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We investigate adaptive resource polymorphism in kokanee (Oncorhynchus nerka) from Jo-Jo Lake, Alaska, by determining whether previously observed niche expansion occurs at the population or individual level. Utilizing morphological, genetic, and stable isotope techniques, we found no evidence of discrete trophic morphotypes as previously described, but instead found evidence for an ontogenetic diet shift. Carbon and nitrogen isotope data indicate a 40% decrease in the proportion of benthic feeding and an increase of one trophic position over the size and age ranges of adult kokanee, corresponding to a diet shift from consumption of macroinvertebrates in smaller individuals to piscivory in larger individuals. This novel piscivory in kokanee may result from predatory and competitive freedom resulting from the lack of limnetic predators in Jo-Jo Lake. Piscivorous feeding despite a phenotype-environment mismatch has resulted in large, piscivorous kokanee having up to 70% of their gill rakers damaged. Observed reductions in gill raker number relative to the putative ancestral population are convergent with expectations for piscivorous fishes, despite a presumed lack of standing genetic variation for piscivory in the sockeye salmon--kokanee species complex. Jo-Jo Lake kokanee are a distinctive example of adaptation in salmonids in response to ecological release. This unusual population highlights the importance of phenotypic plasticity in response to competition in shaping the adaptive landscape and altering evolutionary trajectories. Nous examinons le polymorphisme adaptatif associe aux ressources chez les kokanis (Oncorhynchus nerka) du lac Jo-Jo (Alaska), en etablissant si l'expansion de niches observee precedemment se produit a l'echelle de la population ou de l'individu. Si l'utilisation de techniques morphologiques, genetiques et d'analyse des isotopes stables n'a revele aucun indice de la presence de morphotypes trophiques distincts comme cela a deja ete decrit, elle a plutot fait ressortir des indices d'un changement ontogenetique du regime alimentaire. Les donnees d'isotopes de carbone et d'azote indiquent une baisse de 40 % de la proportion de l'alimentation benthique et une augmentation d' un position trophique sur les fourchettes de tailles et d'ages des kokanis adultes, correspondant au passage d'un regime alimentaire constitue de macroinvertebres chez les individus plus petits a un comportement piscivore chez les individus plus grands. Ce nouveau comportement piscivore chez le kokani pourrait decouler de l'absence de predation et de concurrence associee a l'absence de predateurs limnetiques dans le lac Jo-Jo. Une alimentation piscivore malgre un decalage entre le phenotype et l'environnement s'est traduite par le fait que jusqu'a 70 % des branchictenies des grands kokanis piscivores sont endommagees. Les reductions observees du nombre de branchictenies par rapport a la population ancestrale putative convergent vers les nombres prevus pour les poissons piscivores, malgre l'absence presumee de variation genetique existante associee au comportement piscivore dans le complexe specifique saumon rouge-kokani. Les kokanis du lac Jo-Jo constituent un exemple distinctif d'adaptation en reponse a une liberation ecologique chez les salmonides. Cette population inhabituelle fait ressortir l'importance de la plasticite phenotypique en reponse a la concurrence pour ce qui est de definir le paysage adaptatif et de modifier les trajectoires devolution. [Traduit par la Redaction], Introduction Ecological release from interspecific competition often leads to ecological niche expansion (Van Valen 1965; Robinson and Wilson 1994; Schluter 1996b; Bolnick et al. 2010). Increases in niche width can [...]

Hatcheries are vital to many salmon fisheries, with inherent risks and rewards. While hatcheries can increase the returns of adult fish, the demographic and evolutionary consequences for natural populations interacting with hatchery fish on spawning grounds remain unclear. This study examined the impacts of stray hatchery-origin pink salmon on natural population productivity and resilience. We explored temporal assortative mating dynamics using a quantitative genetic model that assumed the only difference between hatchery- and natural-origin adults was their return timing to natural spawning grounds. This model was parameterized with empirical data from an intensive multi-generational study of hatchery-wild interactions in the world's largest pink salmon fisheries enhancement program located in Prince William Sound, Alaska. Across scenarios of increasing hatchery fish presence on spawning grounds, our findings underscore a trade-off between demographic enhancement and preservation of natural population diversity. While enhancement bolstered natural population sizes towards local carrying capacities, hatchery introgression reduced variation in adult return timing by up to 20%. Results indicated that hatchery-origin alleles can rapidly assimilate into natural populations, despite the reduced fitness of hatchery fish attributable to phenotypic mismatches. These findings elucidate the potential for long-term demographic and evolutionary consequences arising from specific hatchery-wild interactions, emphasizing the need for management strategies that balance demographic enhancement with the conservation of natural diversity., Competing Interests: We declare we have no competing interests., (© 2024 The Authors.)