Dietary evidence of mesopelagic and pelagic foraging by atlantic bluefin tuna (Thunnus thynnus L.) during autumn migrations to the Iceland Basin

Abstract

Atlantic bluefin tuna (ABFT; Thunnus thynnus) is a large highly mobile predator fish species in the North Atlantic Ocean and Mediterranean Sea. Knowledge of its trophic role in marine food webs in summer feeding areas is presently based on recent (1980-2010s) sampling in the Bay of Biscay, Gulfs of Maine and St. Lawrence, and from historical (1950-1960s) sampling in the Norwegian-North Sea-Kattegat. No study has yet investigated the diets of ABFT in Icelandic waters, where it supported an experimental fishery during 1996-2005, nor in any region north of the Bay of Biscay since the 1960s. However, north Atlantic temperatures and fish species distributions, including some ABFT prey species (e.g., mackerel) have been changing in the 2000s. New knowledge of ABFT diets in previously understudied parts of the species range will be useful for understanding factors affecting the trophic role, migration behavior, and bioenergetics of ABFT. Here, we report the dietary composition of ABFT during autumn migrations to the Iceland Basin south of the continental shelf of Iceland. A total of 36 prey species or higher taxa were observed in 421 stomach samples: 17 teleost fishes, 4 squid, 1 octopus, 12 crustaceans, and 2 other invertebrate species. The most important prey species were European flying squid (Todarodes sagittatus) (%N = 16.70, %W = 48.89; %FO = 87.65), barracudinas (Paralepididae) (%N = 14.05, %W = 28.59, %FO = 76.48), and gonate squid (Gonatus sp.) (%N = 9.17, %W = 7.85, %FO = 75.06). Prey sizes were highly variable relative to ABFT sizes indicating highly opportunistic feeding on diverse sizes. The presence of a large proportion of mesopelagic species in the diet indicates feeding in the mesopelagic layer and extensive dive behavior. These results give new baseline knowledge for future comparison with anticipated oceanographic-biological changes in the region in the coming decades and can be used to help parameterize new models of ABFT migration behavior and trophic role.