FEEDING OF PREDACEOUS FISHES ON OUT-MIGRATING JUVENILE SALMONIDS IN JOHN-DAY-RESERVOIR, COLUMBIA RIVER

Diets of northern squawfish Ptychocheilus, smallmouth bass Micropterus dolomieu, walleye Stizostedion vitreum, and channel catfish Ictalurus punctatus from John Day Reservoir were examined to determine the extent of predation on juvenile salmonids during seaward migrations of the salmonids during April-August 1983-1986. Juvenile Pacific salmon Oncorhynchus spp. and steelhead O. mykiss were the most important food group (by weight) of northern squawfish-about 67%-but made up smaller proportions of the food of the other predators: channel catfish, 33%; walleyes, 14%; smallmouth bass, 4%. Seasonal changes in diets indicated that northern squawfish preferred juvenile salmonids in May and August (generally the peak period of salmonid out-migration), and switched to prickly sculpin Cottus asper when numbers of juvenile salmonids declined; walleyes and smallmouth bass showed a preference only for prickly sculpin among the prey fishes analyzed. As judged by dietary composition and prey selectivity, the northern squawfish was the major fish predator on juvenile salmonids in the reservoir; channel catfish also were important predators in the upper reservoir in spring. Walleyes and smallmouth bass were much less important predators on salmonids, and appeared to select subyearling chinook salmon only in August when the distribution of this prey overlapped with that of the predators. Size-selective predation by northern squawfish may also play an important role in reducing survival of the smaller individuals within each run of out-migrating juvenile salmonids.