REPRODUCTIVE CHARACTERISTICS OF THE RUFFE, GYMNOCEPHALUS-CERNUUS, IN THE ST-LOUIS RIVER ESTUARY ON WESTERN LAKE SUPERIOR - A HISTOLOGICAL EXAMINATION OF THE OVARIES OVER ONE ANNUAL CYCLE

The Eurasian ruffe, Gymnocephalus cernuus, is a small percid that has recently been introduced into the Duluth-Superior Harbor at the wester n end of Lake Superior. Because of its adverse impact on native specie s, its presence has caused considerable concern in the North American fisheries community. Ruffe have experienced phenomenal reproductive su ccess in their new home and are rapidly extending their range. The rep roductive success of this species seems to be partly related to the ma nner of spawning. We describe the dynamics of oogenesis of different a ge groups during the 1994 reproductive cycle. Spawning commenced withi n a week or two of ice-out and extended for approximately 8 weeks. His tological observations revealed that different age groups of ruffe spa wned somewhat in succession, with older fish spawning first. As the se ason progressed, earlier spawners engaged in repeat spawning, probably two or three times before the end of the spawning period. These obser vations suggest that the unstable spring weather characteristic of thi s region has a minimal effect on the success of ruffe age groups, in c ontrast with several native species that spawn only once in the spring and that could suffer losses of age groups during a postspawning cold spell.