Diet and consumption rate of Atlantic bluefin tuna (Thunnus thynnus) in the Strait of Gibraltar

- The diet of bluefin tuna was assessed by stomach content and stable isotope analyses.
- The Strait of Gibraltar (SoG) provides enough resources for resident bluefin tuna.
- Unlike most foraging grounds, crustaceans are the most important prey in the SoG.
- Inter-annual differences were found in trophic niche width, and daily meal and ration.

The diet of Atlantic bluefin tuna (ABFT), Thunnus thynnus, caught in the Strait of Gibraltar area (SoG) from late summer through fall was assessed by stomach content (SCA) and stable isotope (SIA) analyses. Our results show that the SoG provides suitable food resources for sub-adult and adult ABFT. The SCA did not show inter-annual differences, and indicated that crustaceans were the most important prey group (alimentary index, AI = 37.47 and AI = 47.22 for 2012 and 2013, respectively), followed by fishes (AI = 28.71 and AI = 25.62, respectively) and cephalopods (AI = 1.55 and AI = 3.03). Diet compositions inferred from mixing models based on muscle and liver isotopic data showed that squid are an important food resource for the ABFT living in the SoG. Feeding patterns were analysed by the graphical method of Amundsen, which suggested that the ABFT exhibits a varying degree of specialisation on different prey. Thus, while most prey species were unimportant and rare, Sergia robusta can be considered as the dominant prey. The trophic niche width, estimated from SCA as the antilogarithm of Shannon's entropy, as well as from stable isotope Bayesian ellipses, indicated that the ABFT sampled in 2012 consumed a more diverse diet than those sampled in 2013. Inter-annual differences in daily meal and daily ration were apparent; whereas the estimated daily food intake was 918.91 g day−1 in 2012 and 1924.95 g day−1 in 2013, the daily ration showed values of 2.52 ± 1.24% body mass (BM) day−1 and 5.84 ± 1.06% BM day−1. Such substantial differences are probably due to differences in water temperatures between the sampling dates of the two years.