Falling Behind: Delayed Growth Explains Life‐History Variation in Snake River Fall Chinook Salmon

Abstract

AbstractFall Chinook salmon Oncorhynchus tshawytscha typically migrate to the ocean as subyearlings (age 0), but a strategy whereby juveniles overwinter in freshwater and migrate to the ocean as yearlings (age 1) has emerged over the past few decades in Idaho's Snake River population. The recent appearance of the yearling strategy has conservation implications for this threatened population because of survival and reproductive differences between the two life histories. Different proportions of juveniles adopt the yearling life history in different river reaches and years, and temperature differences are thought to play some role in accounting for this variation. The specific circumstances under which juveniles pursue the yearling life history are poorly understood. We advance a hypothesis for the mechanism by which juveniles adopt a life history, formalize it with a model, and present the results of fitting this model to life history data. The model captures patterns of variation in proportions of yearling out‐migrants among reaches and years, and it appears robust to uncertainty in a key unknown parameter. Results from fitting the model to empirical yearling migrant proportions suggest that juveniles commit to a life history earlier in development than the time at which smoltification typically begins. Specifically, juveniles that become yearling migrants do so soon after emergence if they are too far behind a typical growth schedule given temperature and photoperiod cues at that time. Our model also offers those interested in the management and conservation of Snake River fall Chinook salmon a useful tool by which to account for life history variation in population viability analyses and decision making.