Tuna and dolphin associations in the North-east Atlantic: evidence of different ecological niches from stable isotope and heavy metal measurements

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 isotope (13C/12C and 15N/14N) analyses were performed in three predator species from the North-east 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. Tuna muscles display higher d15N than in common and striped dolphins (mean: 11.4‰ vs. 10.3‰ and 10.4‰, respectively) which reflects their higher trophic level nutrition. Higher d13C are found in common (-18.4‰) and striped dolphin (-18.1‰) muscles than in albacore tuna (-19.3‰) probably in relation with its migratory pattern. The most striking feature is the presence of two levels of cadmium concentrations in the livers of the tunas (32 mg kg-1 dry weight (DW) vs. 5 mg kg-1 DW). These two groups also differ by their iron concentrations and their d15N and d13C liver values. These results suggest that in the Biscay Bay, tunas occupy two different ecological niches probably based on different squid inputs in their diet