Estimated Loss of Juvenile Salmonids to Predation by Northern Squawfish, Walleyes, and Smallmouth Bass in John Day Reservoir, Columbia River
bass, chinook, Columbia River, juvenile, McNary Dam, migration, mortality, northern squawfish, predation, salmon, salmonids, squawfish, walleye
Journal or Book Title
Transactions of the American Fisheries Society
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.