DNA analysis of scats reveals spatial and temporal structure in the diversity of harbour seal diet from local haulouts to oceanographic bioregions

Abstract

Predation shapes ecosystems, and quantifying the impacts of predation on the distribution and abundance of prey requires substantial effort at appropriate spatial and temporal scales for diet estimation of predators. Here, we present diet estimates of a marine predator (harbour seal Phoca vitulina richardsi) from scat collections (n = 3420) sampled at multiple haulout sites (n = 64) in the Strait of Georgia and other coastal regions around Vancouver Island, British Columbia, Canada, between 2015 and 2019. DNA metabarcoding and hard part analysis were used to identify the proportion and sizes of prey species consumed, respectively. We found that harbour seals consumed 62 primary prey species. Diversity in the diet was highly affected by the number of samples collected and varied at small spatial scales (haulout sites) as well as among broad bioregions. Three to 5 species dominated the diet depending on location, season, and year, including Pacific hake Merluccius productus, Pacific herring Clupea pallasii, and walleye pollock Gadus chalcogrammus. Within the Strait of Georgia, both male and female harbour seals consumed more hake and walleye pollock in areas and seasons in which they were more abundant. Harbour seals consumed a wide size range of prey that also varied by species, season, and region. These results indicate that harbour seal foraging is influenced by both the local abundance and composition of prey. Attempts to model the impact of predation on species of concern risk bias in their estimates and underrepresenting uncertainty if spatial and temporal variation in the diet is not accounted for.