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


Choosing the Right Fly - Part 5

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 more
important points.

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. They
remind me of birds drifting in the air currents high in the sky. They can change
directions and positions with little apparent effort.

I wonder just exactly what it is that triggers the trout into taking the fly or
rejecting the fly. If it is feeding on a certain insect and they are drifting by the
trout regularly, then you would think the trout is making a comparison of your fly
to the real bug. If the trout isn't feeding on any particular insect, they you would
think it is just determining whether or not the fly is something worth eating. The
truth is, we actually don't know. We only guess and assume.

I do know that when you watch a trout feeding on the surface for some time, and
you are not fishing - just watching, you will usually see it reject and select
objects from the surface. I always wonder if the object accepted are real and the
ones rejected are non-food objects such as bits of wood or sticks. We have set
up and video taped several fish with our flies passing by, catching the trout's
reaction to the fly. We have captured several taking the fly and of course, many
that rejected the fly. The only thing we haven't done is figure everything out.

Continued tomorrow

Copyright 2009 James Marsh