10/25/12

Insects and other foods the trout should be eating:
Hatching:
1.    BWOs (Little and baetis BWOs)
2.    Little Yellow Quills
3.    Slate Drakes
4.    Needle Stoneflies

Most available/ Other types of food:
5.    Sculpin, Minnows (Streamers)
6.    Craneflies
7.    Beetles
8.    Grasshoppers
9.    Ants




Following Up On Yesterday's Ending Comments
Yesterday, at the end of the strategy article I pointed out some basic things about fly fishing the
small streams of Great Smoky Mountains National Park that I know for a fact, many anglers fail to
consider important. One was not approaching trout the best way, another not staying hidden from
the trout and others. Email from some anglers fairly new at fly fishing made me think that maybe it
is time to repeat some basic things you should keep in mind, especially when the water is low and
clear as it is now.

Staying Hidden From Trout:
If the trout see you, it will be next to impossible for you to catch them. In order to hide from them,
you need to know a little about how a trout sees the world outside the water and also how trout
position themselves in water when feeding.

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.

Trout's 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
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.

Surface Tension:
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.

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 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 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 does not 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 is not a real 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 does not 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.
Copyright 2012 James Marsh
This instructional DVD illustrates how trout
can see you and your fly better than anything
ever shown on video.  
Stalking Appalachian Trout