Impacts of Hatcheries on Wild Chinook Salmon Productivity: Lessons from Long-Term Monitoring

Captive breeding is a widely used strategy for buffering rapidly declining populations against short-term extinction risk. In the case of Pacific salmon and other exploited species, captive breeding takes the form of supplementation hatchery programs, which are intended to increase population size while minimizing artificial selection in breeding and rearing practices. Supplementation is an increasingly common tool for Pacific salmon management, but substantial uncertainty remains about its effectiveness and potential impacts on wild populations. Directly measuring these impacts is challenging, in part because of the difficulty of estimating the relative reproductive contributions of wild- and hatchery-reared individuals that breed naturally. To address this, we analyzed time series of adult density from 23 populations of spring/summer Chinook salmon (Oncorhynchus tshawytscha) in the Snake River basin, USA, which have experienced a range of supplementation levels (including no supplementation). We fit models that predict total naturally derived recruitment as the sum of contributions by wild- and hatchery-reared adults. We compared alternative hypotheses about density-dependence and the equivalence of wild- and hatchery-reared fish with respect to two key parameters: productivity at low density and carrying capacity.

The data supported models in which carrying capacity is lower on average for hatchery-reared fish than for wild fish. However, the magnitude and direction of this difference varied across populations within the ESU.  In contrast, the intrinsic productivity of hatchery fish relative to wild fish was poorly defined by the data. These results point to underlying differences in the ecology, behavior, or life history of wild and hatchery-reared salmon, despite the efforts by supplementation programs to minimize such divergence. The introduction of hatchery-reared fish into wild populations may reduce natural productivity, and thus supplementation programs may face a trade-off between short-term increases in abundance and erosion of the long-term potential for rebuilding. However, the uncertainty regarding key parameters suggests that even spatially and temporally extensive monitoring may be insufficient to clearly identify the impacts of supplementation, and highlights the need for more powerful adaptive management experiments.