Population structure and foraging ecology of juvenile black sea bass, tautog, and cunner in a southern New England estuary: Habitat and trophic niche overlap amid a species geographic range expansion.

Juvenile black sea bass populations are increasing in southern New England estuaries of the northeastern United States with possible trophodynamic effects on native labrid species: tautog and cunner . In this study, we examined the abundance, size-structure, and feeding ecology of juvenile sea bass and labrids in Narragansett Bay (Rhode Island, USA) from June to October in 2018 and 2019, and evaluated potential interspecific competition using habitat, diet, and isotopic niche overlap indices. Juveniles (age-0+) of each fish species spatiotemporally co-occurred in shallow, polyhaline habitats throughout the estuary. Age-0 cohorts were numerically dominant across species and initially recruited to the study area in July (minimum total length, TL = 15-20 mm), reached peak abundances in mid- to late summer (maximum abundances = 12-140 fish 100 m), and emigrated from field sites in October (maximum TL = 85-96 mm). Stomach content and stable isotope (δN and δC) analyses determined that focal fishes were generalist carnivores that foraged in bentho-pelagic food webs. Focal fishes also overlapped in their respective ambit and core isotopic niche space, but interspecific dietary overlap was size-dependent. Dietary overlap was most pronounced between sea bass ≤ 49 mm TL and labrids ≤ 59 mm TL due to their mutual reliance on gammarid and caprellid amphipods. Ontogenetic dietary shifts then resulted in food niche segregation, such that sea bass 50-139 mm TL consumed decapod crustaceans and fish, whereas labrids 60-181 mm TL mainly fed on amphipods, worms, bivalves, and turf-associated biota, i.e. bryozoans, hydroids, and seaweeds. Habitat and trophic niche overlap between early juvenile sea bass and labrids have not caused opposite population trajectories or food resource limitation in Narragansett Bay, indicating an absence of exploitative competition. Alternatively, realized competition in focal fishes may be too spatiotemporally restricted to alter their population dynamics. Continuous monitoring of the sea bass's bio-ecological effects in southern New England estuaries is warranted, however, given their recent range expansion and increasing abundances at northern latitudes.