Dietary shifts and energetics of Atlantic bluefin tuna Thunnus thynnus in the Gulf of Maine

In recent decades, abundance, spatial distribution, and physical condition of Atlantic bluefin tuna Thunnus thynnus (ABFT) have shifted, in part due to changes in the composition, distribution, and condition of available prey. Dietary studies conducted in the Gulf of Maine (GOM) over the past 30 yr have identified lipid-rich Atlantic herring Clupea harengus (‘herring’) as the primary ABFT prey, but recent stock assessments indicate poor spawning stock biomass and recruitment. Given these declines in abundance, we hypothesized that ABFT diet dominance would shift to an alternative, more abundant prey. Stomach content analysis from 379 ABFT collected in the GOM in 2018 and 2019 showed a reduced reliance on herring and an increased consumption of northern shortfin squid Illex illecebrosus , the dominant prey in frequency of occurrence (56.3%) and weight (23.0%). Atlantic mackerel Scomber scombrus was a secondary prey with similar presence (35.7%) in stomach samples as herring (23.5%). Other clupeids, such as Atlantic menhaden Brevoortia tyrannus , alewives Alosa pseudoharengus , and blueback herring A. aestivalis , were more prevalent relative to previous studies, with menhaden and northern shortfin squid co-dominating the primary prey in 2019. ABFT condition increased seasonally in the GOM and was significantly greater than in ABFT from neighboring foraging habitats where herring were the dominant prey. Our results document a historic diet shift on primary seasonal foraging grounds and suggest that continued monitoring of ABFT diet and prey energetics are essential to understanding future ABFT presence and condition in the GOM.