16 research outputs found
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Estimated Loss of Juvenile Salmonids to Predation by Northern Squawfish, Walleyes, and Smallmouth Bass in John Day Reservoir, Columbia River
We estimated the loss of juvenile salmonids Oncorhynchus spp. to predation bynorthern squawfish Ptychocheilus oregonensis, walleyes Stizostedion vitreum, and smallmouth bassMicropterus dolomieu in John Day Reservoir during 1983-1986. Our estimates were based onmeasures of daily prey consumption, predator numbers, and numbers of juvenile salmonids enteringthe reservoir during the April-August period of migration. We estimated the mean annualloss was 2.7 million juvenile salmonids (95% confidence interval, 1.9-3.3 million). Northernsquawfish were responsible for 78% of the total loss; walleyes accounted for 13% and smallmouthbass for 9%. Twenty-one percent of the loss occurred in a small area immediately below McNaryDam at the head of John Day Reservoir. We estimated that the three predator species consumed14% (95% confidence interval, 9-19%) of all juvenile salmonids that entered the reservoir. Mortalitychanged by month and increased late in the migration season. Monthly mortality estimatesranged from 7% in June to 61% in August. Mortality from predation was highest for chinooksalmon O. tshawytscha, which migrated in July and August. Despite uncertainties in the estimates,it is clear that predation by resident fish predators can easily account for previously unexplainedmortality of out-migrating juvenile salmonids. Alteration of the Columbia River by dams and adecline in the number of salmonids could have increased the fraction of mortality caused bypredation over what it was in the past
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Feeding of Predaceous Fishes on Out-Migrating Juvenile Salmonids in John Day Reservoir, Columbia River
Diets of northern squawfish Ptychocheilus oregonensis, 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 salmon 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