The round skylight is also known as Snell's circle: A cone of light of
about 97 degrees.
a Trout Catch a
Fly?: Marinaro's "Edge of the Window Theory"
In May 2020 this page was updated and extended
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?".
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
the movement of the fly as it is transported
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
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 window increases in size as the fish sinks deeper
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.
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
He observed that trout drift downstream and tilt their head
upwards as they position themselves to take a fly off the
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).
window and the
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
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'
plays an equally important role in the trout's view of the
Remember the description: "The trout lives, as it were, in a
room with a ceiling made of mirrors
for a round skylight in the middle (the window) , through
which the outside
world is visible" Frost and Brown's (1967).
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
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
degrees to the water surface are potentially visible
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 of the fly's wings in its window
the diagram below)
finally, when the insect reaches the edge of the
window, all of its body can be seen (3
in the diagram
The next series of diagrams examines the three
points 1, 2
detail. The diagrams incorporate actual photographs taken by
Clark and Goddard which show how the wings and body of the insect
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
Position#3: legs, wings and
familiar example of a visual
It may help to understand the significance of
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.
"The light's green,
I will start to prepare
in case I need to stop"
"I see insect legs approaching.
This may be food,
I will start to move toward the
water surface to intercept it"
"The light is amber.
I will apply foot brake and
put gears in neutral"
"I see legs and wings.
I will continue to move
"The light is red,
I have stopped.
I will engage the
"This is food.
I will swim towards it,
open my mouth and
eat this insect"
How legs and wings control the rise
Clark and Goddard (1980) consider that
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
Then - once the fish has started to rise - the
wings play an important role in maintaining
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.
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
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
trout's attention on the fly as the fish rises towards the
surface to intercept the fly.
Wing height (inches)
Distance from window
at which wing tips
first become visible to trout
Marinaro's "Edge of the Window Theory":
Marinaro's and Clark and Goddard's
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
Marinaro (1995) summed up his extensive observational studies of
trout feeding behavour as follows: "It is
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
Trout behave in this way in order to judge the exact
the fly. By keeping the fly in a precise position relative to its
body, the trout stands a very good chance of engulfing the
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
We know that:
the trout's window has a width of 97
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
Depth of trout (in feet and inches)
Distance (in feet and inches) between
trout's eye and fly
on edge of window
2 feet 8 inches
1 feet 4 inches
The trout stands a very good chance of successfully ingesting the
fly if they drift downstream keeping the insect on the edge of
The acquisition of this skill may involve learning, maturation
The main message from this analysis is that a successful trout
should present a primary trigger stimulus that
mirror. This engages the trout's attention and initiates the
rise. For example, a dubbed fur body may imitate legs penetrating
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
disturbance in the spatial relationship between the insect and
the edge of the trout's window
Marinaro's "Edge of the Window Theory":
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
a single image when the fly crosses the
edge of the
By keeping the fly on the edge of the window, the trout stands a
very good chance of engulfing the insect.
trout flies to trigger successful rises
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
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 extricating itself from its nymphal
or pupal skin -
Vulnerable insects present strong cues to
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)
below the water surface making a clearly visible footprint which
penetrates the trout's mirror - the primary
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
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
presentation of trout flies?
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:
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
he is expecting to intercept a round, red cricket ball !
The importance of behaviour: Drag
When catching things the behaviour of the
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
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
"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
fly's wake. The trout probably keeps the wake at the edge of
are artificial flies
sometimes ignored by feeding trout?
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
practice in the development of apparently instinctive (unlearned)
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
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
Clark, B. and J. Goddard (1980). The Trout and the Fly.
Ernest Benn Ltd.
Frost, W.E. and M.E. Brown (1967). The Trout. London,
Hewitt, E.R. (1948). A
Trout and Salmon Fisherman for Seventy-Five Years. London,
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.