fly, or ignoring it. The same basic knowledge is necessary in order to understand why generic, attractor and
impressionistic imitations work at certain times and places and don’t work at other times and places. The same is true
of when, where and why specific imitations are more productive than generic or attractor flies. You cannot possibly
understand it without knowing at least some of the basics of the trout’s senses and behavior. Keep in mind that this is
about specific imitations (flies that imitate a specific insect at a specific stage of life) versus generic imitations (attractor
or impressionistic flies that imitate a variety of insects or other trout food). It is not about one fly pattern versus another
Trout learn from birth to accept and reject various objects in the streams as food. They never eat something they don’t
take for food. Even though they have very tiny brains and even by the stretch of one’s imagination are not smart, they
can still learn by experience. They do not depend on intelligence as much as they rely on senses and instincts to eat.
Trout have the ability to hold in slow to moderate current without swimming or expending a great amount of energy.
They manage to do this a lot like birds do when they are gliding in the wind without moving their wings. When they are
feeding, trout find the areas where the current concentrates the insects. Anglers call these places drift lines. Trout will
position themselves somewhere along these drift lines. There they will hold their position and stay focused on what is
referred to as their “window of vision”. They must seek an area of the stream to hold in where the current is not strong.
If they did hold in strong current very long, they would expend more energy than they could acquire from the food they
could take in. Most often, they accomplish this by seeking a depth where the current is slowed down by upstream
obstructions, usually rocks and boulders. Sometimes, they are able to position themselves on the slow side of the drift
line. The current in the drift line above or to the immediate side of the trout may be moving along rather fast, but the
trout is usually positioned in slow to moderately moving water where they can view objects that come into their window
of vision. If that food is in the form of nymphs, larvae or pupae drifting underwater, then the distance at which the trout
can view it depends on several factors. The underwater background, amount of available light, clarity and speed
of the water are just a few of them.
Normally, in very clear water with good light, they are able to detect the movement of objects that are within several feet
of them. Objects on the surface are viewed entirely different.
Window of Vision:
You may hear anglers say “the trout were not looking up today”. I assume they mean that as a figure of speech
because trout are always looking up. Unlike humans, they see almost all the way around themselves. Also, unlike
humans, trout can focus at extremely close ranges. They can focus on a fly that is only an inch or less from their eyes.
However, at long ranges they cannot focus well enough to discern the details of objects in the water. Without going into
unnecessary detail regarding the physics of light, lets look at some facts that affect the trout’s vision of your fly.
The “window of vision” as it is called, is the area of water on the surface above a trout where they can clearly see
objects. Trout can see objects on the surface that are directly above them. If the surface of the water is smooth or not
rough, they can see objects directly above them that are above the water. However, there's a point above them at
which their line of sight will not pass through the surface of the water. It is exactly 48.5 degrees from a point at which a
vertical line extends from a trout’s eyes to the surface of the water. This means that they can see through the surface
of the water in an area formed by a 97 degrees cone. This cone looks like an upside down snow cone cup with the
point of the snow cone extending from the trout’s eyes up to the surface of the water. Using this analogy, if the circle of
the cone (or top of the upside down snow cone) was even with the surface of the water, it would be referred to as the
window of vision. The trout sees everything that is outside of that cone as a mirror image of the underwater
surroundings. The deeper the trout, the larger the window of vision is at the surface of the water. If the trout is only a
couple of inches deep, the window of vision is just over four and one-half inches in diameter. If the trout is two feet
deep, then the diameter of the window of vision is just over four and one-half feet in diameter. In other words, the trout
can see objects at the surface of the water just over two and one-half feet in front of, two and one-half feet behind, and
two and one-half feet on either side of their position. A fly on the surface of the water passing over the trout can only
be seen by the trout for a total distance of four and one-half feet, or the diameter of its window of vision. This window of
vision is caused by light refraction.
Stick the tip of you fly rod down into the water at an angle. Notice the rod appears to bend at the point it penetrates the
surface of the water. This optical effect is caused by the change in speed of light as it goes from one transparent
medium to another or air to water.
Sometimes trout will hold just a few inches under the surface where they can closely inspect their food and at the same
time, expend only a small amount of energy eating. When they are only a few inches deep, the depth of focus only
allows them to see objects that are within a few inches of them. In other words, when they are holding this shallow, their
feeding lane is only a few inches wide. If a dry fly passes by several inches away, to their left or right, they may not even
see it. On the other hand, if the trout is three or four feet deep, the depth of focus is much greater and it has a much
larger feeding lane. Although trout can focus in almost every direction at once, they cannot focus on an object that is
three feet from them the same way they can one that is inches away.
When objects on the surface or beyond first appear in the window of vision, or come in view on the outermost edge of
the circular window, they appear much shorter and wider than they actually are. The more they approach the center of
the window of vision, the more they appear like they should. Objects directly overhead appear exactly as they should.
That means that the appearance of your fly is changing as it comes into the window of vision from being short and wide
to actually looking like the real thing. Now don’t misunderstand this to mean that since the trout sees a distorted view
of your fly when it enter the window, that its appearance of your fly is not important because they see the real insects
on the surface in the exact same manner. They too, appear short and wide near the perimeter of the circle. So, it is
still a fact that the more your fly looks like the real thing, the more the trout are likely to accept it for the real thing.
The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. A
surface "film" is formed which makes it more difficult to move an object through the surface than to move it when it is
completely submersed. If an insect is perched on the surface film its six legs and/or other portions of its body may
protrude through the film. The parts of the insect or fly that extends below the surface can be seen by the trout
even when it is outside its window of vision. Seeing the legs of an insect or other parts of its body may alert the trout
that something is coming into its window of vision. A midge may make such a sight indentation in the surface film
that would be almost impossible to see outside the window simply because the parts penetrating the film are so tiny. A
grasshopper’s legs and maybe even part of its body would be visible outside of the window from much farther away. I
could go on and on explaining light refraction, Snell’s Law and just how it affects the trout’s vision of the fly but I would
be getting away from some of the main points I want to make.
When a trout sees an insect on the surface that has drifted into its window of vision, it determines whether or not to
take the insect. If the trout attempts to take the insect, it moves its fins in such a way that allows the current to assist it
in propelling its upward motion. It takes the insect in its mouth and then moves its fins in such a manner as to propel
back down into its holding position.
Binocular and Peripheral Vision:
When us humans look ahead, our field of vision allows us to see thing that are within a 176 degrees area called the
“field of vision”. Our forward zone of binocular vision is 90 ninety degrees or forty-five degrees on either side of a line
straight ahead. The portion of our vision that is outside of that 90 degrees zone of binocular vision represents the area
of our peripheral vision. Our peripheral vision represents a total of 86 degrees, or 43 degrees on each side of our
binocular vision. To illustrate this, place your finger about two feet directly in front of your face and focus on it. Now,
continue to look directly forward and move your finger to your left until you cannot clearly see it in focus. You should be
able to see it clearly until it is 45 degrees left or right of straight ahead. The area in which you are able to see it clearly
is the area of your binocular vision. If you continue to move it left up to 15 more degrees, you should still be able to see
the finger, but your cannot see it clearly, or in focus. This area represents the area of your peripheral vision. Of
course, things work the same if you move your finger to the right.
Trout have a much narrower width of binocular vision than humans. The trout’s binocular vision allows them to only
focus on things that are within a total of 30 (thirty) degrees directly ahead, or fifteen degrees on either side of a line
directly forward of their eyes. However, they have a much larger field of vision than us humans. It is a total of 330
degrees or represents an area almost completely around them. Of that 330 degrees field of vision, their zone of
peripheral vision represents 300 degrees of it, or 150 degrees on either side of their narrow 30 degrees binocular
zone. When they detect something with their peripheral vision, they must move their eyes towards the object in order to
focus on it. There is only an area of 30 degrees directly behind a trout that is not visible to them. This narrow area is
commonly referred to as their blind zone. The bottom line to this is that although trout can detect movement and
contrast almost all the way around themselves they must look almost directly at an object, or align the object in their
narrow 30 degrees field of binocular vision, in order to clearly see it. Binocular vision is necessary for the trout to see
things in detail. It is necessary for a trout to feed. Peripheral vision is great for detecting movement and contrast, but
things within the trout’s peripheral vision cannot be seen in detail.
The amount of available light also has a huge effect on how a trout sees your fly. Their iris is not adjustable. It is fixed
and cannot be enlarged or reduced. This means that they cannot control the amount of light that enters their eyes
with the iris. Rods and cones allow them to adjust to various light intensities. Trout can detect color and very fine detail
but bright sunlight can eliminate the color that enters their eyes. By the same token, under low light conditions such
as when it is early in the morning, late in the day or at times when the sky is dark, they cannot see the colors of the fly
and well as they can in a well lit situation. Light does not penetrate very deep in water, and the depth of your fly
also affects how the trout sees the color of it. If the trout is deep in the water, flies that are floating on the surface will
not be viewed in the their true colors. The trout must get closer to the fly in order to see it in true color. The bottom
line to this is that under many different lighting conditions, they cannot see the fly very well at all. But the amount of
light is not the only factor in how well a trout sees your fly. There is yet another, far more important factor, in how well
the trout sees an insect or fly. It is the speed of the water and the insects or flies that are drifting in it, or floating on it.
Speed of the Water:
Now lets this important factor is how well a trout is able to see an insect or your fly – the speed of the water. In fast
moving water with a broken surface, the trout must make a very quick decision as to whether to take or reject a fly.
The speed of the water does not only apply to flies drifting on the surface of the water. The same thing applies to a
nymph or larva moving through the water. In fast moving water, the trout cannot take their time in deciding whether or
not to take the fly.
The speed of the water is the number one reason trout can be fooled by generic, impressionistic, or attractor type flies.
In fact, if the water in the current seam is moving fast enough and the trout are holding fairly close to it, they can often
be fooled by a fly that does not resemble much of anything that they have ever seen before. Due to the factors I have
mentioned above, including the fast speed of the fly, they don’t have much opportunity to closely examine anything.
In smooth, slick water where the current is moving at a slow rate, the trout have plenty of time to make a very close
inspection of your fly. For years I have said that you want a fish to see any artificial bait or lure just well enough to think
it's a real creature, but not well enough to determine that it isn't a real live creature. In other words, you want them to
be able to just barely see it – just enough for them to think it is the real thing. The same thing is true of flies. It makes
no difference whether it is a twelve, inch, long marlin lure, jumping in and out of a wave in offshore blue water; a
crankbait passing by a largemouth bass in dingy water, or a nymph passing by a trout in clear water. You want the fish
to see the artificial imitation only well enough to fool it into thinking it is the real thing. The more the lure or fly looks and
acts like the real thing, the longer you can allow the fish to examine it, and the slower the fly can pass by the fish. The
more it looks and acts like the real thing, the higher the odds are that the fish will eat it. Notice I said “acts” like because
that is even more important than its resembling the appearance of whatever you are trying to imitate. A solid brass
nymph cast to perfection that is exactly like the real thing won’t fool a trout very well. It’s abdomen, gills, legs and other
body parts will not move and resemble the movement of the various part of the real nymph.
When you are fishing for trout with flies, the faster the water is moving, the easier it is to fool them. When trout can only
get a quick glimpse of the fly, they are much easier to fool than they are when they have a lot of time to closely
examine the fly. This is especially true when it passes by at a close range where a trout could otherwise get a good lok
at the fly.
Fast Pocket Water Streams:
Now so far, when you are fishing the fast water freestone streams, everything sounds great and in favor of using
generic, impressionistic and attractor flies. I have just explained in detail, the main reasons trout can be fooled into
taking attractor flies for real insects. The typical fast water angler makes all of their presentations in the fast water of
runs and riffles. The biggest decision made is whether to fish a dry fly on the surface, or a nymph below the surface of
the run or riffle. If they don’t get them on one or the other type of attractor or generic flies in the fast runs and
riffles, they excuse their performance by declaring that fishing is poor. Poor success usually occurs when the trout are
not feeding in the fast water of the runs and riffles.
Where Trout Feed When There Is No Hatch to Match:
First of all, and most importantly, trout do not always feed in the faster moving water. They often feed near the bottom
inslow to moderately moving water. This is often below faster water in the upper water column and on the surface of the
stream. They sometimes feed in pools. When the water is fairly cold, they almost always feed in very slow moving water.
Since most of the faster moving water in freestone streams is near the surface, in cold water trout usually choose to
feed in the lower or bottom sections of the stream where the water is moving much slower. Many anglers think they
position themselves on or near the bottom to get warmer. Trout are cold blooded and do not seek warmer water for
comfort. They seek the slower moving water to prevent spending more energy than they can take in from the available
food. It is also a fact that the water temperature changes little, if any, with the depth of stream.
Larger brown trout do not feed on crawfish and sculphin in fast water. They eat them in slow to moderately moving
water on the bottom, either under low light conditions, or from an ambush position. Terrestrial insects are eaten
wherever they happen to fall in the water. More often than not, this is near the banks and often in slow moving water.
Where Trout Feed During Hatches?
Where do the trout feed during aquatic insect hatches? For example, where do midges hatch and the trout feed on
them? Answer: In the slow moving water. Even the clinger nymphs that live in the fast water runs and riffles, move to
the slow moving water of calmer pockets and shallow water that is near banks and behind boulders to hatch. Just about
all the crawlers, burrowers, and swimmers move to slow to moderately moving water to hatch.
Where do mayfly spinners fall? Answer: Usually in the riffles and runs where they hatched, but they are eaten by trout
in eddies, calm pockets, and the slow water at the heads of pools where they collect.
Where do the stoneflies hatch? Answer: They move out of their fast water habitat into slower, shallower water to crawl
out of the water and hatch on the banks and rocks.
Where do the caddisflies hatch? Answer: They just about always move to the slow to moderately flowing water for
their pupae to emerge.
In other words, about everything that hatches does so in moderate to calm water. Often, this slower moving water is
very close to fast water. Often, the calmer water is in pockets distributed throughout the stream within the fast water
of the stream. Sometimes, it is the ends of long runs where the water slows down. If the newly hatched insect stays on
the water and does not fly away quickly, it will most likely be caught up in the fast currents. It depends on the species
Some hatching insects never get caught in the fast water. Examples are stonefly nymphs that all crawl out of the water
in calmer areas of the stream. Slate Drake and Gray Drake mayflies that crawl out of the stream in calm water to hatch.
Most of the many species of Blue-winged Olives, Mahogany Duns, Blue Quills, Pale Morning Duns, Pale Evening Duns,
Small Western Green Drakes, Western Green Drakes, Sulphurs, and other mayflies are able to depart the water from
calmer sections of water, or moderately moving water, before getting caught up in fast water currents.
Some mayflies usually do get caught up in fast water before they are able to depart the water. Quill Gordons, American
March Browns, Yellow Quills, Light Cahills, Wester Ginger Quills, and Dark Red Quills hatch in calm pockets within the
fast water areas of the stream, but often get caught in the fast currents prior to departing the water. However, the facts
are the majority of hatching aquatic insects and egg laying adults, do not usually get caught up in the fast currents.
When trout feed in the slow to moderately flowing sections of the streams; or eddies, pools, the ends of runs and riffles,
and calm pockets that are within the fast flowing freestone streams, they can examine the fly much closer. Given that
opportunity, if the fly is not very imitative of the natural insect, and if it is not presented in such a manner as to behave
like the natural insect, the trout will usually reject the fly.
There is more to the problem. It is not just a matter of how well the imitation looks like the real thing. The way in which
the fly is presented may be an even bigger problem. You have probably heard, over and over, that the presentation of
the fly is more important than the fly itself. That is a very correct statement, but it does not mean that the fly is not
important. It just means that a perfect imitation is not effective unless it is presented to the trout in the same manner
they view the real thing. The fly must drift and act like the real thing, without the trout being able to become alerted, or
alarmed by a tippet, fly line or leader that is attached to it. Again, that is fairly easy to accomplish in fast moving water
but as just mentioned, that is usually not where the trout are feeding. The presentation, and the appearance of the fly,
become even more critical in slow or moderately moving water. When anglers concentrate only on the fast water of
riffles and runs, they are making a big mistake.
The Big Misconception:
When there is no hatch occurring, anglers tend to think they are better off using an attractor or generic imitation that
imitates a variety of things. A fly that imitates no specific insect but rather a variety of them works best when
environmental conditions (water temp, oxygen, stream levels, etc) are near perfect and lots of insects are readily
available for the trout to eat. In other words when anyone that can cast a Royal Humphy twenty feet upstream in a run
can catch trout. When you can hit them over the heads with the line and they still will eat the fly, a Royal Wulff works
great. When there is little challenge in catching trout most anything made of feathers and hair with a hook in it will
produce some good results.
This big misconception came about because book after book about trout fishing lumped things into one of only two
categories - selective feeding or opportunistic feeding. Most anglers think that trout are feeding either one way or the
other. It is true that if trout are not feeding exclusively on one insect, they are categorized as feeding opportunistically,
so by strict definition, I suppose they are. That is fine as far as categorizing them is concerned, but it has little to do
with what is really going on, and it is of little information, or use in catching trout.
For example, lets suppose that there are lots of Little Yellow Stonefly nymphs crawling to the banks to hatch. Don’t
think for a second the trout don’t know it. They view their underwater world 24 hours a day, and they know and see
exactly what is going on. Since these nymphs are crawling across the bottom to get to the banks, they are easy prey
for the trout. Naturally, the trout will focus on feeding on the easy prey.
If the trout are eating these nymphs migrating to the banks and a stray mayfly nymph happens to come along, the trout
may, or may not eat it. Most likely, if the trout does not have to go out of its way to do so, it may very well feed on the
mayfly nymph. If it takes more effort than it does to catch another stonefly nymph crawling to the bank, it most likely
won’t eat it. Now, lets suppose a trout did eat the mayfly nymph. By definition, you would have to categorize the trout as
feeding opportunistically. That is why marine fishery biologist classifies all trout as opportunistically feeders. It makes
sense from a scientific standpoint, but little sense from a practical standpoint of catching fish.
A particular trout may be feeding selectively at the same time one a few feet away may be feeding opportunistically.
One run or riffle may have several trout that are feeding selectively at the same moment another run or riffle, a few
yards upstream, may not. Call this whatever you prefer to call it, but under these conditions, would you rather be
fishing an imitation of a stonefly nymph or a mayfly nymph? I think most anglers would agree that your odds of success
would be greater if you were fishing a specific imitation of a Little Yellow Stonefly nymph.
The Bottom Line:
You are always better off using specific imitations ( a fly that imitates a specific insect at a specific stage of life) to
imitate the behavior of the insects or other food the trout are most likely eating at the particular time and place. Unless
a substantial hatch is underway, this would most likely be the food that is most plentiful and easiest for the trout to
1. The trout’s “window of vision” on the surface of the water is relatively small. Insects and flies pass through it
quickly, especially in fast water.
2. While they can see movement and contrast most all the way around themselves, they only see objects clearly
when they directly face the object to align it in their narrow area of binocular vision.
3. Attractor or generic flies usually work okay in fast water where the trout have little time to examine the fly, but
trout don’t always feed in fast water. Day in and day out, most of the time, they feed in slow to moderate water.
4. Attractor or generic flies work best when environmental conditions (water temperature, oxygen content, water
levels, etc.) are prime and multiple hatches are underway. Simply put, when hungry, aggressive trout have a lot of food
to choose from. When it is easy to catch them.
5. Trout do not have to be feeding “selectively” to be focusing on, or keying in on, a particular insect. Most of the
time they are feeding, they are focusing on one, or no more than a very few insects. Call it opportunistic feeding if you
like. Regardless of how you label it, they are going to feed mostly on the insect or insects that are most abundant and
easiest to acquire at the particular time and place.
6. You are always better off using specific imitations to imitate the behavior of the insects or other food the trout
are most likely eating at the particular time and place. To repeat this important point again, unless a substantial hatch
is underway, this would most likely be the food that is most plentiful, and easiest for the trout to acquire.
How Trout See Flies
n order to understand why trout will take a particular fly at certain times and places, and not take it at other times
and places, you first must understand the basics of how trout sees a fly, and what triggers the trout into taking the
October 2017 Issue
Copyright 2017 James Marsh
by James Marsh