Local conditions drive interpopulation variation in field-based critical thermal maximum of brook trout.

Individual- and population-level responses to thermal change will be pivotal for species' resilience and adaptive responses to climate change. Thermal tolerance of ectotherms has been extensively studied under laboratory conditions, but comparatively few studies have assessed intra- and interpopulation variation under natural conditions or in situ. We measured field critical thermal maximum (CT_max) of brook trout (Salvelinus fontinalis) populations at twenty sites across Ontario, Canada, to assess their thermal tolerance in situ and examine potential factors underlying intraspecific variation in thermal performance. We modelled CT_max as a function of acclimation using short-term stream temperature data to assess interpopulation variation, and used full-season stream temperatures to calculate thermal safety margins (TSM) for each population. CT_max ranged between 27.41 and 30.46°C and acclimation periods between 4 and 40 days were strong predictors of site CT_max, aligning closely with lab-based studies. Seasonal temperature profiles varied substantially among sites, with mean 30-day stream temperature accounting for 66% of the among-site variation in CT_max. TSMs ranged between 0.51 and 15.51°C and reflected differences among site thermal regimes. Streams in watersheds with more urban or agricultural development had the lowest TSMs in addition to those that were fed by lake surface water. This work emphasizes the importance of locally based conservation and management practices that act at or below the population level, as local factors beyond acclimation temperature were partly responsible for variation in thermal tolerance and thus dictate the resiliency of brook trout under climate change.