Marine mammals from the North Sea: Insights in their feeding ecology and nutritional status from stable isotopes ratio and heavy metal concentrations

The relationship between trace metal levels, trophic position and health status has been investigated in North Sea marine mammals. Stable isotope ratio (15N and 13C, mass spectrometry) and trace metal analyses (Cd, Pb, Cu, Zn, Se, Hg, I.C.P Spectrometry and flameless atomic absorption) have been performed in tissues of 52 harbour porpoises Phocoena phocoena, 27 harbour seals Phoca vitulina, 9 white-beaked dolphins Lagenorhynchus albirostris, 6 grey seals Halichoerus grypus, 1 hooded seal Cystophora cristata, 2 white-sided dolphin Lagenorhynchus acutus, 7 sperm whales Physeter macrocephalus and 4 fin whales Balaenoptera physalus, found stranded on the Northern French, Belgian and Southern Dutch coasts. The lowest 15N value has been measured in fin whales, and the highest in white-beaked dolphins, grey and common seals, suggesting a higher trophic position. Sperm whales, white-sided dolphins and hooded seal have strongly negative 13C values and high renal cadmium levels (258, 88 and 62 µg Cd .g-1dw, respectively) indicating that they might feed, at least in part, on oceanic cephalopods. High Hg liver concentrations can be encountered in older animals and reflect a cumulative storage of detoxified HgSe over the whole life span rather than a bioamplification process. Zn, Cu and Hg concentrations were higher in harbour porpoises from the Southern North Sea compared to other areas. Some individuals were severely emaciated as shown by their muscle atrophy and reduced blubber thickness. Hepatic Zn and Se concentrations were significantly higher in emaciated juvenile porpoises than in normal animals suggesting a severe disturbance of the metal homeostasis. To conclude, within the North Sea, trace metal levels in marine mammals display strong intra- and interspecies variations due to geographic origin, age, diet, trophic position but also by nutritional status of the individuals, raising the question of the suitability of marine mammals as valuable bioindicators of trace metal pollution.