Wabash River Freshwater Drum Aplodinotus grunniens Diet: Effects of Body Size, Sex, and River Gradient

The objectives of this study were to describe the diets of freshwater drum Aplodinotus grunniens in the Wabash River in the Midwestern United States. We used a multivariate ordination approach (nonmetric multidimensional scaling) to describe drum diets combined with a generalized linear model to test for covariation of diet with body size, sex, and longitudinal river gradient. Hydropsychidae (trichoptera, caddisfly larvae), pleuroceridae (gastropoda), and heptageniidae (ephemeroptera, mayfly larvae) were the most consumed prey items (∼75% of overall diets). Among all freshwater drum, hydropsychidae, pleuroceridae, and heptageniidae were present in 69%, 23%, and 38% of stomachs, respectively. Freshwater drum diets were similar along an upstream–downstream river gradient spanning 350 river km, but varied with body size and sex. Small- and medium-sized fish tended to consume more diptera and annelids compared with the largest individuals, which fed on mollusks and crayfish. With control for body size, the diets of male individuals were composed of more diptera (chironomidae) and annelid prey items compared with female individuals, whose diets included more molluscs and crayfish. Overall, we interpret the lack of diet diversity in freshwater drum with Wabash River longitudinal gradient as evidence of diet specialization. Alternatively, we propose that a potential dietary–river-gradient signal may be diluted as a function of increased freshwater drum longitudinal movements.