Strong diet and trophic niche overlap of Atlantic cod Gadus morhua and black sea bass Centropristis striata in warming southern New England waters

In the Northwest Atlantic Ocean, climate warming is driving new ecological interactions as temperate black sea bass Centropristis striata abundance increases in the southernmost extent of boreal Atlantic cod Gadus morhua . We examined dietary and trophic niche overlap of black sea bass (n = 69) and Atlantic cod (n = 67) in southern New England using stomach content analysis and bulk and compound-specific amino acid isotope analyses. Both species exhibited highly selective (Levins’ index: 0.17 ± 0.16) and overlapping (Morisita-Horn index: 0.4 ± 0.5) diets dominated by decapod crustaceans (frequency of occurrence: 82-89% of stomachs, total diet wet weight: 72-79%). Evidence of dietary selectivity for juvenile cancer crabs, like Jonah crab Cancer borealis , may constrain recruitment and increase vulnerability of the emerging Jonah crab fishery in southern New England to predation. Both species exhibited overlap in isotopic niche (Bayesian standard ellipse area overlap: 59 ± 19%) and baseline nitrogen isotope values (Atlantic cod: 7.5 ± 1.7‰; black sea bass: 7.7 ± 1.5‰), coupled with functionally identical trophic positions (Atlantic cod: 3.7 ± 0.4; black sea bass: 3.6 ± 0.2) that corroborate stomach content data, indicating that Atlantic cod and black sea bass were foraging on the same diets over relatively small ranges compared to cross-shelf isotope gradients. Our multi-metric foraging data suggest that the niche overlap between these 2 demersal predators has a variety of potential implications for their top-down control on the structure and function of southern New England ecosystems, as well as the management of fisheries that depend on them.