Characterizing the diet of Atlantic bluefin tuna Thunnus thynnus across its spatial range: a metadata analysis spanning over three decades

Atlantic bluefin tuna Thunnus thynnus (ABFT) are highly migratory predators that feed across diverse foraging habitats throughout their range. ABFT diet has been characterized regionally, but a broader analysis is needed to help inform ecosystem-based management of these economically and ecologically important predators, given their environmental and spatial plasticity. We conducted a metadata analysis using assembled diet data from 1985-2020 (n = 4997 total stomachs; 4046 non-empty stomachs) for ABFT (6-328 cm straight fork length, SFL) from 9 Longhurst Provinces (LPs). ABFT trophic ecology was characterized by exploring relationships between (1) diet composition (using classification trees), (2) total stomach content weight (using generalized additive models, GAMs), and (3) predator-prey size (using quantile regression and GAMs) and spatial, temporal, biological, and environmental variables. Diet composition primarily differed by sampling year, LP, and ABFT SFL. Diet over the Northwest Atlantic shelf was distinct due to the prevalence of Clupeidae. Temporal splits in this region were based on consumption of Ammodytidae and Ommastrephidae in the earliest and latest years of our data set, respectively. Diet composition for the other LPs primarily differed based on SFL, with smaller ABFT (<79 cm) mainly consuming engraulids and larger ABFT (≥79 cm) consuming ommastrephids and paralepidids. Total stomach content weight and prey length increased asymptotically with ABFT SFL and varied in relation to environmental, sampling, and spatial variables. Quantifying diet composition by space and ontogeny is useful for developing improved food web models to better understand ecosystem effects of fisheries. Our metadata analysis also highlights future research priorities.