Cephalopods in the diet of young-of-the-year bluefin tuna (Thunnus thynnusL. 1758, Pisces: Scombridae) from the southern Tyrrhenian Sea (central Mediterranean Sea)

Abstract

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.

Growth-related trophic changes of Thunnus thynnus as evidenced by stable nitrogen isotopic values in the first dorsal spine

“…21,31,32 ). In the first life stages (FL in the range 17-48 cm), the low δ 15 N values (mean 5.1 ± 1.4‰) recorded in the YOY tuna specimens may be due to a diet starting on planktonic copepods and euphausiids, that could provide a high-energy food source for juvenile bluefin tuna during periods of rapid growth, and subsequently on small cephalopods 22,30,31,33,34 . Zooplankton may represent an overlooked prey base for YOY bluefin tuna in the Mediterranean Sea but information on the diet of such early juveniles is scanty (e.g.…”

“…Zooplankton may represent an overlooked prey base for YOY bluefin tuna in the Mediterranean Sea but information on the diet of such early juveniles is scanty (e.g. 22,30 ). Moreover, at the best of our knowledge, this is the first study evaluating the relationship between δ 15 N values and Fork Length (FL) in the Young Of Year (YOY) bluefin tuna.…”

“…The bluefin tuna, Thunnus thynnus, is a highly migratory and long-living fish at the top of the pelagic food web and plays a key role in the stability of marine food webs by exerting top-down control on its prey 21 . It feeds on a broad spectrum of prey items, such as small pelagic fish, cephalopods and shrimps [22][23][24][25] ; furthermore, in some Mediterranean and Atlantic feeding grounds, the component of mesopelagic fish is dominant in its diet 26,27 . However, the diet composition of bluefin tuna varies in relation to its growth, seasons and migratory patterns, making it difficult to evaluate spatial and temporal effects.…”

Vii Foro Iberoamericano De Los Recursos Marinos Y La Acuicultura

“…Aunque la longitud furcal de los ejemplares estudiados fue de 131,73±16,32 cm, un valor significativamente superior a los individuos utilizados para este estudio, las presas mayoritarias fueron Engraulis encrasicolus y Sardina pilchardus, mientras que de entre los cefalópodos destacó la dominancia de Illex coindietii. Sin embargo, Battaglia et al, (2013), en su estudio sobre los cefalópodos como presa de juveniles de edad 0+ de atún rojo atlántico en el sur del Mar Tirreno obtuvo Onychoteuthis banksii y Heteroteuthis dispar como las especies con mayor índice de ocurrencia. Según los resultados obtenidos en el análisis de contenidos estomacales, las diferencias encontradas en la composición de la dieta del atún rojo atlántico de edad 0+ indican la existencia de una variabilidad tanto interanual como espacial en la dieta, la cual depende de la abundancia de presas potenciales en el hábitat explotado (Salman y Karakulak, 2009).…”

Importance of the Lunar Cycle on Mesopelagic Foraging by Atlantic Bluefin Tuna in the Upwelling Area of the Strait of Messina (Central Mediterranean Sea)

“…However, the extent of these excursions differs by species, and, according to current knowledge, the deepest dives are found in Xiphias gladius and Thunnus obesus , which probably have a more specialized visual system for dark environment (e.g., [ 26 ]). Recently, some authors observed high percentages of mesopelagic fish and cephalopod prey in the stomach content of Atlantic bluefin tuna (BFT), Thunnus thynnus , in different foraging areas of the Atlantic Ocean [ 27 ] and the Mediterranean Sea [ 23 , 25 , 28 , 29 ]; according to these studies, as well as tagging experiments [ 19 , 20 ], BFT is able to perform vertical excursions to match its food resources and feed on mesopelagic prey. The chance to encounter mesopelagic prey might increase during nighttime, when these species perform nycthemeral migrations (from deep waters toward upper layers).…”