Diet differences among age classes of Arctic seals: evidence from stable isotope and mercury biomarkers

A basic understanding of current food web dynamics and baseline data from which to measure future change is necessary to understand species re-distribution and altered competition for food with climate change. We use mercury (Hg) and carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios as biomarkers to understand species diet differences and age class differences among ringed (Phoca hispida), bearded (Erignathus barbatus), and harbour (P. vitulina) seals in a subarctic marine ecosystem. Adult bearded seals had significantly lower δ¹⁵N and muscle Hg than bearded seal pups, whereas the opposite was observed in ringed seals where pups had lower δ¹⁵N than adults, suggesting age specific foraging differences in trophic food level for both species. For harbour seals, δ¹⁵N did not differ significantly among ages while Hg and δ¹³C did. The δ¹³C in muscle supports that bearded seals in this study are benthic feeders and are part of a separate food web from ringed seals and harbour seals. Harbour seals had the highest levels of mercury and δ¹⁵N, indicating they feed at a higher trophic level relative to the other two seal species. Carbon and nitrogen isotopic ratios and Hg levels illustrate how resources are partitioned among three seal species and offer evidence for separation based on life stages within species.