Blue crab Callinectes sapidus dietary habits and predation on juvenile winter flounder Pseudopleuronectes americanus in southern New England tidal rivers

Abstract

Blue crabs Callinectes sapidus have expanded their geographic range northward in the NW Atlantic with possible trophodynamic effects on benthic communities. In this study, we examined the blue crab’s diet in 2 southern New England tidal rivers (USA) and expounded on their predator-prey interaction with juvenile winter flounder Pseudopleuronectes americanus. Blue crabs (8-185 mm carapace width [CW]; n = 1835) were collected from the Seekonk River, Rhode Island, and Taunton River, Massachusetts, between May and August 2012 to 2016, and their feeding habits were assessed via stomach content, stable isotope, and molecular genetic analyses. Blue crabs were found to be generalist carnivores-omnivores with diets varying throughout ontogeny, yet shifts in prey composition had no effect on size-based nitrogen isotope signatures and trophic position (3.50 ± 0.35, mean ± SD). Carbon isotope values indicated that detritus-macroalgae were the dominant carbon source to the food web, with additional contributions from terrestrially derived organic matter and phytoplankton in oligohaline and polyhaline waters, respectively. The main prey of blue crabs ≤49 mm CW were amphipods, shrimp, and unidentified crustaceans, and larger conspecifics fed on bivalves, crabs, and fish. Winter flounder remains, e.g. sagittal otoliths, were identified in the diet of 2.5% of field-collected blue crabs, whereas PCR-based assays detected winter flounder DNA in 17.7% of crab stomachs. Blue crabs 23 to 160 mm CW preyed on winter flounder ranging from 26 to 66 mm total length, with occurrences of predation most closely associated with increases in crab size. Blue crab predation on winter flounder also varied spatially in the rivers, reflecting site-specific differences in flounder densities, abundances of other preferred prey, and dissolved oxygen concentrations that altered predator-prey dynamics. Lastly, the current predatory impact of blue crabs on juvenile winter flounder is nearly equivalent to other portunid crab species. Anticipated temperature-mediated increases in blue crab densities at northern latitudes, however, will intensify the predator-induced mortality of winter flounder and likely hinder their recovery in southern New England.