标题：Cephalopods in the diet of young-of-the-year bluefin tuna (Thunnus thynnus L. 1758, Pisces: Scombridae) from the southern Tyrrhenian Sea (central Mediterranean Sea)
摘要：Information on distribution and abundance of cephalopods inhabiting pelagic water masses is often not available because of the difficulty of applying conventional methods in this remote habitat. An alternative source of information is represented by stomach content analysis of pelagic predators that are known to be efficient cephalopod prey collectors. This study focuses on the occurrence of cephalopod prey in the diet of young-of-the-year bluefin tuna (Thunnus thynnus L. 1758, Pisces: Scombridae) in the southern Tyrrhenian Sea (central Mediterranean). Overall, 139 fishes were sampled by trolling lines or purse-seine in a Fish Aggregating Devices (FADs) area and as by-catch of the purse-seine sardine fishery, from 1998 to 2000 between July and November. Cephalopods found in the stomach contents were identified to the lowest possible taxon and, when digestion was too advanced, beaks from stomachs were considered as the best taxonomic feature for classification. Of a total of 1233 cephalopod prey, 1144 individuals were identified, belonging to 15 species. Onychoteuthis banksii was the most common prey (%N = 58.39), followed by Heteroteuthis dispar (%N = 23.03). This importance was confirmed also in terms of biomass with O. banksii and H. dispar that respectively reached 39.41% and 21.23% in weight, followed by Todarodes sagittatus (%W = 20.16). In general, young-of-the-year bluefin tunas are able to feed on a large spectrum of cephalopod prey in the study area, usually relying on juvenile specimens and epipelagic and mesopelagic shoaling species. In fact, the identified preys mainly belong to muscular squids (71%) and sepiolids (25%), but pelagic octopuses, buoyant squids and juvenile demersal octopuses were also recorded. These data provide further information on the feeding habits of T. thynnus, relative to a poorly investigated life stage. The improving knowledge on trophic relationships of young tunas and their reliance on cephalopod taxa is an important step in supporting management of this fishery resource.
关键词：Thunnus thynnus; mesopelagic diversity; stomach contents; cephalopods; central Mediterranean Sea
