Diet of larval ruffe (Gymnocephalus cernuus) in the St. Louis River Harbor, Lake Superior

The diet of larval (3-17 mm total length) ruffe (Gymnocephalus cernuus) is described from samples collected in 1994 at two locations in the St. Louis River Harbor, a tributary to Lake Superior. Copepoda, Daphnia spp., and Bosmina longirostris dominated the diet of larval ruffe in the St. Louis River Harbor. Larger Copepoda and Daphnia spp. occurred more often and in larger numbers as ruffe total length increased, whereas smaller Bosmina longirostris occurred less often and in smaller numbers as ruffe total length increased. Ruffe from Whaleback Bay consumed Daphnia spp. rarely and in very small numbers whereas ruffe from Allouez Bay consumed Daphnia spp. often and in large numbers. A general decrease in Copepoda and increase in Daphnia spp. occurred in the second and third weeks of June for ruffe from Allouez Bay, but not Whaleback Bay.