Marine mammals from the North Sea: Approach of their feeding ecology through stable isotope and cadmium measurements

The fertile waters of the North Sea represent one major life site for at least three different marine mammal species: the harbour porpoise, Phocoena phocoena , the harbour seal, Phoca vitulina, and the white-beaked dolphin, Lagenorhynchus albirostris. More occasionally, oceanic species such as the white-sided dolphin Lagenorhynchus acutus, the sperm whale Physeter macrocephalus or the fin whale Balaenoptera physalus can be sighted or found stranded. Very few data dealing with marine mammal diet in the North Sea are available. In order to contribute to the feeding ecology of these North Sea marine mammals, Cd as well as 15N and 13C measurements have been performed in their tissues. Sperm whales and white-sided dolphins display high renal Cd levels (258 µg.g-1 and 88 µg.g-1 dry weight respectively). Mean Cd concentrations measured in harbour porpoise, harbour seal, white beaked dolphin and fin whale kidneys remain low (mean: 5, 0.9, 0.5 and 4 µg.g-1 respectively). However, some adult porpoises display renal Cd concentrations higher than 10 µg.g-1. Mean 13C measured in the muscles of sperm whales, fin whales and white-sided dolphins are more negative compared to harbour porpoise, harbour seal or white-beaked dolphin mean value. Cd contaminated porpoises display significant lower muscle 13C than other porpoises. Both low d13C and high Cd levels suggest a greater reliance of white-sided dolphins, sperm whales on oceanic cephalopods. It appears that some harbour porpoises can also feed on oceanic squids suggesting dynamic interactions between the North Sea and the North Atlantic porpoise populations