Accounting for Density-Dependent Predation in the Survival of Juvenile Salmon and Steelhead during Their Seaward Migration

Predation is a significant source of mortality for juvenile salmonids (Oncorhynchus spp.) during their seaward migration, but the degree to which it affects at-risk populations is largely unknown. In the Columbia River basin, piscivorous birds and fish are abundant throughout the mainstem migration corridor. Bird colony sizes in the mid-Columbia exhibit minimal interannual variation, leading to relatively constant consumption levels. Conversely, migrating juvenile salmonids can exhibit substantial interannual variation in abundance, which is largely dependent on production, juvenile survival, and removal at upstream dams by smolt transportation programs. In the absence of compensatory mechanisms, we expect that mortality rates due to predation would decrease with increasing smolt abundance, and therefore we developed survival models that included a predator functional response as well as the effects of environmental covariates and prey density. We derived survival estimates from mark-recapture data and fit our models with Bayesian methods. We informed prior distributions for model parameters with estimates from other studies where available. We found that higher survival probabilities were associated with greater smolt abundance after accounting for predator abundance, fish travel time, flow, temperature, and dam operations. Improved predictive ability afforded by these models should aid management decisions in the future.