Diet of harp seals (Pagophilus groenlandicus) in nearshore northeast Newfoundland:: Inferences from stable-carbon (δ13C) and nitrogen (δ15N) isotope analyses

Trophic position, and often the source of feeding of predators in food webs, can be estimated using measurements of stable isotope ratios of nitrogen and carbon in predators and their prey. Muscle samples from 60 harp seals (Pagophilus groenlandicus) collected during May 1995 in nearshore waters of Newfoundland, Canada, were analyzed for delta(13)C and delta(15)N values. These values were compared with those for 63 prey samples representing seven species generally collected near the same area. Using diet-tissue isotopic fractionation factors derived from previous studies using captive animals, we infer a greater dependence of harp seals on lower trophic-level prey during April compared with results expected from exclusive diets of Atlantic cod (Gadus morhua), Atlantic herring (Clupea harengus), Greenland halibut (Reinhardtius hippoglossoides), or northern shrimp (Pandalus borealis). Our mean delta(15)N value for harp seals is lower than previous findings for seals collected on the winter whelping parch and may be a function of interannual or seasonal differences in diet. Subadult seals (aged 1-4 yr) had significantly lower delta(15)N values than adults (5+ yr), suggesting that older seals were feeding at a slightly higher trophic level.