Name: Jeffrey M Ulmer
What signals some fish to travel up a waterfall?
Good question, Jeff!
Much is still not known about the spawning drive in fishes,
however, I'll tell you what I have read, and hope it helps!
At a certain age of maturity, a fish's instinct signals it to
spawn. Not much can interfere with this urge it seems, since
fish will swim, crawl and jump over all sorts of obstacles I
including boulders and even waterfalls, as you mentioned. Salmon
have been known to make jumps up to 12 feet in their persistent
drive to the same creeks in which they were spawned.
Tom F Ihde
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Update: June 2012