Population Viability Improves Following Termination of Coho Salmon Hatchery Releases

Recent genetic studies, meta-analyses, and retrospective analyses have documented reduced productivity of wild salmon and steelhead Oncorhynchus mykiss that interbreed with hatchery-reared fish, raising concerns about the long-term viability and recovery of at-risk stocks. In 2007, the Oregon Department of Fish and Wildlife discontinued a Coho Salmon Oncorhynchus kisutch hatchery program at the Salmon River to support recovery of a wild Coho Salmon population in the Oregon Coast Evolutionarily Significant Unit. This decision constituted a unique management experiment, allowing for direct measurement of the wild population's response after the discontinuation of a decades-old hatchery program. We used a before-after, control-impact design to examine whether selected viability metrics of the naturally produced population in the Salmon River changed after the hatchery program ended. We compared metrics for the 2006-2013 broods, representing periods after the hatchery program ended, to those for the 1995-2005 broods, when the hatchery program was still releasing 200,000 smolts annually. We also examined neighboring populations during similar time periods to account for changes or variation due to other factors. Although hatchery-origin spawners previously had accounted for most of the adults returning to the Salmon River, the naturally produced population did not collapse, and two viability metrics improved significantly after the Coho Salmon hatchery program ended: (1) adult abundance increased and (2) spawn timing expanded and moved closer to the historical timing. Recruits-to-spawner ratios in the Salmon River, although initially low, are now approximately equal to those of neighboring populations. The results indicate that hatchery closure can be an effective strategy to promote wild population recovery. However, considerable variability in population trends and environmental conditions will require continued monitoring to verify the long-term resilience and viability of the wild population.