Survival Estimates for Downstream Migrant Yearling Juvenile Salmonids through the Snake and Columbia Rivers Hydropower System, 1966–1980 and 1993–1999

Abstract

This paper examines average annual survival of juvenile spring–summer chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss during migration through the hydropower system of the Snake and Columbia rivers from 1966 to 1980 and 1993 to 1999. In each year, survival was estimated from observations of marked fish in a portion of the hydropower system corridor. We expanded these estimates to calculate an annual estimate of survival 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 to evaluate the effects of dams on survival of downstream migrants. When only four dams were in place (1966–1967), estimates of survival through the hydropower system were 32–56%. Four additional dams were constructed between 1968 and 1975. Survival estimates during the 1970s typically were 10–30% for spring–summer chinook salmon, but less than 3% in the drought years of 1973 and 1977. From 1993 to 1999, after structural and operational changes in the hydropower system, survival estimates of spring–summer chinook salmon and steelhead ranged from 31% to 59%. Smolt-to-adult return rates of Snake River wild spring–summer chinook salmon from the middle to late 1960s generally exceeded 4% but decreased during the 1970s. Although survival through the hydropower system in the 1990s is substantially greater than that in the 1970s, adult return rates in the 1990s have remained low. Thus, in the 1990s, the cause of the continuing low adult return rates does not seem to be related to direct mortality of downstream migrant fish within the hydropower system.