Non-stationary patterns in demographic traits covary with Chinook salmon marine distributions

Abstract

The abundance of many Chinook salmon (Oncorhynchus tshawytscha) stocks has declined despite reductions in harvest. We used state-space models parameterized with data from 57 Chinook salmon indicator stocks, ranging from coastal Oregon to southeast Alaska, to quantify long-term (since 1972 release year) changes in juvenile marine survival rate and mean age-at-maturity, as well as identify stock groupings with coherent dynamics. We found that juvenile marine distribution–rather than freshwater life history, run timing, or adult marine distribution–was the best predictor of trends in both survival and age. Only subyearling stocks that enter the Strait of Georgia showed evidence of transitioning to a low juvenile survival period, other groupings exhibited low and stable or cyclical patterns in survival. Conversely, declines in mean age-at-maturity were widespread and do not appear to have stabilized, suggesting future declines in Chinook salmon population productivity may be influenced by earlier maturation or increased adult mortality, rather than further reductions in juvenile marine survival.