Do prior diel thermal cycles influence the physiological response of Atlantic salmon (Salmo salar) to subsequent heat stress?

We designed two environmentally relevant thermal cycling regimes using monitoring data from an Atlantic salmon (Salmo salar) river to determine whether exposure to prior diel cycles stimulated protective mechanisms (e.g., heat hardening) and (or) resulted in physiological and cellular stress. Wild fish were exposed to 3 days of diel cycling in the lab and then exposed to an acute thermal challenge near their upper reported critical temperature. We measured routine metabolic rate across the time course as well as indicators of physiological status (e.g., plasma glucose and osmolality) and cellular stress (e.g., heat shock protein 70). We observed that thermal cycling altered physiological and cellular responses, compared with an acute heat shock, but saw no differences between cycling regimes. Unique temperature regime and tissue-specific responses were observed in heat shock protein induction, metabolites, haematology, and osmotic indicators. Routine metabolic rate was not affected by the thermal cycling and increased according to Q(10) predictions. While we report unique physiological and cellular responses among all treatment groups, we did not observe a clear indication of a heat hardening response.