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The round skylight is also known as Snell's circle: A cone of light of
width of about 97 degrees.
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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 intercepts a fly drifting towards it on 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 diagram sets the
scene
for our exploration of a deceptively simple question "How does a trout
catch a fly?".
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 "A
problem shared is a problem halved"
How does
a trout
position
itself to ingest a floating insect? It's something many of us see every
day, and there is a temptation to take it for granted. But catching a
fly is no simple feat. The trout's brain has evolved to cope with a
complex situation involving:
- the constantly changing position of the trout
relative to the fly
- the movement of the fly as it is transported
downstream by the current
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.
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 it. 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:
- the window gets smaller the closer the trout
is to the water surface
- the window increases in size as the fish sinks deeper
into the water
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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 into a very small area.
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 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).
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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'
which 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).
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 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) have 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:
- body parts that break through the 'mirror'
- wings appearing in the window
The next diagram shows what part of an insect are visible as it drift
downstream towards a waiting trout.
- At first only the legs are visible beneath the mirror
( ^ ^
in the diagram)
- 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)
- 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
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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.
| Traffic signal |
Human reaction |
Insect signal |
Trout reaction |
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"The light's green,
I will start to prepare
in case I need to stop" |
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"I see insect legs approaching.
This may be food,
I will start to move toward the
water surface to intercept it" |
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"The light is amber.
I will apply foot brake and
put gears in neutral" |
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"I see legs and wings.
I will continue to move
towards them." |
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"The light is red,
I have stopped.
I will engage the
hand brake" |
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"This is food.
I will swim towards it,
open my mouth and
eat this insect" |
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How legs and wings control the rise
 Clark
and Goddard (1980) consider that penetration 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 an important role in maintaining
the rise
Under normal circumstances an insect's
wings do not penetrate the water surface. Because of the laws of
refraction, fish cannot see any part of an insect which lies below an
angle of 10 degrees to the water surface. 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 upstream of the edge of the trout's window.
These
calculations support Clark and Goddard's claim that an insect's wings
are not responsible for triggering the onset of a trout's ascent in the
water to intercept an insect being carried downstream towards it. Wings
appear too late for the trout to use them to trigger an
effective rise. However wings are important.
Wings maintain
the trout's attention on the fly as the fish rises towards the surface
to intercept the fly.
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Wing height (inches) |
0.5 in |
0.4 in |
0.3 in |
0.2 in |
Distance from window
at which wing tips
first become visible to trout |
2.8 in |
2.3 in |
1.7 in |
1.1 in |
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Marinaro's "Edge of the Window Theory": Dry
flies
Marinaro's and Clark
and Goddard's photographs show that when an insect reaches the edge of the trout's window, a crucial 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?"
Trout behave in this way in order 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.
Alternatively the trout may learn to capture prey by using this "edge
of the window effect".
We know that:
- the trout's window has a width of 97
degrees
- and that the radius of the trout's window
is a
precise function of the depth of the trout in the water
- 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
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Depth of trout (in feet and inches) |
3 feet |
2 feet |
1 foot |
6 inches |
| Distance (in feet and inches) between
trout's eye and fly on edge of window |
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2 feet 8 inches |
1 feet 4 inches |
8 inches |
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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.
The acquisition of this skill may
involve learning, maturation and practice.
The
main message from this analysis is that a successful trout fly:
- should present a primary trigger stimulus that
penetrates the mirror. This engages the
trout's attention and initiates the rise. For example, a dubbed fur
body may imitate legs penetrating the mirror
- wings on the artificial may act as
cues during a rise to maintain the
trout's movement towards the fly
- the presence of the merged image of the body
and
wings at the edge of the trout's window allows the trout to precisely
judge and maintain its distance from the fly. A pronounced thorax on an
artificial
fly may enhance this visual cue
- the 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
Marinaro's "Edge of the Window Theory": Wet
flies
The next diagrams show how Marinaro's theory can also account for how a trout intercepts a natural or artificial fly
swimming beneath the surface.
As the sunk fly approaches the fish sees:
- two images:the actual fly and its reflection in the mirror
- a single image when the fly crosses the edge of the window
By keeping the fly on the edge of the window, the trout stands a very good chance of engulfing
the insect.
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Designing
trout flies to trigger successful rises |
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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:
- breaking through surface tension at the
boundary between air and water
- and extricating itself from its nymphal
or
pupal
skin - ecdysis
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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:
- the abdomen, thorax and shed skin (shuck)
remains suspended below the
water surface making a clearly visible footprint which penetrates the
trout's mirror - the primary
trigger
- emerging wings that sit above the surface
and
are visible in the trout's window and may provide a strong distance
cue
- some insects have a protracted
period of
emergence that
gives trout plenty of time to intercept them
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These important features are captured in flies such as Hans van Klinken's
Klinkhamer, Roman Moser's Balloon Caddis, Craig Mathews' Sparkle Dun
and Bob Wyat's Deer Hair Emerger which are effective on west country
rivers.
What
other factors are important in the design and presentation of trout
flies?
Anglers
have successfully designed thousands of
artificial flies that elicit a
'rise' or 'strike' from a trout. A great deal of attention has been
paid to creating flies of the correct:
- size
- shape
- colour and
- behaviour
Datus
Proper (1993) presents a strong argument that these are perfectly
reasonable criteria for designing an effective artificial trout fly.
Common sense tells us that these factors are important. Consider our
cricketer trying to catch a ball. We wouldn't be surprised if a
cricketer made very little effort to
catch a square grapefruit
when he is expecting to intercept a round, red cricket
ball !
The importance of behaviour: Drag
When catching things the behaviour of the
object
is crucial. In games, the flight
of the ball can make it difficult to catch or hit. Likewise for trout:
movement of the fly is critical. The fly will be refused if it drags
on the water surface. Much has been made of the importance of avoiding
drag in books and articles on fly fishing. Many authors claim that drag
actually scares fish. Wyatt and Marinaro offer an alternative view:
- "Drag may not spook the fish; they just go
"nope" and resume watching for the next bite of food" (Wyatt, 2004,
p.44)
- "Many books by competent writers and fishermen
... conclude that a dragging fly frightens the trout. I do not agree
with that at all."
I have to agree with Wyatt and Marinaro. The simplest explanation is
that drag makes it difficult or impossible for the fish to maintain the
fly at the edge of the window.
Of course there are circumstances where fish will take a dragged fly.
But in those situations the trout's rise is triggered
by the fly's wake. The trout probably keeps the wake at the edge of its
window.
| Why
are artificial
flies sometimes ignored by feeding trout? |
Angling
writers tend to focus on success and successful flies. Much
less attention has been paid to why a trout sometimes misses or ignores
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.
- 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 taking a fly from 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.
- Many fishing books concentrate on
ephemeroptera - the upwinged flies. Frankly it is rare to see the duns
of upwinged flies floating down our rivers in vast numbers. On Dartmoor
and South Devon
rivers, large upwinged flies are vastly outnumbered by millions of
little
black
flies "no-see-ums": for example black gnats and glossosoma caddis.
These flies are very small. They are very difficult for us to see on
the water. It sometimes 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".
I don't have an easy solution to this problem. The best I can do is to
make repeated casts in the hope of getting my artificial noticed.
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References
- Clark, B. and J. Goddard (1980). The Trout and the Fly.
London, Ernest Benn Ltd.
- Frost, W.E. and M.E. Brown (1967). The Trout. London,
Collins
- Hewitt, E.R. (1948). A Trout and Salmon Fisherman for
Seventy-Five Years. London, Scribner.
- Marinaro, V.C. (1995). In the Ring of the Rise.
Shrewsbury, Swan Hill Press.
- Proper, D. (1993). What the Trout Said: About the
Design of Trout Flies and Other Mysteries. Shrewsbury,
Swan Hill Press.
- Right Triangle Angle And Side Calculator http://www.csgnetwork.com/righttricalc.html
- Roberts, J. (1994). To Rise a Trout.
Marlborough, The Crowood Press.
- Wells, M.J. (1958). Factors affecting reactions to Mysis by newly
hatched Sepia.
Behaviour.
13:96-111.
- Wilson, D. (1957). Fishing the Dry Fly.
London, Adam and Charles Black.
- Wyatt, B. (2004). Trout Hunting: The Pursuit of
Happiness. Stackpole Books
Online resources
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