Population Dynamics and Food Habits of Bluegill (Lepomis macrochirus) in a Thermally Stressed Reservoir

Bluegill (Lepomis macrochirus) is the dominant fish species in a 67-ha cooling reservoir for a nuclear reactor on the Savannah River Site in South Carolina. Mortality and breeding season of bluegill were strongly influenced by reactor operations, which often produced temperatures of 40–60 °C in portions of the reservoir. Late spring and summer reactor operations caused ~ 90% mortality of bluegill. This mortality was largely associated with reactor restarts. Survivors occupied small thermal refuges in coves. Elevated fall and winter temperatures allowed an extended breeding season, although extremely high summer temperatures appeared to inhibit breeding. Bluegill depended primarily on benthic food resources, including dipterans and microcrustaceans. Despite the great variation in environmental conditions and diets, body condition of surviving bluegill was nearly constant between April 1985 and January 1986. Factors that contribute to the success of bluegill in this system include high thermal tolerance, broad diets, and an ability to breed when thermal conditions are appropriate, regardless of season.