.......................  ......Fly Fishing Yellowstone National Park


Choosing the Right Fly - Part 7

Binocular and Peripheral Vision:
Just to add some more to yesterday's article, let me say that 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

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 look at 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.

Continued tomorrow

Copyright 2009 James Marsh