Understanding how a trout recognises a fly and then intercepts it is key to your choice of fly – and success, says Paul Kenyon

How does a trout see a fly? Learning and understanding the intricacies of this process will prove key to choosing your fly – and your chances of success, says Paul Kenyon.

It may be called Duffer’s Fortnight, but having the right kit and equipment is still key. It is time to take the Mayfly MOT, read essential mayfly kit: demystifying Duffer’s Fortnight. And to learn when to cast and when to stay calm, read how to catch big trout: monster munchers.


Before 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. This short answer avoids a long explanation but, if pressed, the following is my rationale.

I turn the angler’s question on its head to focus on the trout rather than the fly. Let’s explore how a trout recognises and intercepts a fly drifting towards it on the surface of the river. The answer may give an insight into how to design effective trout flies and why our artificial flies are sometimes ignored by feeding trout.

How does a trout see a fly

The merged image of the insect’s body and wings at the end of the trout’s ‘window’ allow it to locate the fly precisely.

Traditionally, angling writers have talked in terms of deceiving a trout into taking an artificial fly. I 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 first recognises and then catches an insect we can select and present an artificial fly so that it can be caught easily by trout.

How does a trout catch a fly? It’s a deceptively simple question about something many of us take for granted. But catching a fly moving quickly downstream is no simple feat, especially for trout lying beside popply water near the head of a pool. 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. Outfielders are expected to make a catch but it’s clearly more difficult for infielders in the slips who – like trout at the head of a pool – must react quickly.

How does a trout see a fly

Because of the refraction (bending) of light rays entering water, only objects that protrude above an angle of greater than 10 degrees on the water surface are visible in the window. Therefore, anything in the red area is not visible in the trout’s window.

It is widely accepted that trout see the world through a circular ‘window’. Think of a skylight surrounded by a ‘mirror’ that reflects the bed of the river and objects in the water column. The diameter of this window varies in size: it gets smaller the closer the trout is to the water surface and increases in size as the fish sinks deeper in the water.

I fish on rapid streams running off Dartmoor. These rivers are relatively shallow. Trout often lie close to the surface. A trout lying one foot beneath the surface has a small window above its head about 11 inches in radius. You may be getting 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 we would rarely catch a rising trout and, more importantly, most trout would starve to death.

How does a trout see a fly

The feet of an approaching insect puncture the mirror and create a ‘light pattern’ that triggers the start of the rise.

Up to now we have concentrated on the trout’s ‘window’. But we must also consider the ‘mirror’ that may play an important role in the trout’s view of the world. It turns out that trout – especially those lying in calmer water – may get ‘advanced warning’ of a fly long before it appears in their ‘window’.

Trout are able to see parts of an insect or artificial fly that rest on, or puncture, this ‘mirror’. The body of vulnerable emerging flies puncture the mirror. The legs of duns resting on the surface rumple the mirror. Therefore, parts of an insect are visible to fish long before the insect has entered the trout’s window.


In their groundbreaking book The Trout and the Fly, Brian Clarke and Dr John Goddard suggest that this distortion of the mirror by an insect’s feet acts as the primary trigger for the trout’s rise. They surmise: “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.”

Once the fish has started to rise the insect’s wings may play an important role in maintaining the rise. Because of refraction – the bending of light rays as they enter the water – only objects above an angle greater than 10 degrees to the edge of the window are potentially visible to the trout. At first only the tips of an insect’s wings are visible, then, as it gets nearer the trout can see more and more of the fly’s wings in its window.

How does a trout see a fly

A wet fly outside the window is reflected in the mirror – the trout sees two flies.

However, wing tips only become visible when the insect is very close to the edge of a trout’s window. I used my imperfect grasp of mathematics to calculate that the wing tips of an insect with wings that are ½in high will only become visible to a trout when the insect is just over 2½in upstream of the edge of the trout’s window.

It could be argued that wings appear too late for the trout to use them to trigger an effective rise. However, wings are important because they maintain the trout’s attention on the fly as the fish rises towards the surface to intercept it. Clark and Goddard’s book contains photographs that show how the wings and body of the insect gradually merge as they get closer and closer to the edge of a trout’s window. These photographs are important. They suggest a way in which the trout can gauge the position of the fly whilst rising to consume it. And they contain important hints for the design of effective artificial trout flies.

In his book In the Ring of the Rise, American fly-fishing author Vincent Marinaro summed up his extensive mathematical and observational studies of trout feeding behaviour 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.”

How does a trout see a fly

The ‘two flies’ merge into one when it enters the trout’s window.

Thankfully, Marinaro did the mathematics showing that the distance between the edge of the window and the insect is a precise function of the depth of the trout in the water. By keeping the fly on the edge of the window, the trout stands a very good chance of engulfing the insect.

I’m not for one moment suggesting that trout do mathematical calculations to catch their prey. They don’t need to. Evolution has provided them with a neat solution. It is called the ‘gaze heuristic’ by scientists. By using this mechanism, predators maintain a constant angle of approach in order to lock on to, and then track, the movements of their prey.

Dr Robert Hamlin gives a detailed and fascinating insight into how animals use, and the military applications of, the gaze heuristic in the paper The Gaze Heuristic: Biography of an Adaptively Rational Decision Process (2017): “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.” Dr Hamlin gives details of how: “the discovery of the gaze heuristic was a major factor in the RAF’s victory in the Battle of Britain in 1940.” After the end of the war, “German [guided missile] technology was combined with the British heuristic to create the Sidewinder AIM-9 missile, the most successful autonomous weapon ever built. There are no plans to withdraw it or replace its guiding gaze heuristic.”


Marinaro’s photographs, calculations and description strongly suggest that trout use the gaze heuristic to catch flies. A trout keeps the fly at the edge of its window to ensure a constant angle of approach during the rise. Of course, maturation and practice may be necessary for trout to perfect their gaze heuristic.

How does a trout see a fly

Before selecting a fly suitable for Dartmoor trout, consider how the trout catches the fly.

The main message so far 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, the thorax and abdomen of Hans van Klinken’s fly the Klinkhåmer and Bob Wyatt’s Deer Hair Emerger hang below the surface of the water to imitate an emerging insect. The dubbed fur body on dry flies, and the prominent hackle on the Adams, produce indentations in the mirror that mimic those caused by the feet of an insect floating on the surface.

Wings on the artificial fly act as cues during a rise to maintain the trout’s movement towards the fly. Nowadays, deer hair and synthetic materials are used to construct most dry-fly wings. These offer a strong profile and also help the angler see his or her fly on the water. The presence of the merged image of the body and wings at the edge of the trout’s window allows the trout to locate the position of the fly precisely. A pronounced thorax on an artificial fly may enhance this visual cue.

To some extent a fly will be buffeted about in broken water. It is reasonable to suppose that a trout can move its body to reacquire the original angle of approach. But severe drag on an artificial fly will disrupt the spatial relationship between the insect and the edge of the trout’s window and confound its gaze heuristic.

How does a trout see a fly

The presence of the merged image of the body and wings of a fly on the edge of the trout’s window enable it to locate the insect precisely.

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 at first sees two images: the actual fly and its reflection in the mirror. Then 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 good chance of catching the insect.


There’s another reason why anglers fishing for wild brown trout on Dartmoor rivers should have confidence in their fly. Local expert and Wild Trout Trust founder Mike Weaver explains that the big, deep, glassy pools where the trout can see you can be a waste of time. Concentrate instead on the quick, broken water at the head of pools. This can be difficult. It is important to follow the position of the fly against the background of this popply water because Devon trout can spit out a fly in the blink of an eye. The chances are, if you have confidence in your fly you are also familiar with how it looks on the water and can track its progress using the gaze heuristic to spot the ‘take’. Remember that the gaze heuristic applies to anglers as well as their quarry. That’s why I tell anglers to: “Watch your fly like a hawk,” and recommend clients use their ‘confidence fly’. What flies give me confidence? I’m sure you can guess. I go for big wings and soggy bottoms.