We examined the trophic roles of two nonindigenous species, ruffe (Gymnocephalus cernuus) and white perch (Morone americana), in the food web of a western Lake Superior coastal wetland, using stable isotope techniques. The δ15N signature of ruffe was similar to published values for YOY yellow perch (Perca flavescens), and intermediate to those of white sucker (Catostomus commersoni), a ben-thivore, and alewife (Alosa pseudoharengus), a planktivore. Ruffe of all sizes sampled had an approximately 4%c enrichment in 15N over published values for benthos, and a 3%c 15N enrichment over values for plankton. A 3–4%c difference is consistent with commonly reported shifts in 5I5N signature between food and prey. These results suggest that ruffe in this food web feed on both benthos and plankton. White perch undergo ontogenetic shifts in nitrogen isotope signatures similar to those reported earlier for yellow perch, and appear to become piscivorous by the time they are 25 cm long. Our data suggest that interactions between ruffe and yellow perch could represent a competitive bottleneck. If yellow perch are able to grow large enough to become piscivorous, they should be able to escape competition with ruffe. In contrast, white perch appear to have the potential to compete with yellow perch throughout their lives.
