Darren P. Reed, Jeffrey J. Herod, and James B. Sickel, Department of Biological Sciences, Murray State University, Murray, KY 42071

Abstract. Zebra mussel veliger densities throughout 1996 were monitored at one site at the Army Corps of Engineers Lock and Dam 52, on the Lower Ohio River near Brookport, IL. The spawning season was determined to have occurred between mid-June and early September. The presence of veligers within water samples taken during the spawning period showed a bimodal distribution. One large peak of 30,000/m³ occurred in late July, with a second smaller peak of 8,000/m³ in mid-August. The presence of multiple peaks indicates the possibility of more than one spawning event within the lower Ohio River for one season. Veligers first appeared at a water temperature of 21^oC. When spawning ended in September, the water temperature was 22^oC. Veligers continued to be found through November at which time the water temperature fell to 10^oC. The lowest temperature at which veligers were observed was 7^oC, in March. Results of this study show that zebra mussels in the lower Ohio River are naturally reproducing and the occurrence of spawning appears to be synchronous. The presence of larvae at such low temperatures in March suggests that veligers are capable of delaying settlement and overwintering in the planktonic stage. Supporting funds for this project were received from the Mussel Mitigation Trust, Murray State's Center for Reservoir Research, and the Shell Exporters of America, Inc.