Seasonal diet shifts and trophic position of an invasive cyprinid, the rudd Scardinius erythrophthalmus (Linnaeus, 1758), in the upper Niagara River

Dietary shifts of invasive rudds Scardinius erythrophthalmus (Linnaeus, 1758) and food web structure of the upper Niagara River were examined. Stable isotope analysis (SIA) of liver tissue was used to test the hypothesis that rudds shifted their diets from piscivory during early spring months when macrophyte availability was low towards herbivory when macrophytes were abundant and warmer water temperatures facilitated digestion. Muscle tissue was used to evaluate the trophic position of rudds and other invasive species relative to native species. SIA revealed enriched delta N-15 and depleted delta C-13 in liver tissue of rudds during early spring months, suggesting a mostly piscivorous diet of pelagic origin when macrophyte availability was low, and depressed delta N-15 and elevated delta C-13 values during warmer summer months when littoral macrophytes were abundant. Analysis of muscle tissue from late summer indicated that rudds and other invasive fishes, including common carp Cyprinus carpio (Linnaeus, 1758) and goldfish Carassius auratus (Linnaeus, 1758), had similar trophic positions that may be attributed to their omnivorous feeding strategies. The ability of rudds to shift their diets from feeding on fishes of pelagic origin towards consuming littoral macrophytes is an adaptation that is likely to both facilitate invasion and create novel pathways of nutrient transfer among habitat types. Our results provide an increased understanding of the feeding ecology of the rudd and the role of this invasive species in the food web of the upper Niagara River.