Scat DNA Sequencing Reveals Harbour Seals in the Strait of Georgia Target Salmon Species of Concern in the Juvenile Life Stage

The species and life stages of fishes consumed by predators are important for understanding the impacts of predation on fish populations, but traditional diet analysis tools often cannot provide that level of information. We combined data from two diet analysis techniques (DNA metabarcoding and morphological prey ID) to quantify the species and life stages of salmon consumed by harbour seals in the Strait of Georgia, Canada. A decision tree approach was developed to merge the two data sets, using the best available information to assign salmon life stage. Comparisons to data from the 1980s revealed that harbour seal salmon consumption in the fall (predominantly adults) has increased substantially since the 1980s. Adult salmon consumed by the seals predominantly consisted of abundant species of lesser conservation concern (chum, pink, and sockeye). However, the opposite was observed in the species composition of juvenile salmon consumed by seals in the spring (ranked juvenile species in seal diet were coho, Chinook, sockeye, pink, and chum), indicating selection for the larger-bodied juveniles of species currently of concern.  Our study shows the usefulness of applying multiple diet analysis techniques, and highlights the necessity of regularly updating predator dietary information because animal food habits are not static.