Ontogenetic shifts in isotope-inferred trophic ecology of Atlantic bluefin tuna Thunnus thynnus in Atlantic Canada

ABSTRACT: Upper trophic level predators can greatly influence the dynamics and productivity of forage fish species. Quantifying this influence requires information on prey consumption; however, establishing feeding habits of highly mobile predators is particularly challenging. Stable isotopes of carbon and nitrogen (δ¹³C and δ¹⁵N) have been applied to characterize the trophic ecology of Atlantic bluefin tuna Thunnus thynnus (ABFT) in several important regions; however, applications in Canada are lacking. Here, we used δ¹³C and δ¹⁵N values of ABFT muscle tissue collected in 2014-2018 on 2 important foraging grounds along the coast of Atlantic Canada to evaluate the temporally integrated trophic ecology of this ecologically and commercially important species. Populations of some small pelagic fish species in these areas have been depleted, and predation by ABFT is considered a potentially important contributor. Isotopic diet reconstructions found that Atlantic mackerel were the dominant prey in the southern Gulf of St. Lawrence, and a combination of Atlantic herring and sandlance were the most dominant prey consumed on the Scotian Shelf. Diet reconstructions identified an ontogenetic shift in prey consumption, with more sandlance consumed by smaller ABFT and a shift to higher consumption of Atlantic mackerel and Atlantic herring as fish increased in size. Isotopic niche overlap indicated relatively high overlap for adults among years, suggesting that ABFT did not show substantial inter-annual variation in their trophic ecology. Overall, the study provides insight into the trophic ecology of ABFT along the Atlantic Coast of Canada that can be integrated into investigations of predator impacts on depleted mackerel and herring populations.