Two bluegill subspecies (common bluegill Lepomis macrochirus macrochirus and coppernose bluegill L. m. purpurescens) were subjected to cold tolerance tests in the laboratory. Juvenile and adult bluegills were exposed to water temperature decreases of 1°C/hour from acclimation temperatures of 20° and 30° C until loss of equilibrium. Response temperatures of both subspecies were similar; temperatures at which 50% oftest fish lost equilibrium were usually <1.0° C lower for common bluegill and overlapping occurred in fiducial limits. Bluegill acclimated to 20° C lost equilibrium at 0.6°-4.2° C; those acclimated to 30° C lost equilibrium at 6.4°-10.4° C. Adults were more resistant than juveniles to low temperatures when acclimated to 20° C but juveniles were more resistant at 30° C.
