Food Habits of Smallmouth Bass, Walleyes, and Northern Pikeminnow in the Lower Columbia River Basin during Outmigration of Juvenile Anadromous Salmonids

I compared the diets of adult smallmouth bass Micropterus dolomieu , walleyes Stizostedion vitreum , and northern pikeminnow Ptychocheilus oregonensis collected in impounded and unimpounded reaches of the lower Columbia and lower Snake rivers during the outmigration of juvenile anadromous salmonids Oncorhynchus spp. from 1990 to 1996. Gravimetric proportions of fish, crayfish, and other prey consumed by each predator in spring and summer differed among the three river reaches for each predator species. Fish prey were generally the largest dietary component for all three predator species, although crayfish were also prevalent in the diets of smallmouth bass and, to a lesser extent, northern pikeminnow. Numerical proportions of salmonids, cottids, and other fish prey differed among predator species and among study reaches. Juvenile salmonids formed the majority of fish prey consumed by northern pikeminnow, whereas cottids, cyprinids, catostomids, and percopsids were more commonly consumed by smallmouth bass and walleyes. Weight of juvenile salmonids consumed by smallmouth bass and northern pikeminnow varied among years, but did not increase from 1990 to 1996. Back‐calculated lengths of consumed prey fish often differed among predator species, and maximum length of consumed salmonids and sculpins was linearly related to smallmouth bass and northern pikeminnow length. Daily rations and numerical salmonid consumption rates differed among river reaches, and consumption of nonsalmonids by smallmouth bass was much greater than that of northern pikeminnow. This study clarified predator–prey relationships between indigenous and introduced predator and prey species in the lower Columbia and lower Snake rivers.