Growth-Temperature Relation for Young-Of-The-Year Ruffe

The ruffe ( Gymnocephalus cernuus) was accidentally introduced into the Great Lakes basin from Eurasia and has established a breeding population in the St. Louis River, a major tributary to western Lake Superior. We captured young-of-the-year ruffe in the St. Louis River; acclimated groups of 90-91 fish to test temperatures of 7, 10, 15, 20, and 25°C; and fed them ad libitum for 42 days at those temperatures. Ruffe grew at all five temperatures, but the optimum temperature for growth was about 21°C. Because the optimum temperature for growth of walleye ( Stizostedion vitreum), sauger ( Stizoste-dion canadense), and yellow perch ( Perca flavescens) is about 22°C, ruffe will probably attempt to share their thermal habitat. A recent survey of the St. Louis River revealed that yellow perch and small forage fish declined sharply as ruffe abundance increased. A similar decline in yellow perch abundance in Lakes Michigan, Huron, and Erie would seriously affect the fisheries in these lakes.