floating on water surface

Dennis Paulson dpaulson@ups.edu
Wed, 19 Jan 2000 17:39:58 -0800

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John Acorn wrote:

"Someone on the list speculated that trout are more of a surface feeding
group of fishes, but trout biologists have shown that actually a small
proportion of their diet comes from the surface."

As that someone was me, I was a little resistant to having a long-held
speculation (actually, it was a belief) so thoroughly destroyed, so I went
to the only book I have that gives some details on freshwater fishes of
this area, Inland Fishes of Washington, by Wydoski & Whitney, U. of
Washington Press, 1979.  This book generally confirms John's statement
about salmonids feeding on aquatic (e.g., underwater) insects.  Rainbow
trout, one of the most ubiquitous salmonids of the region, "feed primarily
on foods that are associated with the bottom."  However, there is obviously
much variation, and some references clearly involve surface feeding.  I
continue the discussion only because I think it does have relevance to
odonate biology.

Under brook trout, "During summer, terrestrial insects such as
grasshoppers, dipterans, beetles, wasps, and bees become principal foods."

Under chinook salmon, "In fresh water the summer food is composed primarily
of aquatic insect larvae and terrestrial insects."

Nothing is said about terrestrial insects for any of the other Washington
salmonids, but adult aquatic insects are listed for some of them,
presumably taken from the surface.  I didn't check the original references
for these summary statements.

I'll have to read Mark McPeek's statements again, as I thought he said that
trout were ineffective predators on larval Enallagma (relative to
centrarchids) because they fed more at the surface, but perhaps I added
that last phrase!   But if I added it, it's because of my many memories of
sitting by mountain lakes in the late afternoon and watching trout popping
surface insects in front of me.

Thanks for the good info on flyfishing, John.  This statement is puzzling,
though:  "Second, an angler tries very hard to get the fly to drift without
"drag"-- in other words, passively with the current.  The more motionless
the "dimples," so to speak, the better the chances are that the trout will
strike the fly."  I'm not questioning this, but it seems odd, as you'd
think that movement against or across the current would be a sure
indication of the presence of a live critter, while passive drifting could
be any number of nonarthropod bits (a huge fall of seeds, for example).
But I know fish are great at sucking in particles and spitting them out if
they don't taste right, perhaps an adaptation for feeding on "dimples."

Dennis Paulson, Director                           phone 253-756-3798
Slater Museum of Natural History                 fax 253-756-3352
University of Puget Sound                       e-mail dpaulson@ups.edu
Tacoma, WA 98416

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