Influence of Ocean and Freshwater Conditions on Columbia River Sockeye Salmon Adult Return Rates

Recently, adult sockeye salmon Oncorhynchus nerka returns to the Columbia River Basin have reached numbers not observed since the 1950s.  To understand factors related to returns, we evaluated productivity changes by estimating smolt-to-adult return rates (SAR) for juvenile migration years 1985-2010.  SAR varied between 0.2 and 23.5%.  Highest values coincided with recent large adult returns.  However, the largest return, in 2012, resulted not from increased survival, but from increased smolt production.  We evaluated 19 different variables that could influence SARs, representing different freshwater and ocean conditions.  We used small-sample corrected AIC to select among 2- and 3-variable models.  The model with April upwelling, Pacific Northwest Index (PNI) in the migration year, and PNI in the year before migration had 10 times the AIC_c-weight as the second-best-supported model, and R² of 82%.  The variables April ocean upwelling and PNI in the migration year were by far the best of the candidate variables to explain variations in SAR.  While our analyses were primarily correlative and limited by the type and amount of data available, changes in ocean conditions in the northern California current system, as captured by April upwelling and PNI, appeared to play a large role in the variability of SAR.