Using Otolith Strontium Isotopes to Quantify Success of Juvenile Chinook Salmon in a Regulated Californian River

California Central Valley Chinook salmon exhibit some of the most complex life histories in the world. We examined variation in juvenile outmigration behavior relative to hydrologic regime in the Stanislaus River, a regulated stream at the southern end of the species range. We used otolith ⁸⁷Sr/⁸⁶Sr and radius measurements to reconstruct the early life histories of returning (i.e. “successful”) adults. Size-at-outmigration and phenotype contributions were compared between juvenile outmigrants and adult survivors from the same cohorts to identify temporal trends in selective mortality. Differences in precipitation patterns and local water operations resulted in contrasting flow regimes among years. In wetter years, most juveniles emigrated as fry, while in drier years, outmigrants tended to be fewer but larger. Metrics of outmigration varied primarily as a function of hydroclimatic regime, while survival rates were driven by conditions in the natal tributary, as well as size- and time-selective mortality. While fry survival is generally assumed to be negligible in this system, here, they represented up to 23% of the reproductive population. We discuss the results in the context of the portfolio effect, and consider the extent to which migration strategies have been (and could be) artificially selected via modifications to the natural hydrograph.