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 an d the question arises how predators requiring similar diet in the same habi tat share their environmental resources. As isotopic composition of an anim al is related to that of its preys, stable isotope (C-13/C-12 and (15)/N-14 ) analyses were performed in three predator species from the North-east Atl antic: the striped dolphin, Stenella coeruleoalba, the common dolphin Delph inus 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 ti ssues of the animals. Tuna muscles display higher delta(15)N than in common and striped dolphins (mean: 11.4, s. 10.3 parts per thousand and 10.4 part s per thousand respectively) which reflects their higher trophic level nutr ition. Higher delta(13)C are found in common (-18.4 parts per thousand) and striped dolphin (-18.1 parts per thousand) muscles than in albacore tuna ( -19.3 parts per thousand) probably in relation with its migratory pattern. The most striking feature is the presence of two levels of cadmium concentr ations 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 the ir delta(15)N and delta(13)C liver values. These results suggest that in th e Biscay Bay, tunas occupy two different ecological niches probably based o n different squid inputs in their diet. (C) 2000 Elsevier Science Ltd. All rights reserved.