Distribution and Winter Survival Health of Asian Clams,Corbicula fluminea, in the St. Clair River, Michigan

We studied die distribution and winter survival of die Asian clam, Corbicula fluminea, in the St. Clair River from the fall of 1988 to the spring of 1990. Between fall of 1988 and spring of 1989, distribution of Corbicula was extended from 5.5 to 11.5 km downstream from an electric power plant. However, total abundance of clams decreased during the winter. By fall of 1989, Corbicula was found 14.5 km from the power plant, and the mean density of clams was 27 individuals/m2. Between fall of 1989 and spring of 1990, distribution was reduced to 7.5 km from the power plant and abundance decreased 97%. During me winter of 1988–1989, we collected clams monthly from one station 2.2 km from the power plant, and we observed that clams survived the harsh winter for two months after the water temperature dropped about 1.5°C below the reported lethal level for Corbicula in midwinter. During the winter of 1989–1990, we held clams at the sediment-water interface in enclosures, and we observed that condition indices (dry body weight: dry shell weight) of clams remained stable (mean = 0.05 ±. 0.01) in December and January and then declined significantly (p < 0.05) to 0.04 ±. 0.01 in February. All clams perished by late March. The deteriorating physiological state of clams, as indicated by declining condition index, seemingly is a factor in late winter mortalities of Corbicula in me St. Clair River. In contrast to the rapid geographic spread and population increases in the southern United States, Corbicula likely will not spread rapidly throughout the Great Lakes beyond shoreline thermal refugia of heated-water discharge plumes from power plants.