Abundance and Diversity of Fishes Migrating to a Hydroelectric Dam in Montana

Abstract

Although many potamodromous fishes make extensive migrations, dams that block their upstream movements are not often mitigated with passage facilities. Over several months in 1998 and 1999 we studied the numbers of all fish species attempting to migrate beyond Milltown Dam, a hydroelectric dam on the Clark Fork River, Montana. Using a radial-gate raceway as a fish trap, we captured 13 fish species known to seasonally congregate below Milltown Dam. In 1998 we captured over 17,000 fish, representing 9 species; in 1999 we captured 12 species and more than twice the 1998 catch by sampling 6 weeks longer. Trapping in both years began on March 10 and was concluded November 17. In 1998 we found that spring migrations were related to spawning behavior for rainbow trout Oncorhynchus mykiss and largescale suckers Catostomus macrocheilus; and in 1999 we additionally noted the spawning-related migrations of westslope cutthroat trout O. clarki lewisi, longnose suckers C. catostomus, and northern pikeminnows Ptychocheilus oregonensis. In contrast, fall movements of northern pikeminnows were not spawning-related and were undertaken by immature individuals in both years. Migrations of largescale suckers in the fall were not related to spawning and are the first reported fall migration of these fish. Redside shiners Richardsonius balteatus, peamouths Mylocheilus caurinus, and longnose dace Rhinichthys cataractae were also captured. Each year some individual fish remained at the dam for extended periods; they were exposed to high water temperatures and often developed injuries trying to ascend the dam. Although the reason for movements of some species is speculative, the spatial and temporal impacts of Milltown Dam on individuals and populations probably have important community level effects.