Influence of ocean and freshwater conditions on C olumbia R iver sockeye salmon Oncorhynchus nerka adult return rates

Abstract

In recent years, returns of adult sockeye salmon O ncorhynchus nerka to the C olumbia R iver B asin have reached numbers not observed since the 1950s. To understand factors related to these increased returns, we first looked for changes in freshwater production and survival of juvenile migrants. We then evaluated productivity changes by estimating smolt‐to‐adult return rates ( SAR ) for juvenile migration years 1985–2010. We found SAR varied between 0.2 and 23.5%, with the highest values coinciding with recent large adult returns. However, the largest adult return, in 2012, resulted not from increased survival, but from increased smolt production. We evaluated 19 different variables that could influence SAR s, representing different facets of freshwater and ocean conditions. We used model selection criteria based on small‐sample corrected AIC to evaluate the relative performance of all two‐ and three‐variable models. The model with A pril upwelling, P acific N orthwest I ndex ( 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 R 2 = 0.82. The variables of A pril ocean upwelling and PNI in the migration year had high weights of 0.996 and 0.927, respectively, indicating they 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 currently available, changes in ocean conditions in the northern C alifornia Current system, as captured by April upwelling and PNI , appeared to play a large role in the variability of SAR .

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