Specialized Feeding Patterns and Marine Resource Use by Nonnative Catfishes in a Coastal River Ecosystem Revealed by Dietary and Stable Isotopic Analyses

Two large predatory catfishes, the Flathead Catfish Pylodictis olivaris and Blue Catfish Ictalurus furcatus, have been introduced into multiple U.S. Atlantic slope drainages in recent decades, with concomitant reductions in native fish abundance and changes in faunal community composition. Here, we report on a study of the trophic ecology of sympatric Flathead Catfish and Blue Catfish in the lower Cape Fear River, North Carolina, to estimate trophic positions and identify possible impacts on marine and estuarine prey resources. We applied both dietary and stable isotope analysis to quantify species‐specific foraging patterns, as well as the extent of variation among tributaries, seasonal periods, and ontogeny. Both catfish species displayed limited dietary breadth and specialized feeding patterns focused on prey resources within a restricted range of trophic levels. Dietary overlap between Blue Catfish and Flathead Catfish was low, and the species were separated by more than one trophic position. Flathead Catfish relied heavily on fish prey throughout ontogeny, with macroinvertebrates (crustaceans) as a consistent secondary prey resource, and showed increased reliance on potential anadromous prey resources at the largest sizes. Blue Catfish diets were dominated by the invasive Asian clam Corbicula fluminea, with contributions from fish prey only increasing for the largest individuals. Catfish trophic ecology varied among tributaries, and to a greater extent for Flathead Catfish, in terms of both the fish fauna eaten and the relative contribution of different invertebrate prey. Isotopic signatures for Flathead Catfish aligned with stomach contents to indicate a greater contribution of marine prey resources, including anadromous fishes, in the main‐stem Cape Fear River compared with the Black and Northeast Cape Fear rivers. Our findings indicate that the predatory impacts of catfishes introduced to coastal river ecosystems will differ considerably between catfish species and may also vary within systems across fine spatial scales.