Choosing the Right Fly: (Part Two)


In 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 the trout sees a fly and what triggers the trout into taking the 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 fly pattern.

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.

Feeding Zones:
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 didn’t they would expend more
energy than they could take in. Most often, they accomplish this by seeking a
depth where the current is slowed down by obstructions upstream, 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 is 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
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
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.

Surface Film:
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
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.

Light Effect:
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 doesn’t 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 insects or flies that
are drifting in it or floating on it.  

Speed of the Water:
Now lets discuss another, huge 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 doesn’t just 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 doesn’t resemble much of anything they
have ever seen before. Due to the factors I have mentioned above and the fast
speed of the fly they don’t have much opportunity to examine anything.
In smooth, slick water where the current is moving at a slow rate, the trout has
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 is
a real creature but not well enough to determine that it isn’t. 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 doesn’t matter if it is a twelve-inch
long marlin lure, jumping in and out of a wave in offshore blue water; a crankbait
passing by a bass in dingy water, or a fly 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. In other words, the slower it can pass
by the fish. Notice I said “acts” like because that is even more important than
“looking like” whatever you are trying to imitate. A solid brass nymph cast to
perfection 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 act like a 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 close ranges where
they can really focus on the details of it.

Smoky Mountain Streams:
Now so far, when you are fishing the fast water freestone streams of the
Smokies, 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 Smoky Mountain 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.
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 in slow to moderately moving water.
They sometimes feed in pools. When the water is fairly cold, they almost always
feed in very slow moving water.
In cold water, since most of the faster moving water in the freestone streams of
the Smokies is near the surface, they choose to locate in the lower or bottom
sections of the stream where the water is moving much slower to feed. 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 food in the cold water. Besides, the water temperature changes little if any
with the depth of the streams.
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 such as eddies, calm pockets and the slow
moving water of pools.  
Where do most mayflies hatch?
Answer: Even the clingers 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 and swimmers move to slow
to moderately moving water to hatch.
Where do the 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
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 doesn’t fly away quickly it will
most likely be caught up in the fast currents. It depends on the species hatching.
Some never get caught in the fast water. Examples are stonefly nymphs that all
crawl out of the water in calmer areas. Slate Drake mayflies that crawl out of the
stream in calm water to hatch. Blue Quill and Mahogany Dun mayflies almost
always hatch in calm pockets along the banks and usually never get caught up
in fast currents before they fly away. Many species of Blue-winged Olives, Little
Blue-winged Olives, Small Eastern Blue-winged Olives and Eastern Blue-winged
Olives are able to depart the water from calmer sections or moderately moving
water before getting caught up in fast water.  
Some mayflies do usually get caught up in fast water before they are able to
depart the water. Quill Gordons and March Browns 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 that most hatching aquatic
insects and egg layers 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
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.
As I just touched on, the problem isn’t just a matter of how well the imitation looks
like the real thing. The way in which it 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 doesn’
t 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 attached to it. Again,
that is fairly easy to accomplish in fast moving water but again, that is not always
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 (which is 98% of the time) 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 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
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. 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.
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 moment when another run or riffle a few yards
upstream may not.
Call it whatever you prefer to call it.
Under these conditions, would you
rather be fishing an imitation of a stonefly nymph or a mayfly nymph?
think most anglers would agree that your odds would be greater if you were
fishing an 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

Example One:
Lets suppose it is the first day of March and nothing is hatching. Everyone
knows that the Quill Gordons and Blue Quills will start to hatch any day.
However, this particular day the water will only reach a high of about 48 degrees
F. In other words, it is still a little too cold and maybe a little too early for the
hatches to start. Under these conditions, the Quill Gordon clinger nymphs have
already begin moving out of their fast water habitat of the runs and riffles into
nearby slower moving water, mainly pockets behind large rocks and boulders.
The little Blue Quill crawler nymphs have began to move from their moderate
water habitat to slow moving water to hatch, usually along the edges of the
stream in calm pockets of water. There are no mayflies hatching on the surface.
The streams are full of nymphs and larvae because most everything is yet to
hatch. That is most everything except the Little Black Caddisflies. They have
formed their pupae and are hatching during the warmest part of the day, usually
mid afternoon. Shortly thereafter the females will begin depositing their eggs
over the same water they are still hatching from. Most anglers are either not
fishing, fishing a generic nymph in the fast runs and riffles or fishing a dry fly
Quill Gordon or Blue Quill imitation. Most are catching few trout and most are
complaining that the water is still to cold for “good” fishing.
If you were fishing this hypothetical day during the morning and up to the
warmest part of the day, a good choice would be to fish an imitation of the Quill
Gordon clinger nymph in the pockets behind boulders and large rocks. A flat
nymph, hock size 14 heavily weighted would be a good choice. You would make
short up and across presentations on the swing or “high stickin” presentations.  
Another option would be to fish an imitation of the small Blue Quill nymphs along
the edges of the stream in slow to moderate water. A good choice would be a
slim, narrow clinger nymph imitation, hook size 18. You would want a long leader
and light 6X tippet. You would make longer than normal upstream cast into
shallow water along the banks. You would make very careful approaches to
keep from spooking the trout that may be holding in the nearby the shallow
Later in the mid-afternoon, you would want to switch to an imitation of the Little
Black Caddisfly pupae in the moderately flowing sections of the stream such as
the ends of the long runs and tailouts of the pools. This would be a hook size 16
imitation of the Little Black Caddisfly pupa. You would present it up and across.
Later in the afternoon, when you started seeing the caddis deposit their eggs
you may want to switch to a dry fly imitation of the adult Little Black Caddis. A
black Elk Hair Caddis, hook size 16 or 18, should work fine.
Using the strategy I just outlined and
fishing imitations of the specific
aquatic insects
I just mentioned would greatly increase you odds of catching
trout this hypothetical March first day.
In fact, this past early March (I am not certain of the exact day but under the
same circumstances)
Angie and I caught over twenty trout in Little River in
less than three hours of fishing when everyone else we talked to
caught few or none.
The “fishing reports” all labeled things as “fair” with an
“any day now” tag to it.
All of the fish were taken within the first half-mile of water below the turn from the
main road to the Elkmont Campground. We did exactly what I described above.
We started fishing about 2:00 o’clock in the afternoon and fished our own
“Perfect Fly” imitation of a Quill Gordon nymph. When we didn’t catch any fish,
we changed to a Blue Quill nymph imitation (a Shane Stalcup swimming nymph
pattern) and caught a couple of nice rainbows in very shallow water along the
bank. When we noticed the Little Black Caddisflies were hatching (about 3:00 o’
clock) we changed to a caddisfly pupa imitation (Orvis model if I remember
correctly) and caught most of the fish we caught during the next hour. About 4;
00 o’clock we changed to a plain black Elk Hair Caddis. During the next hour, we
caught a few more trout. We stopped fishing about 5:00 o’clock, which is fairly
early in the afternoon when the egg laying process was still underway.  
That is not the only time we have been able to accomplish this early in the early
season leading into the Quill Gordon hatch. We have done that several times
during the last three years using the same basic strategy. In most cases, we are
able to catch trout on the Quill Gordon nymph just prior to the hatch. The Blue
Quill nymph works great but is difficult to fish in shallow water edges of the
stream. You will spook a lot of fish trying to make a good presentation along the
banks under the trees.  
Again, the point is unless conditions are prime and multiple hatches are
occurring, you are always better off using specific imitations of the most
prevalent insects than you are using a generic or attractor flies. Attractor and
generic imitations usually work fine as long as conditions are prime and there
are multiple choices of food available for the trout to eat.  

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
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.
Unless a substantial hatch is underway (to repeat the most
important point again)
this would most likely be the food that is most
plentiful and easiest for the trout to acquire.

Coming Up:
In the forthcoming articles (as I have time to write them) I will give several other
examples of how specific imitations of the most prevalent insects outperform the
use of attractor or generic flies. I will use hypothetical scenarios and actual
examples of success we have experienced during those times generic fly
patterns failed other anglers fishing the Great Smoky Mountains National Park.

Copyright 2008 James Marsh