Low Thermal Responsiveness in the Bluegill, Lepomis macrochirus

Fifteen young bluegill, Lepomis macrochirus, acclimated to 5 C for at least 1 mo, died during temperature preference testing owing to apparent voluntary exposure to high temperatures. The mortality rate of 100% for these fish greatly exceeded the 3.8% rate found for 262 bluegill acclimated to temperatures ranging from 20 to 34 C, and tested in the same apparatus. Modifications in the experimental design whereby fish acclimated to 5 C were held at about 19 C for 24 h during testing, enabled all fish to survive. These bluegill selected temperatures only 2 C lower than those acclimated to summer conditions.