Jonathan Cook wrote:

Willi wrote in message ...

Higher water temps are generally the limiting factor in where trout can
survive. As a river flows downstream, average temps generally rise until
they reach the point where the trout can't survive.


Willi, have you ever explored downstream to see if trout move down at
different seasons? I could imagine that trout move down in winter (if
there's enough water) and maybe back again when it gets too warm. You've
talked here before about daily migrations of a hundred yards, but are
there seasonal migrations on the order of miles?

I've never read anything about that except for some reports from the
East where the trout will migrate in the Summer to spring heads to get
relief from the rising temps. I don't know how far they go.

From what I've read, in some Yellowstone rivers, the trout migrate in
and out of certain areas.

Locally what I've observed is that they "hole up" during either high or
low temps. During these low flow periods, I can clearly see the bottom
of all but the very deepest pools and not see a fish. An influx of water
or a moderation in temps will bring them out.



I could also imagine that in high-runoff years, quite a few trout may
find themselves (either "purposefully" in their pea-brains or accidentally)
quite a ways downstream, and not really make an effort to move back up
as the water warms. This could be an opportunity to harvest some decent
trout that would otherwise perish anyways. Maybe in the few weeks after
runoff and before the water really starts warming up.


I think that since they evolved with runoff, they've adapted well to it.
I'm sure there's some fish that end up like you describe. One of the
things they do locally is run up irrigation canals during runoff, but
when the water recedes some of them will get stuck. A few of them find
spots that consistently hold enough water so they can live there. In the
irrigation canal that borders my property, there's a big brown that
lives under the bridge that's been there several years, even though
flows in the Summer can be almost like bathwater.



I guess it should be noted that you live near streams that make a
fairly abrupt change from good trout water to uninhabitable, with
a fairly short amount of "marginal" habitat. In other areas of the
country, this situation may be uncommon (I'm guessing).


I think many streams and rivers share this at some point. They flow from
elevation which keeps them cool enough to support trout, but as they
descend they reach an area where the water becomes too warm to support
them.

Willi