Combining stable isotope and intestinal parasite information to evaluate dietary differences between individual ringed seals (Phoca hispida botnica)

The diet and foraging behaviour of nine individual Baltic ringed seals (Phoca hispida botnica Gmelin, 1785) in the Bothnian Bay were studied by combining results from stable isotope analyses (delta C-13 and delta N-15) with data on intestinal parasites whose occurrence varied among the fish hosts. The patterns of infection with three acanthocephalan parasites, Corynosoma semerme (Forssell, 1904), Corynosoma magdaleni Montreuil, 1958, and Corynosoma strumosum (Rudolphi, 1802), and with a cestode larva, Schistocephalus solidus (Muller, 1776), were examined. The ringed seals become infected with these intestinal parasites by feeding on the fish hosts and hence have different parasite species and different parasite burdens according to their dietary history. delta C-13 and delta N-15 values were determined from diaphragm muscle of the seals and from tissues of potential prey items. A dual isotope plot of delta C-13 and delta N-15 values for individual seals and mean values for key potential prey species, together with the parasitological data from the seals, allowed inferences to be drawn about the feeding of individual seals. It appeared that two seals foraged particularly on fourhorn sculpin (Myoxocephalus quadricornis (L., 1758)) and one seal consumed a high proportion of the isopod Saduria entomon (L., 1758). Three seals apparently preferred coastal benthic prey in their diets, while two other seals fed more on pelagic herring (Clupea harengus membras L., 1761). One older female seal evidently also fed on salmon (Salmo salar L., 1758).