Survival estimates for downstream migrant yearling juvenile salmonids through the Snake and Columbia rivers hydropower system, 1996-1980 and 1993-1999
Columbia River, Hydropower, juvenile, salmonids, survival, survival estimates, chinook, salmon, steelhead, migration, tailrace, dams, Snake River, adult, mortality
Journal or Book Title
North American Journal of Fisheries Management
This paper examines average annual survival of juvenile spring–summer Chinooksalmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) during migration through thehydropower system of the Snake and Columbia rivers from 1966 to 1980 and 1993 to 1999. Ineach year, survival was estimated from observations of marked fish in a portion of thehydropower system corridor. We expanded these estimates to calculate an annual estimate ofsurvival over the entire system (head of uppermost reservoir to tailrace of lowermost dam).Temporal changes in the hydropower system were compared with trends in estimated survival toevaluate the effects of dams on survival of downstream migrants. When only four dams were inplace (1966–1967), estimates of survival through the hydropower system were 32–56%. Fouradditional dams were constructed between 1968 and 1975. Survival estimates during the 1970stypically were 10–30% for spring–summer Chinook salmon, but less than 3% in the drought yearsof 1973 and 1977. From 1993 to 1999, after structural and operational changes in the hydropowersystem, survival estimates of spring–summer Chinook salmon and steelhead ranged from 31% to59%. Smolt-to-adult return rates of Snake River wild spring–summer Chinook salmon from themiddle to late 1960s generally exceeded 4% but decreased during the 1970s. Although survivalthrough the hydropower system in the 1990s is substantially greater than that in the 1970s, adultreturn rates in the 1990s have remained low. Thus, in the 1990s, the cause of the continuing lowadult return rates does not seem to be related to direct mortality of downstream migrant fishwithin the hydropower system.