Gene expression responses in Atlantic salmon (Salmo salar) exposed to chronic thermal stress

The generalized effect of a stressor is denoted by a response that either benefits the individual or induces consequences to habitual functioning. Thermal changes in the external environment mediate the distribution and welfare of commercially valuable ectothermic fish including the Atlantic salmon (Salmo salar). Recently, specific concerns have been raised for the wellbeing and productivity of harvested Tasmanian stocks following exposure to thermal regimes exceeding tolerable limits in summer months. Historically, thermal stress responses in salmonids have been assessed through growth indices and changes in cortisol, cholesterol, and glucose levels. However, these parameters have been shown to be unreliable in determining a state of physiological stress in fish. Although changes in gene expression have been explored as an alternative technique, there remains a limited understanding of chronic thermal stress responses at the transcriptomic level. Thus, the present study investigated gene expression trends in Atlantic salmon reared at 12 °C, 16 °C and 20 °C, across five sampling times (1, 8, 37, 69 and 99). Five significantly up regulated and eight down regulated genes (representing five functional groups) observed in a previous RNA sequencing analysis study were validated through real time quantitative polymerase chain reaction assays. This study aimed to identify gene transcripts as potential candidates of chronic thermal stress biomarkers in S.salar while developing evidence towards an adaptive or maladaptive response. Genes involved in heat shock response (sh1), signal transduction (ppef-2, slap-1) and DNA interaction (prmt-2, sarf-6) were significantly up regulated across 99 days at 20 °C. Down regulated genes were either undetectable when quantified or exhibited no distinct expression trend. Overall, findings related to transcripts that were upregulated during exposure to 20 °C suggest an adaptive response to thermal stress. As such, further studies should seek to investigate expression at the protein level. The present study will aid in mitigating salmon 4 welfare within aquaculture operations and native populations experiencing prolonged periods of thermal stress.