Marine mammals from the southern North Sea: feeding ecology data from δ13C and δ15N measurements

Abstract

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 δ 13 C and δ 15 N in their muscles as well as in 49 invertebrate and fish species collected from the southern North Sea. The δ 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 19.2‰ respectively). The harbour porpoise occupies a slightly lower trophic position (mean δ 15 N value of 16.2 ‰), reflecting a higher amount of zooplanktivorous fishes in its diet (mean δ 15 N of 14.7‰); 2 suckling harbour porpoises displayed a significant δ 15 N enrichment of 2.2‰ compared to adult females. Adult females are δ 15 N-enriched compared to adult male harbour porpoises. Fin whales, sperm whales and white-sided dolphins are 13 C-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.