Limited variability in upper thermal tolerance among pure and hybrid populations of a cold-water fish

As climate warming threatens the persistence of many species and populations, it is important to forecast their responses to warming thermal regimes. Climate warming often traps populations in smaller habitat fragments, not only changing biotic parameters, but potentially decreasing adaptive potential by decreasing genetic variability. We examined the ability of six genetically distinct and different-sized populations of a cold-water fish (brook trout, Salvelinus fontinalis) to tolerate acute thermal warming and whether this tolerance could be altered by hybridizing populations. Critical thermal maximum (CT(max)) assays were conducted on juveniles from each population to assess thermal tolerance, and the agitation temperature was recorded for assessing behavioural changes to elevated temperatures. An additional metric, which we have called the ‘CT(max)–agitation window’ (CT(max) minus agitation temperature), was also assessed. The CT(max) differed between five out of 15 population pairs, although the maximal CT(max) difference was only 0.68°C (29.11–29.79°C). Hybridization between one large population and two small populations yielded no obvious heterosis in mean CT(max), and no differences in agitation temperature or CT(max)–agitation window were detected among pure populations or hybrids. Summer variation in temperature within each stream was negatively correlated with mean CT(max) and mean CT(max)–agitation window, although the maximal difference was small. Despite being one of the most phenotypically divergent and plastic north temperate freshwater fishes, our results suggest that limited variability exists in CT(max) among populations of brook trout, regardless of their population size, standing genetic variation and differing natural thermal regimes (temperature variation, minimum and maximum). This study highlights the level to which thermal tolerance is conserved between isolated populations of a vertebrate species, in the face of climate warming.