Large pelagic predators could jeopardize the recovery of endangered Atlantic salmon

Abstract

Long-term population viability of Bay of Fundy Atlantic salmon (Salmo salar) is threatened by high levels of marine mortality during migration. Pop-up satellite archival tags on repeat spawners provide direct evidence of extensive natural mortality of migrating salmon in coastal zones attributed to predation by large pelagic fish and no evidence of fishing mortality. Ingested tags show that salmon with a coastal migration are eaten by porbeagle shark (Lamna nasus) inside the Bay of Fundy, whereas distant migrants are consumed by Atlantic bluefin tuna (Thunnus thynnus) and other apex predators along the Scotian Shelf. Mortality is clustered in a few zones because of similar predator–prey habitat preferences and overlapping migration paths. The extent of predation in salmon populations with different migration strategies can account for observed decline rates in neighbouring populations with different life histories. The impact on endangered salmon populations that rely on multiple repeat spawners for population stability may be sufficient to hamper ongoing recovery efforts.