FEEDING RESPONSE BY NORTHERN SQUAWFISH TO A HATCHERY RELEASE OF JUVENILE SALMONIDS IN THE CLEARWATER-RIVER, IDAHO

We collected gut contents from northern squawfish Ptychocheilus oregon ensis captured in the Clearwater River, Idaho, 0-6 km from its conflue nce with the Snake River, following the release of 1.1 million yearlin g chinook salmon Oncorhynchus tshawytscha from the Dworshak National F ish Hatchery. Before the hatchery release, northern squawfish gut cont ents (by weight) in the study area were 38% crayfish Pacifastacus spp. , 26% insects, 19% nonsalmonid fish, and 16% wheat kernels Triticum sp p. Juvenile salmonids constituted 54% of gut contents about 24 h after the hatchery release, 78% after 5 d, and 86% after 7 d. The mean numb er of salmonids per gut (1.2) after release was higher than typically seen in guts from northern squawfish collected in mid-reservoir areas away from hydroelectric dams on the Snake and Columbia rivers. Length- frequency distributions of juvenile salmonids eaten and those captured in a scoop trap 4 km upstream of the study area indicated that northe rn squawfish were selectively feeding on the smaller individuals. We a ttribute the high rates of predation in the study area to the artifici ally high density of juvenile salmonids resulting from the hatchery re lease and to the physical characteristics of the study area in which t he river changed from free flowing to impounded. Our results suggest t hat northern squawfish can quickly exploit hatchery releases of juveni le salmonids away from release sites in the Columbia River basin.