Assessment of trophic dynamics of cephalopods and large pelagic fishes in the central North Atlantic Ocean using stable isotope analysis

Abstract

Pelagic ecosystems in the central North Atlantic Ocean support numerous commercially-exploited tuna, shark, and billfish species, which rely largely on cephalopod as well as fish and crustacean prey. Carbon and nitrogen stable isotope analyses were performed on tuna and billfish predators as well as cephalopod prey species sampled during two research longline cruises (2001–02) to study their trophic structure. Nitrogen stable isotope (δ15N) analyses revealed similarity in trophic position (TP) among sampled fish predator species, with large swordfish occupying the highest TP. Species with wider vertical distributions (swordfish and bigeye tuna) had higher δ15N values than species more constrained to the epipelagic zone (yellowfin tuna and dolphinfish). Analysis of tissue nitrogen isotope values showed an ontogenetic increase for swordfish and white marlin but no effects for other sampled fish species. For cephalopods as a group, δ15N increased with size. Smaller cephalopods sampled in this study had δ15N values that were about one TP below co-occurring tunas and billfishes, confirming their importance as a prey resource. Larger cephalopods had similar δ15N values to tunas and billfishes, indicating that these large cephalopods occupy a comparable TP to their fish predators. Both carbon and nitrogen stable isotope values of large pelagic fishes showed spatial gradients relative to conspecifics analyzed in coastal regions, which can be used to trace large scale movements.