If they are anadromous ("Migrating from the sea to fresh water to
spawn" WordWeb) I've read that there is a combination of salinity (or
lack of in degrees) and the perceived strength of the current (also in
degrees) that the fish respond to. This is important in the arguments
that are on in "American Antiquity" and elsewhere about "eco-niches"
along the Pacific Northwest coast, permitting human travel along the
coast at the edge of glaciation, versus the inter-montane migration
hypothesis posited for the various settlements of the Western
Hemisphere from Northern Asia. ONe might argue the glacial outwash
current was too strong to produce short run migrtations, which may be
true but varied.

George Myers