Downstream movement of rainbow trout past Kootenai Falls, Montana

This report addresses the question Why do rainbow trout move downstream over Kootenai Falls, allegedly impassable to upstream migrants/ In 1982, we compared tissues of rainbow trout (Salmo gairdneri) populations from above and below the falls electrophoretically, tagged 2,000 trout in a 7-km reach above the falls (of which 163 were subsequently recovered), and reviewed the history of trout stocks in this area. Tag recoveries (normalized for angler effort and catch) indicated upstream, no, and over-the-falls movement percentages of 7, 71, and 22, respectively; unadjusted recovery percentages were 6, 87, 7, respectively. Tagging results from earlier work (1978-1981) had yielded similar results. The data on movement did not include small fish. Isozymes indicated no similarity of rainbow trout from Kootenay Lake and populations near the falls, but rainbow trout above and below the falls were similar. We cannot prove that fish do not move upstream over the falls, but we visually assessed the falls as impassable. Rainbow trout populations largely replaced cutthroat trout (Salmo clarki lewisi) from the 1940s to 1950s. Regulated flows from Libby Dam led to rapid expansion in rainbow trout numbers (ninefold) and biomass (threefold) from 1972 to 1982 in the dam-to-falls area. A colonizing stock, combined with interactive pressures, probably accounts for the movement downstream over the falls