Exploring the diet of harbour seals (Phoca vitulina)

Abstract

Changes in fish species abundance in the marine environment, can influence the prey composition of harbour seal diet and ultimately cause a decline in somatic growth and pregnancy rates. Thus, diet analysis enables early detection of changes in the marine ecosystem with consequences for seal health. The present study aims to assess both the overall diet of harbour seals and to elaborate several statistical methods to describe what we can learn from different methods of analysis. In particular, the variation in consumption is compared when the samples are treated individually (per sample) or at the population level (pooled samples). Diet composition was determined by identifying remains of the prey species in scat samples. To assess the variance in diet composition of individual samples, the data was tested for Local Contribution to Beta Diversity. To further explore differences among scats, two different distances were applied, the Jaccard similarity and the Bray-Curtis dissimilarity. Wrasse, Trisopterus spp., haddock/pollack/saithe, flatfish and cod were shown to be the most important prey species when accounting for abundance and haddock/pollack/saithe, wrasse, cod, Trisopterus spp. and flatfish when accounting for biomass. The results showed moderate to high variation in diet composition among scat samples, and the data treatment (individual samples or pooled) did affect the results significantly regarding prey species abundance estimated to be consumed by seals. However, when the biomass of prey species was estimated, the two methods produced similar results regarding the importance of different fish prey species in the diet. The results imply that former studies treating diet samples pooled can be used for comparisons with recent samples, when it comes to relative importance regarding biomass in the seal diet. The result showed that herring, which historically was highly important in the diet of harbour seals, showed low abundance in the present study. This could be explained by their migratory patterns or the decline in the fish stocks due to overfishing. The variance in prey species composition among scats can indicate that harbour seals show individual habitat- or prey species preferences or random events in encountering prey while foraging.