Changes in the Species Composition of the Fish Community in a Reach of the Kootenai River, Idaho, after Construction of Libby Dam

ABSTRACT

I evaluated fish community structure and the density and growth of mountain whitefish (Prosopium williamsoni) downstream of Libby Dam in a 1.0-km reach of the Kootenai River, Idaho, in 1994 and compared the results with those of a similar study in 1980, after closure of the dam. In 1980 seven species of fish were collected; mountain whitefish comprised 70% of the sample (42% by weight), and largescale sucker (Catostomus macrocheilus) represented 19% of the sample (49% by weight). In 1994 of the eight species caught, mountain whitefish represented only 40% by number (19% by weight), and the largescale sucker was 65% of the sample (70% by weight). Growth of mountain whitefish was also slower in the early 1990s compared to the later 1970s. Reduced productivity because of the nutrient sink effect of Lake Koocanusa, river regulation, the lack of flushing flows, power peaking, and changes in river temperature may have led to the changes in the fish community structure.