Evaluating benefits of stocking on sockeye recovery projections in a nutrient-enhanced mixed life history population

Abstract

Dam construction often blocks migration of anadromous sockeye salmon (Oncorhynchus nerka), resulting in a residualized population that is often managed as landlocked kokanee. Anadromy resumes when a reconnection to the ocean is established; however, there may be selective pressure acting on threshold trait(s) affecting smolt timing and probability. While there may be interest in predicting persistence of anadromous and residual sockeye forms, this is difficult because the heritability of smolting in these populations is poorly known. We develop a fully density-dependent age-structured model to project abundances for both anadromous and resident sockeye. The model considers trophic interactions due to nutrient variation and the density-dependent consequences for smolting and adult returns. Moreover, it asks how each life history type will persist if a hatchery were used to promote anadromous sockeye through artificial selection. We show hatchery supplementation is unlikely to impact anadromous or resident sockeye significantly, although there is substantial prediction uncertainty suggested in projections. Our study suggests that providing passage for previous land-locked anadromous populations will lead to the return of anadromous fish in the short term, but long-term prospects are far from certain.