called "hunting by search image" by the ethologist  Jacob von Uexkull  in 1934. It enables an animal to concentrate attention on one prey item at a time. Most importantly, a search image is short-lived. It has a temporary existence. This distinguishes a search image from the relatively permanent effect of a sign stimulus. Predators develop a search image based on the prey that is encountered most frequently. It can be replaced by a different search image as the animal shifts from one type of prey to another. Selective feeding in trout is probably the result of adopting a search image.

    Search image is related to  'optimal foraging theory'. From a fly-fishers point of view problems can arise when deciding
  • what is the trout feeding on at a particular point in time?
  • why does a trout stop feeding on one type of fly and switch to another insect?
  • Once a prey type becomes less abundant a new search image will be developed. Ethologists have investigated several 'stopping rules'. "The best stopping rule is based upon the number of prey found as a function of time spent searching.." (McFarland, 1985 p. 473). This may explain why stopping occurs at different times for individual trout in what anglers call a mixed or masking hatch .

    It is possible that some animals retain and re-use successful search images.  Blough (Fig 8)   reports that she found "evidence for improved detection as a result of repeated encounters" in her research using pigeons. But I haven't encountered similar reports for fish.

    McFarland concludes that "Animals may make decisions on the basis of simple rules-of-thumb designed to fit particular environments" (ibid p. 479) i.e. an adaptive rational heuristic (Hamlin, 2017).

    What's in a Search Image?

    It's all very well me saying that trout probably use a search image to detect prey, and that we stand a good chance of catching them if our artificial fly mimics that search image, but what feature(s) of the natural should we include?

    I have a copy of this wonderful book by Dave Hughes (1999). I enjoy browsing through its 443 pages of trout fly patterns and how to tie them. But Hayes and Stazicker (2019) express the problem faced by many fly-fishers when choosing which of these artificial flies to use as follows:

    "Now, that database of food items will, by the time we are trying to catch our trout in its maturity, amount to millions. And it is that database that we are trying to fit our imitation into in such a way that it matches the images of successfully eaten items, and does not jar. The fly tyer needs to pay attention to the construction and materials of an imitation to get these matches right. There is another database of prey items, we believe, in each and every trout (though it may vary by species). And that database is not solely experiential, it is genetic. We cannot prove it. It may be illusory. But every trout fly fisher in the world will agree if you say that you think they have wired-in prey images of ants, beetles and worms. In rainbows it probably includes salmon eggs and the colour orange. "

    I doubt that trout have a database with millions of entries corresponding to food consumed, not least because it would have many duplicate items. But I do agree trout may have several prey images acquired through experience rather than as a birthright !

    What's in a search image. We just don't know. But the scientists who have studied visual search have given us a clues that it might contain:
  • One feature that makes the natural fly stand out from all the background 'clutter' in the trout's environment. Why? Because if the search image contains two or more features that must be present to define the prey item, and any of these features is shared by some other thing(s) in the drift, then making the correct choice between potential food and clutter takes more of the trout's time, even humans find this a difficult task - the scientists call this a 'conjunction display' (Eckstein, 2011).
  • Incidentally, this may account for why an artificial fly that precisely imitates the natural insect will be effective, but unnecessary - the feature in the prey image is present in the artificial imitation, but the rest of the fly's dressing may be immaterial to the fly's effectiveness.

  • Location feature:  Performance in a search task can improve when a restricted location in the search space contains the target.
  • This may assist trout searching for particular stages in an insect's development: e.g. nymph, dun, spinner

    Every day we spend a surprising amount of time searching for things. We can pick out a familiar object without checking it closely. We recognise familiar things on the basis of very limited scrutiny.

    This ability has been described as a Recognition Heuristic: a simple and fast way of making a decision based on limited information. (Pachur et al 2011). For non-human animals, search is more vital than for humans since survival depends on finding food and avoiding predators (Eckstein 2011).

    A trout's search image may have some of the properties of a recognition heuristic.

    Scientists have spent years trying to understand how humans spot things (Narbutas et al 2017). It started off with simple search tasks e.g. picking out a red circle from a group of green circles, getting a bit more difficult when the letter L was hidden in a group of the letter T, and then using a more difficult task involving two features - a search for a vertical red bar among horizontal red bars and vertical green bars.

    They were asking if search is a serial or parallel process. Is search a relatively slow serial process where every item is examined one by one? Or is it a faster - parallel process - where everything is considered at the same time until the search item is found?

    This diagram illustrates the difference between an easy search task that can employ a fast, efficient, parallel search strategy, and a more difficult search task that requires the use of a slower, inefficient, serial search strategy.

    Thus, in the image on the left, people are able to quickly find the grey fish among white fish, no matter how many white fish are present. The grey fish pops out. However, in order to find the grey fish that is facing right in the other image, people need to individually search through each fish. The more fish there are, the slower they’ll be at finding the correct fish. (Fazio 2020)

    The scientific term   'parallel visual search' is an explanation for our ability to quickly find our favourite brand on supermarket shelves, our car in a car park, a book on a bookshelf etc. It's an unconscious ability - an   heuristic  if you will -that we take for granted.

    Speed is one important characteristic of parallel visual search. We don't have to examine in turn each car, face or cereal packet, the object we're looking for seems to  ' Pop-out'  from the background.

    An important feature of   Pop-out  is its speed- it happens quickly and does not increase as the number of distracting stimuli in the environment is increased. A trout lives in an environment containing a steady flow of distracting features. Some is edible, some inedible and the proportions  vary across the day.

    Until recently it was thought that only mammals and birds were capaible of parallel visual search. In 2015 Ben-Tov et al reported that fish shared this   Pop-out ability.

    It is generally agreed by scientists (see Wolfe & Horowitz) studying 'visual search 'that

  • colour
  • movement
  • orientation
  • size
  • and probably shape
  • are attributes that enable an object to be quickly spotted or   'Pop-out'  from background 'clutter' during parallel visual search.

    In addition, previous research has shown that "Your ability to find a target in the current search is affected by what you have been searching for previously. In general, you are faster searching for a given target if you found that same target on a recent trial" (Wolfe & Horowitz). This idea is used in the popular books "Where's Wally?"

    Thus a trout feeding on a particular type of natural fly, is more likely to quickly spot the next similar one that floats towards it.

    During a rise, a feeding trout has very little time to inspect an artificial fly. This leads me to the conclusion that a Recognition Heuristic based on Pop-out may play a role in opportunistic as well as selective feeding.

    This "Pop-Out" heuristic is ephemeral because a search image is short-lived. It has a temporary existence. If it persisted, previously searched-for objects would continue to pop out as distractions - visual clutter. This is important for trout as well. When a hatch comes to an end, the trout needs to search for another now more abundant prey. There will be a transition period as the original seach image wanes, and is replaced by a new search image. As we saw earlier, there is some evidence that animals  retain and re-use  successful search images

    Are trout selective feeders?

    In the fly-fishing literature the term 'selective' generally applies to the behaviour of trout living in rivers with a variety of abundant food ( chalk streams in the UK, limestone rivers in the USA). In those rivers selective trout may focus on one particular type of fly in preference to other species. This preference is thought to be influenced by relative abundance at a particular time (i.e. a 'hatch') or ease of capture (e.g. 'emergence') of the insect. Scientists call this behaviour  optimal foraging based on a search image. The trout's behaviour involves a cost benefit analysis; securing the most energy (benefit ) for the lowest expenditure of energy (cost) in capturing the prey. Effective deployment of this strategy will maximise the trout's energy gain.

    This can cause a problem for fly-fishers choosing the right artificial fly if more than one species, or stage, of fly is available to the trout, i.e. a multiple hatch situation of dun, nymph, or spinner. It may be a problem for fly-fishers, but it makes sense for the trout. In an experiment, Dukas and C. Kami (2001) found evidence for lower prey detection rates when birds searched for two prey types at the same time compared to searching for just one prey, suggesting that it's better to attend to one 'search image' at a time.

    I've always had a problem with interpretation of the phrase 'selective trout'. I get the impression that sometimes in the fly-fishing literature it implies that trout are making an "educated" choice from a selection of options that includes the angler's fly alongside several other types of natural fly.

    This sounds like the meaning of selection in everday life. For example selecting a favourite chocolate from the box - a slow serial process where items in the box are carefully inspected until a favourite chocolate is found. If trout are using a search image, then they are not making a selection - in this sense - from a series of perceived options.

    A search image using  parallel rather than a serial  visual process is an efficient and fast way of making a choice from items moving past trout in the drift. The searched-for insect will pop-out from the background. Insects in the background will have much less visual impact whilst a particular search image is controlling the trout's behaviour.

    In this analogy there are chocolates in the background, but one stands out from the rest and can be quicky spotted. The search image may act like a visual filter.

    If your favourite is missing attention switches to using a search image for your next preferred chocolate.

    There are important differences between a trout's search image and this human analogy. Humans may have a list in memory of preferred chocolates in a selection box. So if their favourite is missing they can quickly search for a second favourite.

    Switching between  search images  for animals is a slower process governed by the relative availability of different prey items i.e.  optimal foraging theory. Once a prey type becomes less abundant a new search image will emerge. Search images can trigger an animal's behaviour but - strictly speaking - they are not sign stimuli in the ethological sense.

    Fly-fishers should take heart, their fly has not been - in the human sense - "rejected" by an "educated" trout. The trout just hasn't perceived, recognised or noticed your fly. After all, how much attention do you pay to other cars in a car park when you are searching for your vehicle?

    In freestone rain-fed rivers where food is less abundant trout are said to feed opportunistically and less likely to focus on one type of insect when others are also available. On chalk streams optimal foraging theory may be an efficient method of feeding. But in freestone rivers focussing on one food item is not efficient if that item is sparse, and there is a larger density of other potential food in the river (Dukas and C. Kami 2001)

    Are trout in freestone rivers selective feeders? Almost certainly.  Why?  Because their environment is filled with junk, and it's not junk-food!

    Trout in less fertile freestone rivers are selective in one respect - the challenge of selecting between edible flies and non-edible debris in the river.

    Neuswanger et al, (2014) used video cameras to record in 3D the feeding behaviour of juvenile Chinoook salmon during a four month period after they emerging as fry in an Alaskan river with potential trout food familiar to fly-fishers: chironomids, mayflies and stoneflies.

    The fish displayed the ability to ingest, visually inspect and reject items in the drift. The research also revealed the importance of considering the impact of non-food debris is studies of salmonid feeding strategies.

    They reported: Among all potential food items fish pursued:
  • 52 % were captured and quickly expelled from the mouth
  • 39 % were visually inspected but not captured and
  • only 9 % were ingested
  • clip from Smelly Flies Work Better for Trout

    The most interesting finding was the high percentage (52%) of items captured and quickly spat out after capture (see frequency chart), compared with the low percentage (9%) retained. Interestingly 39% of items were visually inspected and rejected.

    Trout are probably not born with the ability to recognise and differentiate chironomids, mayflies and stoneflies. This is not surprising given geographical variation in available food items across salmonid habitats. But - as these results show - they do exhibit a set of simpler innate behaviours: approach objects in the drift, suck them in, spit them out, or ingest them. There was no evidence of increased discrimination between food and debris as the fish matured across the study period.

    Neuswanger et al, (2014) make a very useful comment "Our results help motivate a revised theoretical view of drift feeding that emphasizes prey detection and discrimination, incorporating ideas from signal detection theory and the study of visual attention in cognitive ecology."

    Trout in freestone rain-fed rivers may develop a search image that differentiates edible items from stream debris. This search image may not be only visual, it could involve a separate tactile element. Schullery (2006) describes how rapidly adult trout spit out something they have sucked into their mouths. This can happen repeatedly to the same fly without the angler being aware. This confirms that trout don't always make the right decision. Behavioural scientists describe this as the ability to distinguish between  signal and noise and classify behaviour as falling into one of four categories depending on whether the signal is present or absent, responded to (a hit) or ignored (a miss).

    The signal in this case is an item of prey. The noise is flotsam coming downstream alongside potential food. The trout is faced with a decision whether or not to rise to intercept the item.

    Decision is:
    Item is: Rise Don't rise
    Food Food eaten 9% Meal missed
    Not food Debris spat out 52% Correct rejection: debris in drift 39%

    There are two factors that influence behaviour on a signal detection task: sensitivity and bias. Bias and sensitivity are independent of each other.
  • Sensitivity refers to the ease of  detecting prey .How distinctive is it? Does it   Pop-Out for trout?
  • Bias is the extent to which one response is more probable than another.
  • For example, if the angler makes a clumsy cast or is visible, the fish may not rise to any fly, natural or artificial, because the response to predators (fleeing) takes over from the feeding response.
  • In contrast, if there is hatch of fly, or an increase in  invertebrate / behavioural drift  a trout will be more likely to rise in preference to conserve energy by resting.
  • I'm writing this under Covid-19 "lockdown" restrictions, so with time on my hands I fed Neuswanger et al's, (2014) data into an  online program  to calculate the 'sensitivity' of their subjects to items in the drift. I was curious about the extent to which consumed objects stood out from inspected but rejected objects. I made the following assumptions:
  • consumed objects were judged as 'prey' by the fish
    i.e. 9% (eaten) + 52% (spat out) = 61% (perceived to be prey)
  • 39 % were visually inspected but judged as not 'prey' and correctly rejected by the fish
  • The resulting graph shows that the trout acted on a perceived difference between prey and non-prey objects in the drift. Their behaviour is not just random sampling of items in the drift. This is shown by the just noticeable separation of the two distributions shown on this page. And they acted on this perceived difference. Of course they quickly realised their mistake and spat out 52% of them.

    An image of the results with the Receiver Operating Characteristic (ROC) curve is  available here The term ROC refers to tests of the ability of World War II radar operators to determine whether a blip on the radar screen represented an object (signal) or noise. Radar was essential to the   wartime development  of the Tracking Heuristic. ROC curves now have an important role in medical diagnostic testing (Fan, 2006)

    I find signal detection theory a useful way of thinking about trout behaviour and motivation, as well as fly effectiveness:
  • the four quadrants cover possible trout behaviour : (fly hit, fly miss, fly present and fly absent)
  • trout motivation (bias)
  • sensitivity (an artificial fly effectiveness in eliciting a trout's rise)
  • The next section considers what makes an artificial fly attractive to trout.

    Helping anglers deal with selective trout. Did Halford take his finger off a Trigger?

    The idea of the 'selective trout' and consequently the need for precise imitation of the natural fly by fly-tyers, has led to a long running debate amongst fly-fishers.

    Belief in precise imitation has survived over the years. The earliest proponent was probably the influential fly-fisher Frederic M.Halford (1844-1914) who was inflexible in his view that exact imitation of natural flies was essential for tying, and de rigueur for fishing with, artificial flies.

    Halford shone a searchlight on natural and artificial flies leading to the modern concept 'matching the hatch', and encourages monitoring insect abundance and diversity in initiatives such as the  Riverfly project.

    A less helpful consequence is a focus on the angler selecting the "right fly" as a panacea for our failures, rather than basing our approach on an understanding of the trout's behaviour. I must confess to often being a victim of the "right fly" virus. It's difficult to shake off, it's always in the background, part of fly-fishing culture that keeps magazine editors and tackle vendors in business so I suppose, on balance, it's worth living with.

    But an unhelpful consequence of Halford's approach was expressed 70 years ago by McCaskie (1950 p 78) "The belief, or delusion, that the trout is a highly intelligent creature is of comparatively modern origin, since it is a by-product of the development of the dry fly". McCaskie explains how followers of Halford found that when ever more precise imitations "..failed something had to be done in defence of injured pride, and the thwarted angler evolved the theory of a highly educated and shrewdly reasoning adversary ." McCasky realised that the trout's brain is incapable of these cognitive feats based on anthropomorphism. The same observation and explanation appears in John Gierach's much admired book "Trout Bum. 2013 Pruett".

    Marinaro (1970 p 62) was a critic of Halford's strict insistence on precise imitation "Every detail of the natural fly's anatomy must be included, no matter how absurd the result. For example "... the segments of the natural fly were counted and the same number of turns of ribbing were included in the artificial"

    Marinaro uses a strong word, 'absurd', to describe this  E pluribus unum   approach. The angler is casting an artificial fly during a hatch of naturals - unless this is done with pinpoint accuracy, at the tempo of the fish's rise, the statistical odds are stacked against the angler.

    In Halford's time the GRHE was, and continues to be, a very effective pattern. It doesn't look like any natural fly. Halford added wings to make it acceptable to dry fly purists, without any deterioration in its attractiveness to trout. In his books Halford regarded it as "the most successful [fly] of modern times" .

    Perhaps a little cruelly,  Skues  wrote: "At one time the late Mr. F.M. Halford was a great advocate of the Gold-ribbed Hare's Ear [GRHE], but I believe that latterly his enthusiasm for precise imitation induced him to give it up, successful pattern though he knew it to be, because he could not explain its success to his satisfaction."

    Halford discarded the GRHE because it simply did not fit into his view, and that of his followers, of fly fishing as the presentation of a dry fly imitating an identified floating dun.

    Robert Smith  traces the history of the use of hare's ear in artificial flies from Walton’s fifth edition of the Compleat Angler in 1676, through variations of a GRHE culminating in a Hare’s Ear Comparadun.

    Bob Wyatt's Deer Hair Emerger is essentially a modern winged version of Halford's abandoned fly - a winged Gold-Ribbed Hare's Ear tied as an emerger on a Klinkhamer hook.

    This composite image of a Gold-Ribbed Hare's Ear (GRHE), and a natural fly photographed near the edge of a trout's window, offers one explanation for the success of the GRHE - the fly ultimately rejected by Halford. The shaggy artificial may present a sufficiently exaggerated representation of the insect's feet to aid its location and interception by a trout.

    It does't take much insight into the human condition to understand Halford's dilemma. He chose to give up the GRHE in favour of his theory. But the theory of precise imitation lives on in best selling books and modern arguments over   selectivity.

    Originally published in 1971 as 'Selective Trout' , and republished in 2018 as 'Selective Trout The Last Word on Stream Entomology and Aquatic Insect Imitation'. The authors Doug Swisher and Carl Richards seem to perpetuate Halford's attitude to exact imitation of the natural fly: "The right fly is one that resembles the natural so closely that the fish   seem   to prefer it to the real thing."

    Are trout selective? In certain places, at certain times, trout are reported to concentrate on specific insects when there is a prolific hatch. But that doesn't mean that it's a good idea to present an artificial fly that doesn't stand out from the crowd.

    Swisher and Richards make the point powerfully: the artificial fly should resemble the natural, but be sufficiently different that it is preferred to the real thing.

    Of course this also applies to trout that adopt a 'whatever-comes-along' feeding strategy. These are the type of trout   opportunistic feeders  that most of us encounter whenever we go fishing on acidic upland rivers.

    Ethology & the design of trout flies

    Nobel Laureate Nikolaas Tinbergen   was one of the founders of ethology - what he described as "watching and wondering " about animal behavior. "Tinbergen found, it is often the case that quite crude tricks suffice, itself perhaps a reflection of animals’ greater reliance on simpler rules of thumb." i.e. heuristics (Hutchinson & Gigerenzer, 2005)

    Simplicity has always occupied a room in the attic of the house named Selectivity. The writer of "A Modern Dry Fly Code", Vince Marinaro expressed it thus: "I am continually astonished by the fact that the most killing flies in fly-fishing history are of very simple construction" (Schullery, 1987 p232)

    Tinbergen's approach to studying animal behaviour and discovery of   sign and supernormal stimuli  is having a refreshing influence on our understanding of trout behaviour towards artificial flies.

    So the question becomes :"What feature(s) makes an artificial effective so that it is Recognised (Pops-Out) and triggers the rise controlled by the Tracking Heuristic?"

    Increasing attention is being paid to ethological concepts such as   sign and supernormal stimuli  or 'triggers' in the fly-fishing literature.

    Bob Wyatt has introduced these concepts to a wide audience in his important book "What Trout Want: The Educated Trout and Other Myths".

    Bob is critical of precise imitation, but most of all to burdening trout with human cognitive and emotional processes : intelligence, suspicion, fastidiousness and capriciousness. To put it bluntly - the trout's brain structure is not able to support these complex psychological processes (Rose, 2002).

    It's possible that effective artificial trout flies contain a feature that in the natural attracts the trout's attention. This is called a sign stimulus by Tinbergen.

    Bob Wyatts Deer Hair Emerger (DHE) is a conscious attempt to design a trout fly based on what ethologists / behavioural ecologists call triggers or sign stimuli. According to Wyatt:

    "Borrowing the essential features of Fran Better’s Haystack and Usual, Al Caucci’s ComparaDun, and Hans van Klinken’s Klinkhamer Special, the DHE is designed to present a strong prey-image. It incorporates a couple of primary stimuli, or ‘triggers’: a visible wing and a sunk abdomen. While suggesting natural aspects of the insect, these exaggerated features ensure that the fly will be noticed - what behavioral ecologists call a super-normal stimulus." The DHE is essentially a winged version of  Halford's abandoned fly  - a winged Gold-Ribbed Hare's Ear tied on a Klinkhamer hook.

    Maybe this simple fly may be effective for a number of reasons:
    1. The body is made of spiky hare's ear fibers that penetrate the trout's mirror to create the star-bursts of light that Clark and Goddard stressed as being the   most important triggers   to elicit a rise.
    2. The wings and body merge together when they are on the  edge of the window.
    3. The conspicuousness of the wings and body enable trout   to track the position of the fly   by keeping it close to the edge of the window during the rise.

    Wyatt is one of the first fly-tiers to specifically refer to ethological concepts in designing artificial trout flies. Previously some flies were tied to include what are now referred to as triggers or sign simuli. For example, Threadgold's footprint fly system - inspired by Marinaro's thorax flies - and Clarke and Goddard's Upside Down (USD)Paradun. But these flies were complicated to tie and therefore did not enjoy commercial success.

    Wyatt's flies are simple to construct and are available to buy, but - in some examples I have seen - not always in the style of the originator.

    This image is from a  Youtube video  of Bob tying his Snowshoe Emerger. It emphasises how scruffy or 'buggy' Bob makes the abdomen and thorax on his emergers.

    Is there a property of hare's ear that can account for its long use in flytying? Robert Smith puts it this way: "Quite what makes the hair fibres from a brown hare’s skin so attractive to trout and grayling is unknown. Maybe it is the subtle mix of natural hues that are evident within individual hair fibres. However, even when the fur is dyed it still has an unknown attractiveness. Which leads me to believe it is the spiky contrasting guard hairs within these mixed fibres, dyed or otherwise, that lead to the almost magical quality of this fur." Robert makes a crucial point: the attractiveness of hare's ear survives a colour change.

    Ralph Cutter makes a subtle point about another property of hare's ear, it can trap air bubbles. "The first time I rubbed beeswax into a hare’s ear and tossed it in the water, I knew I’d found the answer. The nymph glittered like a diamond, and the buoyancy of the air crust caused the bug to swim and drift in the current like a creature come to life. Ralph Cutter,Fish Food (p. 43)

    Pete Hidy (1973) introduced the term 'flymph' to describe the transition from nymph to adult i.e. emergence. In this extract from his Open Letter to the International Society of Flymph Fishermen he explains his interest in mimicry before advocated the use of hare's ear to add mimicry to flymphs.

    The somewhat quaintly named website  International Brotherhood of the Flymph  contains a wealth of information including this collection of flies tied by Hidy that illustrate his use of hare's ear and tinsel to mimic the bubbles of air surrounding emerging insects - the transitional stage from nymph to dun.

    I love this   comment   about flymphs: "If you feel that you can improve this pattern by putting a bead on it's head you are missing the point. Golf awaits you!"

    This definition of 'flymph' - and how to present them - appears in an article written by his son Lance Hidy (2018) that makes clear that his father made a clear distinction between flymphs and other wet flies. But appreciated the similarities and differences between flymphs and the nymphs tied by Skues.

    "For possibly the first time in fishing literature, [Pete] Hidy publishes a [this] photograph of the bubbles on a submerged artificial fly." (Hidy, 2018).

    The next section takes this important observation forward and considers the possibility that gaseous bubbles play an important role in triggering the trouts' rise to mayflies, midges, caddis and beetles.

    Jumpin' Jack Flash, It's a gas, gas, gas !

    Writing in 1920 Francis Ward expressed the opinion: "Flash" no doubt plays an important part in making many unconventional wet flies attractive. Many a time when the trout are off their feed an odd fish has been picked up on a "Butcher" fished deep. The trout has been unable to resist the flash of the silver body.

    Nowadays there is increasing use of synthetic materials to create 'flash' as a trigger in artificial trout flies. But what does this sparkle represent?

    Ephemera (mayfly)

    Ed Engle (2004) devotes a chapter "A Little Flash" to trace the history of fly-tyers using these materials to add sparkle to their flies.

    Flash may be a trigger / sign stimulus. Many years ago Harris (1990 ) pointed out that as the dun emerges from its exoskeleton, the small gas-filled opening at "The tail end of the hatching nymph immediately assumes a much increased lustre, and in fact it strongly resembles a section of glass tube which has been filled with mercury" (Chapter 5, p42).

    Note the tail on Craig Mathews' Sparkle Dun to represent the nymphal shuck of the emerging mayfly at a vulnerable stage - and therefore subject to predation - in its lifecycle.

    Clarke and Goddard (p 120) "established conclusively " - and confirmed in photographic evidence (p 80) - that several baetis lay their eggs underwater. They were observed spending up to 30 minutes underwater sustained by an air bubble trapped between their wings.

    This is confirmed in this photograph from Ralph Cutter: "The Baetis does not lay eggs on the water like a normal mayfly, but instead crawls underwater to affix eggs directly on the streambed. The insect has no gills and breathes directly from the bubble trapped between its wings." (Cutter, Fish Food. p. 80).

    Under favourable light conditions - a shiny layer of gas beneath the nymphal skin is easily seen as it presents a glistening appearance.

    This layer of gas buoys the mayfly nymph to the surface. This can be seen in a segment of video from "Mayfly Life Cycle: trout view" available here

    Yvon Chouinard uses Peacock Hareline Ice Dub to create the thorax of his Pheasant tail & Partridge wet fly this may represent the air bubble beneath the outer skin of the ascending nymph, as well as the air bubble trapped between the wings of the egg-laying female.

    Trapped air / gas bubbles may account for the success of the 'flashback' feature in modern nymphs as well as Chris Dore's Glister Nymph 

    Two entomologists, Edmunds and McCafferty (1988), present a detailed review of hydrofuge (water-repellent) properties in mayfly larvae (nymph), pharate subimago (emerger / dun), and adult (spinner). In summary, the hydrofuge surface of the body, legs, and wings of the dun protects them from 'drowning' during emergence: "Larvae swim or float to the surface and slightly expose the dorsum of the thorax. A gaseous layer that forms between the larval cuticle and the pharate subimago provides buoyancy and is evidenced by several small extruding bubbles...Underwater emergence requires that the subimago be hydrofuge."

    In contrast, adults (spinners) are not generally hydrofuge: "For example, the female adults of certain species of Baetis actually crawl into the water to oviposit; owing to their small size and weak legs, this would probably be impossible if they were hydrofuge."  Edmunds and McCafferty (1988)

    The authors report simple informal experiments "throw them in at the deep end!" on Ephemeroptera duns and spinners (Siphlonuridae, Baetidae, Heptageniidae, Leptophlebiidae, Ephemerellidae, and Tricorythidae), that support the conclusion that duns were hydrofuge, but this property was absent in adults (spinners).

    Chironomids, nonbiting midges

    Harris (1990) points out that "This effect is even more noticeable in the pupae of those long-legged midges, the chironomids ... and I think that it explains the added attraction which a flat tag of gold or silver gives to many artificial flies".

    The gas bubble of ascending midge pupae can be seen in a segment of video from "Midges: Chapter 5 of Bugs of the Underworld" available here

    Caddisflies: The gas bubble debate revisited

    Gary La Fontaine was a behavioural psychologist with a masters degree in trout selective feeding. Later he used scuba gear to study emerging caddis pupae underwater. After 10 years of field research he wrote Caddisflies published in 1981.

    He was   interviewed  by Craig J. Oberg before he died in 2002. "We took scuba gear, went underwater, and were the ones who actually realized what was happening with emerging caddisflies. We saw the bright air bubbles created by the caddisfly and discovered the Antron that would match that insect. We developed the emerging sparkle pupa of the diving caddis."

    His Sparkle Pupa fly was designed to hold a bubble of air to represent a gas that the pupa uses to aid its ascent to the surface.

    It proved to be a remarkable artificial fly on two counts: it caught trout, and sparked a heated debate. The problem was that LaFontaine had seen the sparkle effect underwater, but was unable - at that time - to provide photographic evidence, or research papers to confirm his observation. Unfortunately this provoked critical articles in the UK angling press, and from Bob Wyatt in his book "What Trout Want" who - after consulting universities and the British Natural History Museum - could not find scientific reference to this effect. Wyatt gives a full and fair assessment of LaFontaine's observation, but concluded that "the Sparkle Pupa theory dovetails perfectly with the popular conventional view of the selective trout."

    Image from R. Cutter Fish Food 2005

    Sadly it was shortly after LaFontaine's death that Dr. Ian Wallace  (2003) made this comment about caddis pupae that confirms Gary's underwater observation: " It is assisted in reaching the surface by a bubble of gas secreted between the adult and the skin. This however makes them appear as a silvery bubble that is very conspicuous to fish. Large numbers of caddis are predated at this time. Many species emerge at night when the fish cannot see them – but waiting Daubenton’s bats can detect these juicy mouthfuls."

    Dr Wallace has written extensively on UK caddis.

    Image from Cutter (2005)

    Inspired and encouraged by Gary LaFontaine's underwater observations, Ralph Cutter devotes a chapter in his book  Fish Food (2005) to the range of insects that display gas bubbles.He describes how to create gas bubble effects in artificial flies, and how to present them to trout. He introduces Chapter 9  All That Glitters  as follows:

    Most aquatic insects create or trap bubbles of gas at some time in their lives. They use these bubbles for respiration, buoyancy, and as an aid for escaping the subadult form. Whatever reason these insects use bubbles, all reveal themselves to trout as dazzling, quicksilver images that appear to glow with an inner light. Bubble-encrusted insects look like living jewels.

    Personally I think LaFontaine and Cutter's observations are consistent with the central theme of Wyatt's book "What Trout Want".

    Coleoptera, beetles

    An  ecological survey  on a river I fish in South Devon that runs off Dartmoor, found abundant numbers of Elmis aenea beetles in summer kick samples. Elmis aenea is a very small (2 mm) riffle beetle that is equipped with strong claws to enable it to grip in strong currents. It is a dark coloured species with very deeply ridged wing cases.It does not need to surface for air as it breathes the trapped oxygen in submerged bubbles but it does leave the water at times and can fly. If disturbed it will float to the surface.They may move downstream by drifting in the current.

    Of course finding an insect in a river does not mean that it is eaten by trout. Dr JM Elliott (1967) studied the food of trout in the Walla Brook a tributory of the East Dart. His results highlighted the importance of beetles in trout diet.16% of of various food items consumed by Year 2+ (>15cm) trout over a one year period were terrestrial invertebrates e.g. helmis maugei riffle beetle. Limnius beetles, latelmis beetles.

    The importance of beetles may have been overlooked because they are relatively unimportant on chalk streams, but "vital to summer fishing on rain-fed (freestone) rivers". Mike Weaver fishes a beetle as a dry fly (Deerhair beetle) in broken popply water at the head of pools. He advises using a sinking beetle (his Black Bug) especially when rivers are low and clear under low-flow summer conditions on very smooth water beneath overhanging trees.

    This is "usually taken with a visible swirl within a second of its hitting the water if it is going to be taken at all," ( Weaver, 1991, 1992).

    After much experimentation  Tim Roston  ties his (beetle-like) corixa with silver or pearl crystal flash to represent the air trapped in hairs around their bodies. "The fly on each slow strip gives a little semaphore flash of its underside, a little winking beacon that seemed to pull the trout in from yards away."

    Thorpe, W. H. & Crisp, D. J. (1949) described the hydrofuge hairs that enable aquatic beetles (including Elmidae) to capture air and hold it in a bubble (plastron) outside their body. This image shows the location (stippled areas) of the plastron on the dorsal surfaces of Elmis maugei. These bubbles serve as an oxygen store, and also allow the insects to absorb oxygen from the surrounding water. They also facilitate re-surfacing. (Flynn M. R. and John W. M. Bush 2008)

    These shiny properties of natural insects - ascending nymphs, caddis pupae, hatching duns, egg-laying Baetis and diving beetles - as well as Clarke and Goddard's "star-bursts" produced by the feet of duns resting on the meniscus, and even the wake created by an artificial surface lure suggest to me that shining with a sparkling light, glisten or flash, may act as a trigger to trout.

    This feature may be an example of a single-element  in a search image

    What is the advantage of a single-element search image? Speed of recognition  - the feature Pops-Out from its background.

    Image from Peter Hayes and Don Stazicker 2019

    How can this single-feature search image based on a light pattern be picked out from an underwater environment filled with bubbles?

    Trout face a similar problem to that confronting a  radar operator  recognising an enemy aircraft against background noise on a display screen. Location and movement of the target may hold the key.

    Bubbles are beneath the surface and passive; they are influenced exclusively by currents.

    In contrast, ascending and descending insects move under the influence of currents plus their own efforts to ascend or descend in the water column. And a bubble on an insect, or artificial fly - unlike a free-floating bubble - is framed by the darker body of the insect that bears it.

    By definition the wake produced by a surface lure moves against the current.

    The light patterns associated with duns resting on the surface, and spinners with wings flush with the film, are located within the meniscus.

    "Is Shape a Search Image? The ubiquitous Pheasant Tail"

    Frank Sawyer introduced the Pheasant Tail nymph - a simple fly construced from copper wire and dark pheasant tail fibres to imitate Baetis nymphs - which is cast upstream of the trout so it sinks to trout's level.

    Sawyer (1979) commented: "General shape and colouration,together with the right sizeis of greater importance than an exact copy. My two universal patterns, as I call them, are the Pheasant Tail and the Grey Goose. The Pheasant Tail serves for the darker coloured nymphs and the Gray Goose for the lighter ones."

    The effectivenees of simple flies constructed from pheasant tail fibres is not restricted to English chalk streams.
  • Arthur Cove's Pheasant Tail was developed to imitate 'buzzers' (chironomid midges) on Eyebrook reservoir.
  • The American Al Troth based his Pheasant Tail nymph on Sawyer's original pattern but used peacock herl as thorax material.
  • The Teeny Nymph is another example of a simple but effective trout fly which may imitate a shrimp. Size and colour variations of Jim Teeny's basic pattern have been responsible for catching 25 IGFA (International Game Fish Association) fresh and saltwater world records.
  • The simplicity of these flies suggests that they may share a common property that dovetails with a trout's search image. Here are some possibilities:
  • movement - these patterns tend to be fished with some form of movement
  • colour - colour is often varied to match the colour of the natural nymph
  • thorax - is present but construction materials vary (Sawyer and Teeny used pheasant tail; Cove used rabbit fur and Troth used peacock herl)
  • body shape - designed to represent shape of natural (Sawyer and Troth tied a straight body to represent a mayfly nymph; Cove tied around the hook bend to represent chironomid pupae)
  • the ratio between body size and thorax may be important when representing particular insect groups
  • This table presents the design elements in several 'classic' artificial flies used for sub-surface presentation to trout in rivers and stillwaters.

    All of these successful classic trout flies have the following design elements in common:
  • body made of pheasant tail fibres
  • thorax made of pheasant tail fibres
  • movement imparted by the angler
  • Application of Lloyd Morgan's Canon would suggest that an artificial fly constructed with a straight body from pheasant tail fibres and some form of thorax which is moved in the water should catch trout.

    It is interesting that the flies constructed by Sawyer, Cove, Troth and Teeny are more elaborate than this simple pattern.

    For example, Sawyer's nymph has a tail. However it is possible that the tail is a part of the search pattern when trout are feeding selectively on mayfly nymphs.

    Likewise, the curved body in Cove's fly may be part of the search pattern when trout are feeding selectively on chironomid pupae.

    It's perhaps not surprising that selective feeding is the result of the operation of a "search pattern" for particular types of insects.

    If an ethologist was asked to investigate which of these features (body, thorax, movement) elicit the feeding response, they would construct even simpler flies which incorporated just one element. No ethologist has attempted this task. But fly-fishers may have done so.

    The writings of several   experienced anglers suggest that the stimuli involved may be:

    1. the outline of a nymph represented by a thorax composed of a few wire wraps
    2. movement of the 'model' in the water - the 'induced-take' technique
  • Raymond Baring found that a Pheasant Tail nymph increased in attractiveness as it became more and more bedraggled and finally lost all of its original dressing
  • Ed Zern (1979) described how he caught trout on a pheasant tail nymph that was " a bare size 18 hook with three turns of fine copper wire around its short shank and nothing else - no fur, no feather, no silk, no tinsel."
  • Oliver Kite also reported success with his ' bare hook nymph' which consisted of a few turns of wire wrapped around the hook shank. He was also able to catch trout whilst blindfolded by using the  'induced-take' technique
  • Inspired by Kite's success,  Roy Christie developed his Copper Wire Hare's Mask fly with which he has "..spent many hundreds of hours using this system and caught thousands of trout with it." But he adds:" Does it always work? Well, no."
  • Shape+Flash: A marriage made in heaven?

    In this video Yvon Chouinard  ties a fly with conventional materials, Pheasant Tail & Partridge together with a modern twist to create sparkle / flash to mimic the air bubble in the natural fly. It brings together the two elements discussed above: shape and flash or sparkle. It's worth reading this  article  in which he discusses his approach to fly-fishing - simplicity and finding simplicity not only in flies, but also in fly-fishing tackle.

    Less is more: Simplicity in fly-tying

    I've noticed that some fly-tyers are producing simple flies with triggers. Simplification by removing unnecessary clutter in the dressing may actually increase the effectiveness of the fly. And it may help to isolate triggers.

    For example, Tim Rolston removed hackle from his  Compar-ant pattern  that was obscuring the ant shape of the artificial.

    Local angler Luke Bannister uses just one material (poly yarn) to tie Kenneth Boström's Rackelhanen to represent small mayflies, emergers and even terrestrials.

    It's worth reading Kenneth Boström's  advice  on fishing his fly.

    These videos shows Luke  tying  and  fishing  the Rackelhanen

    Chris Dore's  Glister Nymph is also tied with one material -Glister

    Supernormal triggers / stimuli

    An especially effective artificial fly contains a feature(s) that - paradoxically - makes the artificial more attractive than the natural fly.

    This is called a supernormal stimulus by Tinbergen.

    This diagram illustrates a supernormal stimulus in the signal detection model (full results  available here): the distribution of the natural and supernormal flies barely overlap.

    Here are some examples of supernormal stimuli discovered by Tinbergen.

    In this experiment the supernormal stimulus received about 25% more pecks from gull chicks than the natural head, a model of an adult head, or a model of the adult's bill (Tinbergen and Perdeck, 1950)

    In another experiment songbirds that laid light blue grey-dappled eggs preferred to sit on a bright blue black polka-dotted dummy so large they slid off repeatedly.

    There is a remarkable difference between the normal and supernormal stimuli.

    Earlier I introduced Hugh Falkus'  Surface Lure.  On dark nights brown trout and sea trout (Salmo trutta) will enthusiastically take a dragged fly, but refuse the same lure if it does not drag.

    It has supernormal stimulus properties:
  • the wake, not the 'fly' elicits the rise
  • it attracts sea trout that normally  do not feed  in freshwater
  • Exploiting vulnerability in designing artificial trout flies

    Note the preparatory movements to reposition the trout closer to the approaching fly prior to the rise

    It makes sense for trout to have evolved mechanisms to select flies when they are most vulnerable. Flies that are making the transition from water to air - emergers- would seem to be particularly vulnerable. At this stage in their life cycle the insect has to overcome two problems:
    1. breaking through surface tension at the boundary between air and water 
    2. and extricating itself from its nymphal or pupal skin - ecdysis

    In this clip from Ozzie Ozefovich's video Underwater World of Trout Part Two | Feeding Lies trout are feeding on the small mayfly Tricorythodes (Tricos)

    Vulnerable insects present strong cues to potential predators. The emerging insect is temporally trapped at the water surface. Part of the body hangs below the mirror and provides a primary trigger that could tempt the fish to begin to rise. During emergence, the head, thorax and wings gradually rise above the surface. It is reasonable to argue that trout exploit this vulnerability. An emerging fly has special characteristics that make it attractive trout food:
    1. the abdomen, thorax and shed skin (shuck) remains suspended on or below the water surface making a clearly visible footprint that penetrates the trout's mirror - the primary trigger
    2. emerging wings that sit above the surface and are visible in the trout's window and may provide a strong secondary trigger
    3. some insects have a protracted period of emergence that gives trout plenty of time to intercept them

    The Importance of Angler-Confidence


    I hope this page has gone some way to dispelling any concerns you may have had that  'drag' scares trout :
  • In  daylight drag  interferes with trout intercepting a dry fly on the surface
  • In  darkness drag  acts like a supernormal stimulus for brown and sea trout
  • I think it's very unlikely that drag will have opposite effects on trout by day and night.

    In my opinion, it's more likely that drag disrupts the  Tracking Heuristic  by day, and drag provides the target for the Tracking Heuristic at night.

    Dealing with drag

    Drag can be a serious problem. With experience you will be able to pick out your fly on the water. You will become habituated to the distraction of ever-moving surface currents. Then you will find that you are unconsciously using a gaze or tracking heuristic to follow your fly's progress. If you have a problem finding where your fly has landed take a look at this section

    Choice of fly: Detect, Inspect, Select or Reject

    anabolia nervosa caddis

    It's understandable why we spend a great of time pondering what fly to use. But look at it from the trout's point of view. It has to eat.

    The things that it eats are camouflaged to protect them from being eaten.

    I was amazed by the amount of "stuff" fish ingest and quickly spit out. Among all potential food items fish pursued: 52 % were captured and quickly expelled from the mouth 39 % were visually inspected but not captured and only 9 % were ingested (Neuswanger et al, 2014)

    And the spitting out was done in the blink of an eye.

    The good news is that it's very likely that your fly will attact the attention of a trout, it may even be ingested. Here's a way I increased my confidence. Westcountry rivers are crossed by many bridges. Stand on the bidge. Look down and you are pretty certain to spot a trout. Take a small bit of stick about the length of the nail on your little finger, and drop it a few feet to the side of the trout. Very often a trout will move to take a look at this "manna from heaven". The lesson from this version of "Pooh Sticks" is that it's only a small step from your stick to a fly a trout on our rivers will eat.

    When I'm guiding anglers on South Devon rivers I am often asked, “What fly should I use?” I advise them to start with a fly they have confidence in. If they don't have a favourite fly, I'll open the box and invariably find a Adams or a parachute pattern with a post that can be easily seen in fast moving water. They give a good opportunity to catch a fish on our local rivers. They are popular flies with well deserved widespread reputations. This short answer avoids the long explanation that I've presented on this page. It's a bit like an iceberg; 90% of my rationale is, and should stay, beneath the water.

    Some fly designs seem to pass the test of time. Most lists of recommended flies contain the Adams and the Gold-Ribbed-Hare's ear. They're there for a very good reason, year in year out, they catch trout and inspire confidence in anglers. And it's that confidence that is important above all else.

    Here's an article about fly designs that should be read by anyone who buys flies, or reads fly fishing magazines  A Failure Of Modern Fly Design by John Juracek.He concludes: "There are still plenty of fishing problems awaiting solutions. Undoubtedly, some will be solved by better fly designs. But if our focus remains stuck on creating flies to solve business problems instead of fishing problems, it’s a safe bet that those solutions will be a long time coming.". It's worth looking through Juracek's other articles on his website for some forthright opinions borne out of experience: e.g. "In Search of Low Line Speed" in which he explains the problems with most modern fly rods;"For The Classics" in which he laments the lack of interest in classic fly-fishing books such as Nymph Fishing for Chalkstream Trout by G.E.M. Skues and A Modern Dry Fly Code by Vince Marinaro; and "How to Cast 20 Feet" - more difficult than it sounds.

    We all have ups-and-downs of confidence, or a lack of it, in a particular fly . I suspect it may even get stronger with fly-fishing experience. Here are two revealing opinion pieces from experienced fly-fishing guide Tim Rolston on   fly confidence   and  fly tying. I must confess for me it can get frankly totally irrational. The test came today in his Lockdown Day 12 blog post where he asked the question: "So how then does one explain the effectiveness of the rigid Perdigon?" Maybe it's a left- right-brain conflict for me. The right side of my brain says it looks horrible; the left side explains that it will get to the required depth more quickly than a conventional fly, present an uncluttered shape with a prominent trigger colour. If a client has confidence in a Perdigon that's fine with me, but I wouldn't use it on a guide's day off ... yet !

    Some flies tend to be simple and may capture, or present in exaggerated form, the essence of the natural fly. For example Frank Sawyer's nymphs, Comparadun / Sparkle Dun

    I think they may have a sign stimulus trigger in an exaggerated, or an uncluttered form, of a feature of a natural fly that causes them to   Pop-out and be recognised as edible.

    Unfortunately was can only guess what is or is not a sign stimuls for a trout, here is   a word of caution about triggers.

    I have a preference for dry flies with a big wing and a soggy bottom !

    Take your time ...

    Video of a small trout on the Tory Brook
    courtesy of Graeme Webster

    It can help fish and angler to simply sit still for a few moments and "let nature come back to you". It will, and you'll see little and larger fish within a few feet and maybe a fish rising a short distance away. Remember that the  trout's visual system  has evolved to eat and not be eaten by predators. So they are going to respond to overhead predators and their monocular vision on each side of the head is focused at infinity.

    I think it helps if anglers appreciate that a trout's main concerns in life are to eat, reproduce and stay away from predators - and round here predators are mostly on two legs, and reproduction can wait until the season is over - everyone needs peace and quiet for that! We tend to think that having the right fly is the most important thing to get right. It helps, but remember a trout wants to eat, sometimes eat anything that looks edible - remember those young  Alaskan chinook  who would have a go at virtually everything that came their way. What really can ruin your chances is clumsy wading. It helps to walk upstream like a ghost and hopefully see small fish a few feet ahead.

    Fly lines crashing down on the water can startle fish - they will slide away. False casting is said to have a similar effect. I call it the 'Carshalton dodge' - the original name for false casting. It was originally invented as a way of drying a waterlogged dry fly. Nowadays there is a wealth of powders and potions to overcome that problem. And around here overhanging trees, I swear, actually move their position when they see your backcast coming !

    Remember trout make mistakes too ...

    Angling writers tend to focus on success and successful flies. Much less attention has been paid to why a trout sometimes misses a fly. Most of us have experienced occasions when a trout rises to an artificial fly but fails to ingest it. Various excuses are offered; the trout is said to be 'rising short', or we blame ourselves for not striking soon enough. But maybe the trout has simply misjudged the position of our fly on the surface, or micro-drag on the leader has taken the fly out of the window.

    And remember the speed at which those Chinook salmon fry in Alaska spat out non-food items: "Among all potential food items fish pursued: 52 % were captured and  quickly expelled  from the mouth."

    Peter Hayes and Don Stazicker (2019) reported that: "We have timed a spit-out at 1/5 of a second. A trout often ejects an artificial fly faster than it ejects an untethered inedible item such as a seed pod or piece of debris."

    Young trout on fast moving water seem especially prone to these 'mistakes'. In contrast, older and bigger trout - especially on small stillwaters - rarely miss the fly. Juvenile fish may need to practice the skill of tracking a fly on the surface. Wells (1958) found that young cuttlefish needed to practice catching their prey. If you have nothing better to do you can read my   article on the role of maturation and practice in the development of apparently instinctive (unlearned) behaviours. Here is more discussion on  why a trout may miss a fly

    These tables shows possible outcomes of a trout's decision making  process to natural and artificial flies. We saw earlier that trout don't always make the right decision. Sometimes they don't rise to a natural fly. Sometimes they rise to debris in the drift and spit it out. The message is that ignoring or spitting out your fly is part of their daily life - it doesn't show that they have  human-like cognitive and emotional processes  : intelligence, suspicion, fastidiousness and capriciousness. Drag and refusal of a fly won't scare fish. They do it all day long. "The fish is not frightened by the artificial fly, he simply recognises that it is not what he wants and so ignores it (Ward, 1920, p 141). What will 'scare' them is acting like a clumsy predator.

    Decision is:
    Item is: Rise Don't rise
    Food Food eaten Meal missed
    Not food Debris spat out Correct rejection: debris in drift

    This table shows possible outcomes when an angler's artificial fly is added to the trout's decision making  process :

    Decision is:
    Item is: Rise Don't rise
    Food Food eaten Meal missed
    Artificial fly I'm hooked Correct rejection:
    poor presentation?
    lack of stealth?

    Why can't I see trout that my friend spotted ...

    First check if they're wearing expensive rose-tinted polaroids !

    Try to get elevation above the river, preferrably shielded by vegetation. If there's a tree trunk that provides cover, go some way behind the tree and approach the riverbank using the tree trunk as concealment. Focus on the bed of the river. Don't just glance, look carefully because a trout at rest is often camouflaged against its background. Move your head slowly from side-to-side. If you're not used to seeing trout, spend some time looking at it rather than looking for more. This will help develop your 'prey image'. Have a look at this page with some tips on spotting sea trout and the aircraft recognition system that I found useful.

    Size of trout. The 'grass is always greener' belief

    Trout in South Devon rivers appear smaller compared to their colleagues who appear on Facebook and are thrust before the camera on outstretched arms ! This diagram shows the size of our resident brown trout. But some of these fish migrate to saltwater, spend time goodness knows where, and return to home rivers having put on weight and with a new moniker 'sea trout'. But they're still  salmo trutta  under that bright shiny coat that fades - like a holiday tan - after they return home. And they're pretty snooty about eating freshwater fly life. But they have retained a love of the nightlife which can be their downfall.

    This page discusses the size of  South Devon trout.

    This page discusses salmo trutta's lifestyle choices

    This page describes  day- and nightime tactics  for sea trout.

    What insects do trout feed on in South Devon?

    Many fishing books concentrate on ephemeroptera - the upwinged flies that can hatch in large numbers on chalk streams.. Frankly it is rare to see vast numbers of upwinged flies floating down our freestone (rain fed) Dartmoor and South Devon rivers, large upwinged flies are vastly outnumbered by little black flies "no-see-ums": midges, beetles and caddis.

    Here is a page that draws on a  wide range of reliable sources: 
  • a survey of the insects found in a Dartmoor river
  • an examination of Dartmoor trout stomach contents
  • advice from an experienced local angler
  • to build a picture of what flies and tactics may be successful on a Dartmoor river.

    Some of these flies are very small and difficult for us to see on the water. It helps to squat down and look across the water surface.  Small insects do not present a large footprint in the trout's mirror. Also they have small wings which give a very indistinct target in the window. When trout are feeding on small flies they seem to lie very close to the surface. As we saw above the window is very small in diameter when the trout is close to the surface. Therefore it is important to try to present the fly very close to the fish.

    But as we have seen it can be misleading to assume that a fish is lying where a rise is spotted. Maybe that's why small flies are called the "Anglers' Curse".

    Ed Engle has written two books, one on tying, the other on fishing small flies. They are packed with useful information. For example: "Drag is especially challenging for small-fly fishermen because its effects can be more difficult to detect when fishing a fly that may not be even visible on the water's surface. The best course of action is to assume that drag exists on anything but the shortest of casts and act accordingly. Consider your position before the cast, use casts that put slack in the leader, and if necessary, mend the fly line once it's on the water to counteract drag." This page has my   review of these excellent books.


    I am grateful to   Prof Wheeler for pointing me in the direction of relevant papers on Gaze Heuristics.

    To Dr Eleanor Caves and Dr Jason Neuswanger for their refreshing enthusiasm for all things 'fishy', and their patience in helping me understand their research areas.

    To  John Shaner,  Rob Smith  and Paul Schullery  for their interest in this project, and sharing their knowledge of the fly-fishing literature.

    To Aidin, Geoff and Mike for their support and encouragement during the writing of this long page

    To Graeme Webster for his video of a trout on the Tory Brook a small tributory of the  River Plym

    I would like to acknowledge the publishers who provide free access to academic papers that formed an essential resource for my exploration of Heuristic research...

    ... including “The Gaze Heuristic:” Biography of an Adaptively Rational Decision Process by  Dr. Robert Hamlin  that disabused me of the widespread belief "Fly fishing is not rocket science" - it is in a most unexpected way!"

    About the author

    Paul guiding ITV News reporter in June 2019

    with sea trout in camera range ...

    Paul Kenyon lives in Ivybridge on the southern edge of Dartmoor about 6 miles from the Upper Yealm Fishery.

    Paul devotes more time than is reasonable to his love of all things associated with fish, fishing, instruction and guiding on Dartmoor rivers.

    He retired in 2006 from the Department of Psychology, University of Plymouth where he lectured in behavioural neuroscience and evolutionary psychology.


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    92. Rosenbauer, Tom. Orvis Guide to Prospecting for Trout, New and Revised: How to Catch Fish When There's No Hatch to Match. The Lyons Press. 2008.
    93. Sawyer,F. "Nymphing in the Classic Style" in Migel Wright (Eds) "The Masters on the Nymph", 1979, published by Ernest Benn, London.
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    Online resources

    1. Pennsylvania Fly Fishing Museum Association  Vincent Carmen Marinaro (1911-1986) Collection
    2. Link to the full Ozzie Ozefovich's video Trout Vision & Refraction