| Summary: | In southern New England (SNE), a new ecological interaction is occurring as temperate black sea bass abundance increases in the home range of the iconic Atlantic cod as a result of climate-induced ocean warming. In this study, we examined the diet and trophic dynamics of both SNE Atlantic cod and black sea bass using a multi-tracer approach of stomach content analysis as well as bulk and compound-specific amino acid isotope analyses. We reveal a highly selective diet (0.01-0.43 Levin’s index) with decapod crustaceans occurring in 82-89% of stomachs and making up 72-79% of the total wet weight for both species, resulting in strong overlap in diet (0.34-0.99 Morisita-Horn index). The most prominent decapod crustacean seen in stomachs were juveniles of the Cancer genus, which has cascading implications for Jonah crab (Cancer borealis) population dynamics and the burgeoning SNE fishery they support. We also reveal high overlap (30-85%) in isotopic niche between species. We show a seasonal shift in ?15N Phe, which reflects changes in regional productivity but also indicates that both species are foraging locally in relatively small ranges relative to cross shelf isotope gradients and in the same locations seasonally. Our trophic position estimations corroborate our diet information and further reveal that cod (3.5-4.1) and black sea bass (3.4-3.5) generally occupy the same trophic position in the food web with high diet selectivity for crustaceans. This work reveals the potential for future shifts in SNE food web dynamics and the fisheries they support, while serving as a case study for future studies in other regions of the world seeking to examine the impacts of climate-driven shifts in species distribution on food web structure and ecological integrity.
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