Examining the impacts of pinnipeds on Atlantic salmon: The effects of river restoration on predator-prey interactions

Seal populations in the Gulf of Maine have been recovering from historical bounty-driven declines since they received federal protection in 1972. In the past few decades, their population growth has sparked renewed concerns over conflict between pinniped and fish conservation. In the Penobscot River in Maine, USA, where restoration efforts over the past decade have aimed at restoring diadromous fish runs, seal predation on the endangered Atlantic salmon (Salmo salar) has become a growing concern. Quantitative and qualitative data for individual salmon, as well as the presence of putative seal-induced injury, have been collected from salmon passing upriver at dams in the Penobscot River since 2012. An analysis of seal-induced injury rate with demographic data, river herring returns, estuary fish biomass estimates, and timing of peak estuary fish biomass revealed that the seal-induced injury rate declined in the Penobscot River from 2012 to 2019, coincident with increasing river herring returns and estuary fish biomass. Despite a potentially increasing seal population, these results suggest that predator swamping from increasing forage fish may provide protection to salmon against seal predation. The presence of multiple injuries on an individual salmon was also significantly associated with the probability of a salmon having a seal-induced injury. Lamprey wounds and lacerations on salmon were commonly associated with seal-induced injury, suggesting a confounding effect of multiple stressors. This assessment illustrates how multi-species diadromous restoration activities may alter predator-prey interactions in ways that support salmon conservation efforts. As demonstrated in the Penobscot River, a focus on overall ecosystem health and restoration can benefit multiple species that use the river. Insights from this study may help inform future management decisions in other human-impacted systems where protected predators come into conflict with endangered prey.