Marine mammals stranded on the Belgian and Dutch coasts: Approach of their feeding ecology by stable isotope and heavy metal measurements

The harbour porpoise Phocoena phocoena, the harbour seal Phoca vitulina and the white-beaked dolphin Lagenorhynchus albirostris are commonly found stranded on the Belgian and Dutch coasts. More occasionally, oceanic species such as the white-sided dolphin Lagenorhynchus acutus, the sperm whale Physeter macrocephalus or the fin whale Balaenoptera physalus are also found stranded. For the last 10 years, trace analyses (Cd, Pb, Cu, Zn, Se, Hg) have been performed in their organs as well as complementary stable isotope ratio determinations (d15N and d13C) in order to trace their respective metal level and trophic position.
Sperm whales, fin whales and white-sided dolphins display strong differences in their d13C and d15N values compared to harbour porpoises, harbour seals or white-beaked dolphins. The isotopic data from white-sided dolphins, sperm whales and fin whales suggest that they do not feed mainly within this area. On the contrary, porpoises, seals and white-beaked dolphins display close isotopic data reflecting a similar nutrition within the North Sea. However significant differences appear between these species: harbour porpoises display a significantly lower d15N (mean: 16.4 0/00 against 18.70/00 and 18.8 0/00 respectively) and d3C (mean-17 0/00 against –16 0/00 and –15.70/00 respectively) than harbour seals and white-beaked dolphins reflecting a lower trophic position of the porpoises.Moreover, Harbour porpoise isotopic compositions seem to have changed over the last five years, which could reflect a switch in their diet.
In marine mammals, heavy metals are mainly absorbed through the diet and so potential relations between stable isotopes and heavy metals have been investigated. For example, it is suggested that harbour porpoises occasionally feed on cadmium contaminated preys as squids and that harbour seals and white-beaked dolphins are more restrictive on fish.