Adaptive potential of Puget Sound Chinook salmon seawater tolerance

Pacific salmon have evolved to use estuarine habitats as they transition from freshwater to ocean environments. These estuary habitats, however, are severely reduced compared to historical conditions and are rapidly changing due to environmental change and human development. To assess whether at-risk populations have the plasticity or evolutionary capacity to adapt, we quantified phenotypic and genetic variability in size-at-migration and seawater tolerance across three Puget Sound Chinook populations in a common-garden environment. We found (1) strong evidence for population differences in size-at-migration and subsequent size-selective seawater survival, (2) a size threshold of 49 mm required to achieve 50% survival in seawater that was consistent across populations, and (3) moderate heritability of size-at-migration and moderate-low heritability of seawater survival with strong maternal effects. Combined, our results suggest that evolutionary adaptation is a viable pathway for Puget Sound Chinook salmon populations to adapt to rapidly changing estuary conditions.