Marine mammals from the southern North Sea: feeding ecology data from delta C-13 and delta N-15 measurements

The harbour porpoise Phocoena phocoena, grey seal Halichoerus grypus, harbour seal Phoca vitulina and white-beaked dolphin Lagenorhynchus albirostris are regularly found stranded along southern North Sea coasts. Occasionally, offshore species such as the fin whale Balaenoptera physalus, the white-sided dolphin L. acutus and the sperm whale Physeter macrocephalus are also found stranded. In order to trace their diet, we measured delta(13)C and delta(15)N in their muscles as well as in 49 invertebrate and fish species collected from the southern North Sea. The delta(15)N data indicate that the harbour seal, grey seal and white-beaked dolphin occupy the highest trophic position, along with ichtyophageous fishes such as the cod Gadus morhua (mean muscle values of 18.7, 17.9, 18.8 and 19.2parts per thousand respectively). The harbour porpoise occupies a slightly lower trophic position (mean delta(15)N value of 16.2parts per thousand), reflecting a higher amount of zooplanktivorous fishes in its diet (mean delta(15)N of 14.7parts per thousand); 2 suckling harbour porpoises displayed a significant delta(15)N enrichment of 2.2parts per thousand compared to adult females. Adult females are delta(15)N-enriched compared to adult male harbour porpoises. Fin whales, sperm whales and white-sided dolphins are C-13-depleted compared to southern North Sea particulate organic matter and species, suggesting that despite regular sightings, they do not feed within the southern North Sea area.