Diet and trophic position of Atlantic bluefin tuna (Thunnus thynnus) inferred from stable carbon and nitrogen isotope analysis

Stable C-13 and N-15 isotope analyses of scale, bone, and muscle tissues were used to investigate diet and trophic position of North Atlantic bluefin tuna (Thunnus thynnus Linnaeus) during residency in the northwest Atlantic Ocean off the northeast coast of the United States. Adult bluefin tuna scales collected from fish between June and October 2001 were significantly enriched in C-13 compared to both muscle and bone across all months, while muscle was significantly enriched in N-15 compared to either bone or scale throughout the same period. In muscle tissue, there was evidence of a shift over the summer from prey with delta C-13 values (-17% to -18%) that were characteristic of silver hake (Merluccius bilinearis) to species with delta C-13 values of -20% to -21% that were similar to Atlantic herring (Clupea harengus) and sandlance (Ammodytes americanus). Depletion of N-15 values in adult scales and bone compared to muscle tissue may be explained by bone and scale samples representing juvenile or life-long feeding habits, isotopic routing, or isotopic differences in amino acid composition of the three tissue types. Adult bluefin tuna were estimated to be feeding at a trophic position similar to pelagic sharks in the northwest Atlantic Ocean, while the trophic positions of yellowfin tuna (Thunnus albacares), albacore tuna (Thunnus alalunga), and juvenile bluefin tuna were indicative of a diet of up to a full trophic position below adult bluefin tuna. The close relationship between the juvenile bluefin delta N-15 values and those of suspension feeders suggests that nektonic crustaceans or zooplankton may contribute significantly to the diet of bluefin tuna, a food source previously overlooked for this species in the northwest Atlantic Ocean.