Harbour seals target juvenile salmon of conservation concern

Abstract

Knowing the species and life stages of prey that predators consume is important for understanding the impacts that predation may have on prey populations, but traditional methods for determining diets often cannot provide sufficient detail. We combined data from two methods of scat analysis (DNA metabarcoding and morphological prey ID) to quantify the species and life stages of salmon (Oncorhynchus spp.) consumed by harbour seals (Phoca vitulina) in the Strait of Georgia, Canada, where juvenile Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon survival is poor. Harbour seals primarily consumed adult salmon of lesser conservation concern in the fall (August–November): chum (Oncorhynchus keta: 18.4%), pink (Oncorhynchus gorbuscha: 12.6%), sockeye (Oncorhynchus nerka: 7.4%), Chinook (7.1%), and coho (1.8%). However, the opposite species trend occurred during the spring when seals preferred juvenile salmon of greater conservation concern (April–July): coho (2.9%), Chinook (2.9%), sockeye (2.5%), pink (1.4%), and chum (0.8%) — percentages that can equate to many individuals consumed. Our data suggest that harbour seals select juveniles of salmon species that out-migrate at ages >1 year and provide evidence of a potential causal relationship between harbour seal predation and juvenile salmon survival trends.