Water-borne cortisol is correlated with acclimatization temperature but not metabolic rate in a teleost fish

This study investigated the effect of seasonal acclimatization temperature on metabolic rate and water-borne cortisol in a marine teleost, the European seabass (Dicentrarchus labrax). Individuals from the Atlantic population were reared for three years in three thermal regimes (i.e. control Atlantic, moderately warm and warm), following natural seasonal variations in temperature and photoperiod. Subsamples of fish from each regime were taken at different acclimatization temperatures throughout the year and swim tunnel respirometry used to measure oxygen uptake and accumulated water-borne cortisol concentrations. Linear mixed models demonstrated that both metabolic rate and water-borne cortisol levels increased significantly with increasing acclimatization temperature, with similar thermal sensitivity across all thermal regimes. When acclimatization temperature was accounted for, metabolic rate did not differ significantly among the regimes, but water-borne cortisol levels were consistently higher in the control regime compared to the warm one. Further, once corrected for individual fish mass, there was no clear correlation between metabolic rate and water-borne cortisol levels. This is the first investigation of relationships among acclimatization temperature, metabolic rate and water-borne cortisol in a fish species, and opens avenues for further research on this topic.