Review of reintroduction as a conservation tool for Pacific salmon in western North America

Abstract Objective Reintroduction of Pacific salmon Oncorhynchus spp. into historically occupied habitat is a widely used management strategy in western North America. Since the last major review, new information on donor populations, fish passage, dam removal, ecological interactions, monitoring, and cultural and societal considerations has emerged. We conducted a comprehensive review to synthesize these advances and inform best practices. Methods Co-authors representing federal, state, tribal, academic, and private organizations with expertise in Pacific salmon reintroductions convened to synthesize literature, evaluate case studies, and update key findings, lessons, and best practices. We developed a framework to assess program status using biological and cultural benchmarks and analyzed adult abundance trends for 26 reintroduced populations of Chinook Salmon O. tshawytscha, Coho Salmon O. kisutch, Sockeye Salmon O. nerka, and steelhead O. mykiss across 16 reintroduction sites in the Pacific Northwest, USA. Results We reviewed 51 Pacific salmon reintroductions, along with relevant studies providing insights into reintroduction protocols and outcomes. Of the programs that have been implemented to date, 48 (94%) remain active and 27 (53%) have operated for at least 20 years. Tribal involvement has been instrumental in the initiation and advancement of many of these efforts and is focused on restoring cultural, spiritual, and ecological connections to salmon. Most populations (81%) evaluated in a trends analysis exhibited stable or increasing abundance, demonstrating that reintroduction is effective at reestablishing populations in historical habitats. The restoration of anadromy to these habitats has ecological benefits that are commonly recognized, such as restoring marine-derived nutrient transport, providing access to cold water in headwater tributaries, contributing to fisheries, and benefiting other species of concern (e.g., sea lions and orcas Orcinus orca) that rely on salmon for prey. Benefits of reintroduction also include cultural reconnections between salmon and Indigenous Peoples. Salmon are considered “relatives” and are central to the way of life in many Indigenous cultures, but this connection has been lost in places where salmon have been extirpated from their historical ranges. Such losses have resulted in great harm to Indigenous communities, and reintroduction has the potential to begin restoring what has been lost. While we acknowledge the importance of identifying and mitigating the risks associated with reintroduction, we also maintain that there are inherent risks of inaction, such as ongoing harm to communities that rely on Pacific salmon. Conclusions This synthesis updates the existing state of knowledge for Pacific salmon reintroduction. Our review indicates that reintroduction efforts are supporting many stable or increasing populations throughout western North America in addition to restoring culturally important species for Indigenous communities.