Variation in Thermal Adaptive Potential Among Brook Trout Populations

With respect to global warming, coldwater salmonids are becoming more vulnerable as changing thermal conditions threaten the future sustainability of species and populations.  Thermal stress and habitat loss from increasing water temperatures are expected to impact population viability, particularly for inland populations with limited adaptive resources. Although the long-term persistence of brook trout (Salvelinus fontinalis), an archetypal coldwater salmonid, will depend on their ability to cope with and adapt to changing thermal conditions, very little is known about the scope and variation of thermal tolerances within and among brook trout populations and evolutionary lineages.  This work assessed differences in comparative thermal performance and stress within and among several populations of brook trout from different thermal environments in the northern part of the species range, to investigate their potential to cope with and adapt to predicted increases in water temperatures.  Brook trout were assessed in terms of population of origin and temperature acclimation, utilizing swimming performance, standard metabolic rate, enzyme analysis, and heat-shock protein (HSP) responses to quantify performance and variation (adaptive potential) within and among populations.  The results showed that the three populations varied significantly in their ability to tolerate increasing water temperatures, as well as resting metabolic rate and swimming performance.  The relative performance of the three strains appeared to reflect both their phylogeographic ancestry and native environmental conditions, suggesting both historical and contemporary influences on potentially adaptive traits.  This work provides insight into the physiological basis for potential differences in thermal tolerance within coldwater salmonids, as well as early steps towards quantifying the adaptive potential of salmonid populations to predicted selective pressures in a warming world.