DIET AND FEEDING PERIODICITY OF RUFFE IN THE ST-LOUIS RIVER ESTUARY, LAKE-SUPERIOR

Ruffe Gymnocephalus cernuus, a percid native to Europe and Asia, is es tablished in the Lake Superior drainage and could have negative impact s on native fish through competition for forage and predation on fish eggs. We investigated the diet of ruffes in the 4,654-ha St. Louis Riv er estuary in May-October 1989-1990 and the feeding periodicity of ruf fes in two adjacent habitats during five 24-h periods in summers 1990- 1991. Ruffes were primarily benthophagous. Age-0 ruffes fed mostly on cladocerans and copepods in early summer and midge larvae (Chironomida e) in late summer and fall. Adult ruffes less than 12 cm fed mostly on midges and other macrobenthos but also consumed large numbers of micr ocrustaceans. Adult ruffes 12 cm and larger fed mostly on midges, burr owing mayflies Hexagenia spp., and caddisflies (Trichoptera). Ruffes c onsumed few fish eggs. Adult ruffes in deeper waters and all age-0 ruf fes fed throughout the day as indicated by weight patterns of stomach contents. However, adult ruffes generally moved to shallower waters al night to feed most heavily. Results of this study indicate that ruffe s will probably compete with other benthic-feeding fishes such as yell ow perch Perca flavescens and trout-perch Percopsis omiscomaycus.