Peter Charles wrote:

How do you go about knowing "what the fish will take at any given
time"?


That is THE question.

The natural inclination is to simply use a fly that looks most like the
natural to OUR eyes. A trout eyes are mechanically different from ours
and perceive light in different ways from ours - they can see further
into the ultra violet spectrum and infrared scale than ours and (as
Scott showed me) they can also utilize polarized light. In addition,
like Mike said, how something is perceived also relies on brain
processing and I'm pretty confident that a trout's brain is going to
interpret things differently than ours' does. I do think we judge size
in a similar manner as a trout but beyond that ?????? , I think it's a
crap shoot.

So where does that leave us? I don't think we can or should reject
what our vision tells us, but we also shouldn't rely on it too much.
What I think is FAR more important than how well a fly we're using
matches a natural to our eyes, is presentation. Presentation, as I look
at it, goes farther than how you as an angler "present" the fly.
(Although that's VERY important) It also involves fly selection.
Different flies present themselves to the fish in different ways. With
dries, there are a wide range of characteristics - hackle length and
density, no hackle ties, body material and weight, tail material and
length etc etc that will determine how a fly will sit on/in the surface
and how it presents itself to the fish. With "wets" - weight,
absorption, texture, flexibility, water resistance, etc etc are going to
effect how a fly will act in the water. IMO, this "attitude" that the
fly takes when it is fished, is much more important than using a fly
that's a precise color or profile match to our eyes.

Willi