README_Stewart_BT_CTmax.pdf

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 <i>in situ</i>. We measured field critical thermal maximum (CT_max) of brook trout (<i>Salvelinus fontinalis</i>) populations in the field at twenty sites across Ontario, Canada, to assess their thermal tolerance <i>in situ</i> and examine potential factors underlying intraspecific variation in thermal performance. We modelled CT_max data as a function of short-term stream temperature data to assess plasticity and local adaptation, and used full-season ambient temperature data to calculate thermal safety margins (TSM) for each population. CT_max ranged between 27.41 °C 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 65% of the among-site variation in CT_max. TSMs ranged between 4.15 °C 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 both watershed characteristics and local adaptation drive intraspecific variation in thermal tolerance and thus dictate the resiliency of brook trout under climate change.