Behavioural responses to acute warming precede critical shifts in the cellular and physiological thermal stress responses in a salmonid fish (brook trout, Salvelinus fontinalis)

From a conservation perspective, it is important to identify when sublethal temperatures begin to adversely impact an organism. However, it is unclear whether, during acute exposures, sub-lethal cellular thresholds occur at similar temperatures to other physiological or behavioural changes, or at temperatures associated with common physiological endpoints measured in fishes to estimate thermal tolerance. To test this, we estimated temperature preference (15.1 +/- 1.1 degrees C, mean +/- s.d.) using a shuttle box, agitation temperature (22.0 +/- 1.4 degrees C), defined as the point where a fish exhibits a behavioural avoidance response, and the upper thermal limit (CTmax, 28.2 +/- 0.4 degrees C) for 1 year old brook trout (Salvelinus fontinalis) acclimated to 10 degrees C. We then acutely exposed a different subset of fish to the mean temperatures associated with the pre-determined physiological endpoints and sampled tissues when they reached the target temperature or after 60 min of recovery at 10 degrees C for transcriptomic analysis. We used qPCR to estimate mRNA transcript levels of genes associated with heat shock proteins, oxidative stress, apoptosis and inducible transcription factors. A major shift in the transcriptome response occurred once the agitation temperature was reached, which may identify a possible link between the cellular stress response and the behavioural avoidance response.