Competition between Atlantic cod and black sea bass due to warming southern New England waters

The Atlantic cod (Gadus morhua) fishery has played an important role in the ecosystem, economy, and history of the Northwest Atlantic. Broadly speaking, populations of this cold-water predator in the region are at historic lows due to overfishing and environmental stressors, such as rapid ocean warming. However, in the past two decades, southern New England cod fisheries have reported an increase in landings despite the expectation that this subpopulation, at the southern extreme of the species’ global range, would be experiencing increasing thermal stress due to climate change. In addition, warm-water competitors moving northward into the region, such as black sea bass (Centropristis striata), are also a threat as temperatures rise. Based on previous diet analyses, black sea bass are expected to be one of the top competitors with Atlantic cod in southern New England. In this study, we examined the feeding and trophic dynamics of both southern New England Atlantic cod and black sea bass using stomach content analysis, bulk isotope analysis, and compound specific isotope analysis of amino acids (CSIA-AA). Together, these approaches allowed us to examine specific dietary preferences, integrated trophic level, and source nitrogen as a proxy for foraging location along a nearshore-offshore isotopic gradient. We found significant overlap in carbon and nitrogen isotope values for black sea bass and southern New England cod, indicating strong competition between the two species. Both species registered as top predators in the region (trophic level >4) foraging on offshore production. Gut contents suggest strong preferences for decapod crustaceans (e.g., cancer crabs) and shrimp, though cod appeared to rely more heavily on fish (e.g. flatfish and hake) than black sea bass. With Atlantic cod already exhibiting evidence of declining productivity due to rising temperatures, the added stress of warm-water competitors, such as black sea bass, may have negative effects on the southern New England subpopulation and could counteract the population boom seen in recent decades.