To assess the potential for introgression and competition between hatchery-produced (H p ) and natural-origin (N o ) chum salmon Oncorhynchus keta, we quantified a suite of fitnessrelated morphological and life-history traits on the spawning grounds of a small creek in Southeast Alaska that has high rates of immigration from nearby hatchery programs. Using thermally marked otoliths to distinguish between H p and N o fish, we estimated that 51.4% of individuals examined in 2015 were H p strays.

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... ompared with their N o counterparts, H p males and females entered the creek significantly later, were younger at maturity, smaller in body length, and smaller for a given age. On average, H p females lived 2 d less on the spawning grounds than N o females and also had higher rates of egg retention (47 vs. 19% observed in N o females). The observed phenotypic differences between the local N o and H p individuals suggest barriers to introgression through combinations of selection against strays or spatio-temporal segregation on the spawning grounds, though the phenotypic baseline of the N o population prior to potential hatchery influence is not known, nor is the extent to which environmental plasticity may be influencing trait expression. Although rates of interbreeding may be sufficiently low to currently maintain phenotypic differences between N o and H p strays, at least some spawning did occur by the majority of H p individuals. Ultimately, it remains unclear how long the presumably adaptive phenotype of the N o population may persist given the consistently large numbers of strays appearing annually on the spawning grounds.