Transcriptional profiling provides insights into sublethal thermal stress thresholds in juvenile bull trout

Bull trout ( Salvelinus confluentus) inhabit mainly cold-water streams in their Western North American range. Multiple anthropogenic stressors, including climate change, have caused population declines and range reduction, leading to classification of bull trout as threatened. Characterizing how fish respond to increasing temperature can identify key thresholds beyond which fish health is impacted, that are useful for developing species recovery strategies. Juvenile bull trout were acclimated to a range of relevant temperatures (6–21 °C), after which mRNA transcripts involved in responding to thermal stress were measured using high-throughput qPCR, and their upper thermal tolerance (critical thermal maximum, CT max ) was determined. Beyond 18 °C appears to be a critical sublethal threshold for juvenile bull trout: capacity to increase CT max through acclimation was lost, growth and survival decreased, and transcriptional data suggest widespread activation of cellular stress and growth suppression. Below 18 °C, transcriptional data suggest subtler metabolic adjustments in response to acclimation temperature, preceding changes in whole-animal performance. Altogether, these data provide insight into thermal responses in bull trout, and their capacity to cope with climate change-related warming.