Trophic position and cadmium content of dolphins and tunas from the Northeast Atlantic

Associations of tunas and dolphins in the wild are quite frequent events and the question arises how predators requiring similar diet in the same habitat share their environmental resources. As isotopic composition of an animal is related to that of its preys, stable isotopes (13C/12C and 15N/14N) analyses were performed in three predator species from the Northeast Atlantic: the striped dolphin, Stenella coeruleoalba, the common dolphin Delphinus delphis, and the albacore tuna, Thunnus alalunga, and compared to their previously described stomach content. Heavy metals (Cd, Zn, Cu and Fe) are mainly transferred through the diet and so, have been determined in the tissues of the animals. Albacore tunas muscles display higher 15N than in common and striped dolphins (mean: 11.4 0/00 vs. 10.3 0/00 and 10.4 0/00, respectively) which reflects its higher trophic level. Higher 13C are found in common (-18.4 0/00) and striped dolphin (-18.10/00) muscles than in albacore tuna (-19.3 0/00). Differences also appear in heavy metal contents, especially cadmium in muscles, lower in albacore tuna than in dolphins (0.37 and 0.1 ppm DW for striped and common dolphins vs.<0.07 ppm DW for albacore tuna) which probably reflects in dolphins a cadmium contamination through squid assimilation.
These results suggest that, despite a close hunting association, the trophic position of these three predators is quite well distinct.