Andrew N. Dick, Charles J. Gagen and Joseph N. Stoeckel, Fisheries and Wildlife Biology Program, Arkansas Tech University, Russellville, AR 72801

Abstract. Zebra mussels (Dreissena polymorpha) are known to alter limnological conditions and adversely impact man-made structures in heavily infested waters. We have sampled all life-stages of zebra mussels in Lake Dardanelle, AR since 1993. Following their discovery in 1992, the population has grown exponentially. Peak densities of settling juveniles increased from below detection in 1993, to millions/m² in the spring of 1997. However, a die-off of approximately 80% all size classes coincided with unusually high water temperatures (>30^oC for one week) during July of 1997. In more southern reservoirs, high adult mortality in the summer produces a zebra mussel population dominated by juveniles. In more northern latitudes, adult zebra mussel populations can reach higher densities, but fluctuate considerably due primarily to die-offs of the oldest cohorts. Thus, mesotrophic reservoirs in the mid-south provide unique conditions for zebra mussels; where, years of favorable conditions punctuated by episodes of high water temperature, can produce exponential population growth followed by massive die-offs. We expect future exponential increases in zebra mussel densities to be periodically disrupted by sustained high water temperature. We hypothesize that a model incorporating air or water temperature could be used to predict the magnitude of thermal-induced zebra mussel die-offs.