Effects of Hydroelectric Development and Fisheries Enhancement on Spring and Summer Chinook Salmon and Steelhead in the Columbia River Basin

Abstract

Trends in abundance of spring and summer chinook salmon Oncorhynchus tshawytscha and steelhead Salmo gairdneri returning to the Snake River and mid-Columbia River above Priest Rapids Dam were determined by analyzing the percentage of adults returning from the smolt out-migrations of 1962–1984. Runs declined as a result of hydroelectric development of the river; the main cause for the decline was the mortality of juveniles migrating downstream through as many as nine dams and impoundments en route to the ocean. Mid-Columbia River summer chinook salmon runs experienced the greatest decline because of higher mortalities incurred during their migration to sea as subyearlings in July and August. Mortality was lower for remaining races of fish that migrate to sea as yearlings in the spring during higher river flows, more spill at dams, and cooler water temperatures. Enhancement measures to offset dam-related mortality of smolts began in 1970 on the Snake River and in 1975 on the mid-Columbia River. These measures included increased numbers of smolts released from hatcheries, spillway deflectors to reduce dissolved gas saturation, fingerling bypasses at dams, transportation of smolts around dams, supplemental river flows to minimize delay for smolts passing through reservoirs, and supplemental spill at dams to minimize turbine mortality of smolts at dams without fingerling bypasses. These actions have reversed the decline of steelhead but not of salmon. Enhancement has improved the rate of return of wild spring chinook salmon, but wild fish contribution is minimal at this time because stocks were reduced by earlier hydroelectric development. Presently, runs are mostly of hatchery origin and have not responded well to enhancement. Mortality of hatchery fish may be due to activation of bacterial kidney disease by stresses encountered during downriver migrations, transportation, or subsequent transition into seawater.