Critical and chronic thermal maxima of northern and florida largemouth bass and their reciprocal F1 and F2 hybrids

Thermal responses varied among four genetic stocks of juvenile largemouth bass: Northern largemouth bass Micropterus salmoides salmoides (NLMB), Florida largemouth bass M. s. floridanus (FLMB), and both reciprocal F1 hybrids, NLMB ♀ × FLMB ♂ (N × F) and FLMB ♀ × NLMB ♂ (F × N). Thermal responses for each stock were measured as the critical thermal maximum, the temperature at which death occurred during a temperature increase of 0.2°C/min, for fish at various acclimation temperatures (8, 16, 24, 32°C); and chronic thermal maximum, the temperature at which death occurred during a temperature increase of l°C/d, for fish acclimated to 32°C. The rank order of values for both response measurements was identical for the four stocks: F × N > FLMB > N × F > NLMB. Increased acclimation temperature resulted in significant, proportional increases in the critical thermal maxima for all stocks. In addition, second-generation hybrid stocks (N × F F2 and F × N F2) that were acclimated at 24°C produced similar critical thermal maximum values (38.4°C and 37.8°C, respectively) that were intermediate between those of the NLMB (37.3°C) and FLMB (39.2°C). Subsequent genetic analysis revealed no correlation between critical thermal maximum and genotype at the three enzyme loci studied (Mdh-B, Idh-B, and Aat-B). Thus, the subspecific origin of the alleles at these three loci had no detectable influence on the thermal responses of hybrid offspring.