Biological, freshwater, and marine drivers of age at maturity in wild Chinook Salmon

Understanding variation in age at maturity is important for endangered species recovery because older, larger adults contribute disproportionately to the next generation. Conditions in early life stages may have underappreciated impacts on age at maturity. Our study objective was to associate adult age of individually tagged wild, spring/summer Chinook Salmon (Oncorhynchus tshawytscha) outmigrating from the Snake River (Idaho and Washington, USA) in 1998-2020 with covariates measured during juvenile and subadult stages. We used a hierarchical Bayesian ordinal probit regression model to estimate statistical effects of juvenile body length, seasonal migration timing or river temperature, transported or in-river hydrosystem passage, river flow, and a large-scale ocean index. Results indicated notable carryover effects consistent with underlying biological mechanisms related to growth and development, in which shorter juvenile length and later seasonal migration timing were associated with older adults. These biological and behavioural factors were more important than riverine or marine environmental conditions examined. Our study suggests that managers and decision makers should consider carryover effects from the juvenile life stage on age structure in conjunction with survival.