Harbour Seal Consumption of Juvenile Salmon in the Strait of Georgia

Historically, Chinook and coho were two of the most valuable commercial and recreational salmon species in western North America, particularly in the Salish Sea. However, marine survival rates of these species have declined dramatically since the 1970s as numbers of harbour seals increased. This correlation has led some scientists to hypothesize that seal predation may be impeding the recovery of Chinook and coho salmon, primarily because reduced exploitation rates over past decades have not returned them to historic abundances. While seal consumption of adult salmon has been quantified in past studies, few attempts have been made to estimate predation on out-migrating smolts. Most existing estimates of predation on juveniles are based on the presence/absence of hard structures in seal fecal material, which cannot determine salmon species and are susceptible to biases associated with survival of diagnostic hard structures during the digestion process. Here, we fit models to DNA-based diet data to estimate annual seal consumption of juvenile and adult salmon in the Strait of Georgia. Results suggest that seal predation on juvenile Chinook and coho during the early marine life stages is substantial in some areas.