RELATIVE DENSITY AND DISTRIBUTION OF SMALLMOUTH BASS, CHANNEL CATFISH, AND WALLEYE IN THE LOWER COLUMBIA AND SNAKE RIVERS

We used two indices derived from catch and effort to compare density and relative abundance of smallmouth bass Micropterus dolomieu, channel catfish Ictalurus punctatus, and walleye Stizostedion vitreum vitreum among reaches (forebays, mid-reservoirs, tailraces) and reservoirs of the lower Columbia and Snake Rivers, and among unimpounded sampling zones in the Columbia River downstream from Bonneville Dam, Smallmouth bass density was highest in forebay and mid-reservoir reaches of Snake River reservoirs. Density was greatest in lower Snake River reservoirs, intermediate in Columbia River reservoirs, and lowest in the Columbia River downstream from Bonneville Dam. Smallmouth bass were most abundant in Lower Granite Reservoir. Channel catfish were distributed throughout all reaches within reservoirs. Density was far greater in the Snake River than in the Columbia River. Walleye density was low throughout the lower Columbia River, and walleye were absent in the Snake River upstream from Ice Harbor Dam. Distribution and relative abundance of introduced piscivorous fishes indicate that predator-prey relationships differ among reservoirs, between impounded and unimpounded reaches, and between the lower Snake and Columbia rivers. Differences in the relative abundance of introduced predators should contribute to variation in losses of juvenile salmonids to resident predators.