J. M. BATTLE, T. B. MIHUC, F. G. SPIESS, AND C. F. BRYAN, Louisiana Cooperative Fish and Wildlife Research Unit, School of Forestry, Wildlife, & Fisheries, Louisiana State University, Baton Rouge, Louisiana 70803, USA

Previous studies on the zebra mussel Dreissena polymorpha have examined tolerance limits of dissolved oxygen and temperatures that can be used in controlling this exotic pest. We wanted to determine if colonization of the zebra mussel in the Atchafalaya Basin would be deterred by the natural seasonal phenomenon of increased temperatures and decreased oxygen saturation. The Atchafalaya Basin is a large river-floodplain in Louisiana that represents the current most southern distribution of D. polymorpha in North America. During 1995-96, adult mussels were counted monthly on colonization samplers placed in riverine and floodplain habitats. We found a seasonal colonization dynamic with mussels settling in fall when minimum daily temperatures dropped below 31°C. By midwinter mussels colonized floodplain habitats as far as 10 km from the main stem river. In August, adult mussels experienced mortality as minimum daily temperatures rose above 29°C in the floodplain and above 31°C in riverine sites. Low dissolved oxygen levels and high temperatures in the floodplain prevented establishment of zebra mussel populations; however, small populations remained in riverine sites even during unfavorable summer conditions. Seasonal colonization of zebra mussels in the Atchafalaya Basin has implications for predicting zebra mussel distribution in warmwater systems.