Using an unplanned experiment to evaluate the effects of hatcheries and environmental variation on threatened populations of wild salmon

Efforts to conserve depleted populations of Pacific salmon (Oncorhynchus spp.) often rely on hatchery programs to offset losses of fish from natural and anthropogenic causes, but their use has been contentious. We examined the impact of a large-scale reduction in hatchery stocking on 15 populations of wild coho salmon along the coast of Oregon (USA). Our analyses highlight four critical factors influencing the productivity of these populations: (1) negative density-dependent effects of hatchery-origin spawners were ∼5 times greater than those of wild spawners; (2) the productivity of wild salmon decreased as releases of hatchery juveniles increased; (3) salmon production was positively related to an index of freshwater habitat quality; and (4) ocean conditions strongly affect productivity at large spatial scales, potentially masking more localized drivers. These results suggest that hatchery programs’ unintended negative effects on wild salmon populations, and their role in salmon recovery, should be considered in the context of other ecological drivers.