Thermal Physiology of Warm-Spring Colonists: Variation among Lake Chub (Cyprinidae: <italic>Couesius plumbeus</italic>) Populations<xref ref-type="fn" rid="fn1" ptype="f665539" citart="citart1">*</xref>

In northern Canada, lake chub (Cyprinidae: Couesius plumbeus) have colonized a variety of thermal springs that differ substantially from the ancestral environment in both mean temperature and thermal variation. To examine whether this environmental change is associated with differences in physiological traits, we compared the thermal breadth, capacity for acclimation of thermal tolerance, and metabolic enzymes in populations of lake chub from three habitats: a warm but variable hot spring, a thermally constant warm spring, and a seasonally variable temperate lake. Thermal breadth was generally lowest in fish from the constant environment, and this difference was statistically significant in fish acclimated at 10 and 25 C. Critical thermal maximum (CTmax) increased with increasing acclimation temperature in all populations. CTmax was similar among populations when acclimated at high temperatures but greater in the variablespring population acclimated to low temperature (10 C). Critical thermal minimum was also dependent on acclimation temperature in all populations but differed among populations such that fish from the stable-spring habitat were not as tolerant to cold temperature when acclimated to 25 C. Temperateand variable-spring populations showed an increase in mitochondrial enzyme activities (citrate synthase and cytochrome c oxidase) with decreasing acclimation temperature, but this response was absent in the stable-temperature population. Protein content did not change with acclimation temperature in the stable-temperature population, while it increased with decreasing acclimation temperature in both variable thermal habitat populations. Our study suggests that interpopulation variation in thermal physiology is associated with habitat thermal variability.