This page explores in greater depth the ideas introduced in our article in the May 2020 edition of The Field magazine.
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'.
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.
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.
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Hewitt in a list of previous authors of books on fish vision that - in their opinion - contained misunderstandings, or out of date scientific knowledge (p 60, 1980 edition)
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.
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.
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'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 !
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)
| 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 |
| 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 |
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.
"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.
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.
"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'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.
| 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 ∞ |
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.
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.
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.
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 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 .
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).
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:
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.
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....
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.
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).
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.
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 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.
| Effect of drag on the Tracking Heuristic: | ||
|---|---|---|
| Time of day: | Drag present | Drag absent |
| Daytime | Disrupted | Enabled |
| Nightime | Triggered | Disrupted |
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.
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.
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)
It's not unusual for predators to fail to catch their prey.
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.
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.
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 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.
If the aircraft deviated from the required height of 60 ft (18 m):
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 . 
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.
Fishing a wet fly downstream is not always as simple as it appears on first sight.
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!
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.
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 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.
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.
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.
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.
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.
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."
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.
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.
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)
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.
The difficulties emerged when radar was used in World War II to identify enemy aircraft (prey) in the Dowding aircraft interception system
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.
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:
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
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.
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.
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.
If your favourite is missing attention switches to using a search image for your next preferred chocolate. 
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)
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.
| 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% |
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: 
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.
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" .
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."
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.
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."
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)
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:
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.
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.
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).
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.
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.
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.
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.
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. 
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."
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:
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.
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.
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.
How can this single-feature search image based on a light pattern be picked out from an underwater environment filled with bubbles? 
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.
The effectivenees of simple flies constructed from pheasant tail fibres is not restricted to English chalk streams.
This table presents the design elements in several 'classic' artificial flies used for sub-surface presentation to trout in rivers and stillwaters.
The writings of several experienced anglers suggest that the stimuli involved may be:
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.
Local angler Luke Bannister uses just one material (poly yarn) to tie Kenneth Boström's Rackelhanen to represent small mayflies, emergers and even terrestrials.
Chris Dore's Glister Nymph is also tied with one material -Glister
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)
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.
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:
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.
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)
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 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
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."
| Decision is: | ||
|---|---|---|
| Item is: | Rise | Don't rise |
| Food | Food eaten | Meal missed |
| Not food | Debris spat out | Correct rejection: debris in drift |
| Decision is: | ||
|---|---|---|
| Item is: | Rise | Don't rise |
| Food | Food eaten | Meal missed |
| Artificial fly | I'm hooked | Correct rejection: drag? poor presentation? lack of stealth? |
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.
