Insights Into the Feeding Ecology of Young-of-the-Year Bigeye Tuna (Thunnus obesus) in the Southwestern Atlantic Ocean

Bigeye tuna (Thunnus obesus) is among the most prized tuna species sustaining fisheries worldwide. However, information on the feeding habits for the species is mostly restricted to the size range harvested by commercial fisheries. In this study, we report the diet composition of juvenile bigeye tuna schooling near the surface at night in the oceanic area of the southwestern Atlantic Ocean. We combined short-term dietary information derived from stomach content analysis (n = 14) with time-integrated diet information from carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analysis (n = 8) to characterize diet composition and estimate trophic position (TP_bulk) of the individuals. The prey-specific index of relative importance (%PSIRI) indicated that Teleostei (79.1%) was the most important taxon, followed by Cephalopoda (7.6%) and Crustacea (3.9%). Myctophidae was the most frequent Teleostei in the diet (frequency of occurrence: 85.7%). The δ¹³C values on juvenile tuna muscle tissue ranged from −18.6‰ to −17.4‰ (mean ± SD: −18.1 ± 0.5‰) and δ¹⁵N values ranged from 7.2‰ to 8.4‰ (mean ± SD: 7.8 ± 0.3‰). TP_bulk (4.15 ± 0.47) results corroborated a diet based on lanternfishes and the predation upon relatively small-sized oceanic squids. Our results address a knowledge gap by highlighting important trophic links for bigeye tuna before they reach a size suitable for exploitation.