Salmon hatcheries can demographically boost wild populations at the cost of diversity: quantitative genetic modelling of Alaska pink salmon

Authors: May, Samuel; SHEDD, KYLE - Alaska Department Of Fish And Game; GRUENTHAL, KRISTEN - Alaska Department Of Fish And Game; HARD, JEFFREY - Conservation Biology Division, Northwest Fisheries Science Center, National Marine Fisheries Servic; TEMPLIN, WILLIAM - Alaska Department Of Fish And Game; WATERS, CHARLES - Auke Bay Laboratories, Alaska Fisheries Science Center, National Marine Fisheries Service, National; ADKISON, MILO - Alaska Department Of Fish And Game; WARD, ERIC - Conservation Biology Division, Northwest Fisheries Science Center, National Marine Fisheries Servic; HABICHT, CHRISTOPHER - Alaska Department Of Fish And Game; WILSON, LORNA - Alaska Department Of Fish And Game; WERTHEIMER, ALEX - Consultant; WESTLEY, PETER - University Of Alaska

Submitted to: Royal Society Open Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2024
Publication Date: 7/3/2024
Citation: May, S.A., Shedd, K.R., Gruenthal, K.M., Hard, J.J., Templin, W.D., Waters, C.D., Adkison, M.D., Ward, E.J., Habicht, C., Wilson, L.I., Wertheimer, A.C., Westley, P.A. 2024. Salmon hatcheries can demographically boost wild populations at the cost of diversity: quantitative genetic modelling of Alaska pink salmon. Royal Society Open Science. 11(7). https://doi.org/10.1098/rsos.240455.
DOI: https://doi.org/10.1098/rsos.240455

Interpretive Summary: Salmon hatcheries are widely used to support fisheries by releasing millions of juveniles into the ocean, where they grow and are harvested upon return migrations. Some of these fish, termed 'strays,' end up making their way into wild populations. Many studies have shown how these strays can be different from wild fish, which has caused concerns over potential impacts to wild populations. However, measuring how these fish may be impacting wild populations is incredibly difficult in wild systems. In this study, we used simulation models, based on real-world dynamics, to show some of the ways in which hatchery strays might impact wild populations. Most importantly, we identified a trade-off between demographic impacts and evolutionary impacts. Hatchery fish made the wild populations larger, because they contributed offspring, which may be viewed as beneficial. However, hatchery fish also caused the wild populations to evolve to be more like the hatchery fish, and as a result the wild populations were less diverse. Diversity is important for long-term stability and resilience to changes. This study provides an important framework to help managers think mechanistically about the complicated interactions between hatchery and wild populations.

Technical Abstract: 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.