Hatchery supplementation may support salmon fisheries but hatchery strays can also have significant impacts on natural populations. We developed a demographic model to explore how hatchery contributions to ocean harvest versus straying onto natural spawning grounds was affected by hatchery production, maturation, fecundity and straying rates of hatchery-origin fish. We evaluated model outputs based on widely used metrics such as the proportion hatchery fish in the catch and the proportion hatchery-origin spawners as well as a newly developed index of hatchery-origin catch per stray spawner that measures the trade-off between supplementing harvest and limiting natural-area impacts; higher values of this index indicate success in increasing hatchery ocean harvest contributions relative to strays spawning in natural areas. When evaluating practices at a specific hatchery, this index has the advantage that it can be evaluated a hatchery-specific performance metric which is not obscured by variability the productivity of natural populations and is independent of the impacts of other hatcheries.
Results/Conclusions
Earlier maturation of hatchery fish reduces catch per stray (and reduces proportion hatchery-origin catch while increasing proportion hatchery-origin spawners if all other factors are held constant) by shifting the age distributions of both catch and spawners toward younger ages. Although the numerical impact of strays can thereby be reduced by delaying the maturation of hatchery-origin fish, age-dependent fecundity may complicate predicted demographic effects of changing maturation schedules. Increased straying does not affect catch but increases hatchery-origin spawning in natural areas and decreases catch per stray. Differences in hatchery production affect hatchery contributions to both catch and natural area spawning, exacerbating the tradeoff between these conflicting goals but with no net effect on catch per stray. Fishery intensity magnifies the effects of altered maturation schedules of hatchery fish by reducing the spawning contributions of older fish. These results demonstrate how a careful consideration of life history variation may assist hatchery and fishery managers in accommodating conflicting goals for harvest supplementation and conservation of naturally spawning stocks.