Cross-sectional scat sampling reveals intrapopulation feeding diversity in a marine predator

Harbor seals (Phoca vitulina) have substantial impacts on species of concern. To understand and predict the impact that harbor seals have in their communities, we need to describe their level of individual specialization because it can affect food web dynamics, responses to changes in prey availability, and the accuracy of predictive models. I estimated intrapopulation feeding diversity, a proxy for individual specialization, of P. vitulina in the Salish Sea relative to sex, time, and location using repeated cross-sectional sampling of scat. Based on 1,083 scat samples collected from five haul-out sites over the course of four, non-sequential years, diet was quantified using traditional and metabarcoding techniques, and sex was determined using a molecular assay. Though variable spatially and temporally, high levels of specialization at a short-time scale (24 - 48 hours) ( = 0.392, 95% CI = 0.013, R = 100,000), combined with previous knowledge of P. vitulina feeding strategies, suggested that specialization was pervasive in Salish Sea populations. Males showed less specialization than females, particularly in the summer and fall, and demersal and benthic prey species were correlated with higher levels of diversity. These results suggest that although females consumed a wider range of prey species than males, they had a higher degree of specialization, likely driven by consumption of benthic species. Further, this finding also suggests benthic species likely require more specialized foraging strategies and that there are trade-offs between a pelagic and benthic foraging style for P. vitulina. Differential specialization on prey species as well as between sexes of P. vitulina indicate that predator-prey interactions are not well understood. Therefore, the likelihood of specialist versus generalist interactions with a prey species should be considered when management decides how to address P. vitulina influence on prey of conservation concern.