Cross-sectional scat sampling reveals diet heterogeneity in a marine predator

Harbor seals (Phoca vitulina) are the most abundant marine mammal in the Salish Sea and have a large impact on species of conservation and economic concern. It is important to accurately describe and predict the impact harbor seals have in their communities, including their level of diet heterogeneity, which can affect food web dynamics, responses to changes in prey availability, and the accuracy of predictive models. To estimate heterogeneity at large spatial and temporal scales, I used repeated cross-sectional sampling of scat to gather information about the diet and sex of multiple haul-outs of harbor seals in the Salish Sea. Using 1,083 scat samples collected from five haul-out sites over the course of four, non-sequential years, the diet of harbor seals was quantified using traditional and genetic techniques. My results confirmed diet heterogeneity among and across the different combinations of factors (sex, season, location, and year), suggesting that specialization is pervasive in Salish Sea harbor seals (Heterogeneity (PSi) = 0.392, 95% CI = 0.013, R = 100,000). Further, males showed less heterogeneity than females, particularly in the summer and fall, and demersal and benthic prey species were correlated with higher levels of heterogeneity. We hypothesize that the last two findings reflect the fact that females ate a wider range of prey items than males, which often include more benthic and demersal species. These findings indicate the need for further research on the level of heterogeneity in Salish Sea harbor seals to accurately understand predator-prey relations and predict prey consumption