Evaluating the long-term per capita productivity benefits and associated costs of supplementation in upper Yakima River Chinook salmon

Numerous studies in salmonids have demonstrated a fitness cost of producing and releasing hatchery-origin fish into the natural environment. One approach to reduce these fitness costs is to incorporate natural-origin fish into the hatchery broodstock, but this is not always feasible and may not consistently buffer against domestication. In this study, we used 15 years of spawning and genetic data from the upper Yakima River spring Chinook salmon ( Oncorhynchus tshawytscha) population to successfully assign approximately 50 000 returning adult progeny to their parents, allowing us to reconstruct a two-generation pedigree and evaluate reproductive success (RS). We identified consistently lower RS of hatchery-origin compared to natural-origin fish when spawning in nature. However, the hatchery broodstock demonstrated higher per capita productivity than natural spawners into the second generation even after accounting for lower RS by hatchery-origin progeny in the intermediate generation. We also identified fork length and returning timing, in addition to origin, as important components of individual RS in this population. We then discuss the significance of these results in the context of future salmonid supplementation studies.