Behavioural responses to acute warming precede critical shifts in the cellular and physiological thermal stress responses in fish

From a conservation perspective, it is important to identify when sub-lethal temperatures begin to adversely impact an organism. However, it is unclear whether, during acute exposures, these cellular thresholds occur at similar temperatures to other physiological or behavioural changes. To test this, we estimated temperature preference (15.1 ± 1.1 °C) using a shuttle box, thermal optima for aerobic scope (10—15 °C) using respirometry, agitation temperature (22.0 ± 1.4 °C) as the point where a fish exhibits a behavioural avoidance response and the CTmax (28.2 ± 0.4 °C) as the upper thermal limit for 1 yr old Brook Trout (Salvelinus fontinalis) acclimated to 10 °C. We then acutely exposed a different subset of fish to these temperatures and sampled tissues when they reached the target temperature or after 60 min of recovery at 10 °C. 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 near the agitation temperature, which may identify a link between the cellular stress response and the behavioural avoidance response. ### Competing Interest Statement The authors have declared no competing interest.