Effects of elevated temperature on gene expression, energy metabolism, and physiology in brown trout, Salmo trutta

Given the imminent threat of global warming and rising water temperatures in Austria, this study investigated the effects of elevated temperature on gene expression, energy reserves, and cellular energy status in brown trout (Salmo trutta), a species particularly sensitive to increasing water temperature. A total of 250 fish were placed in four stream channels under flow-through conditions. Two channels were maintained at 9 °C as controls, while the other two had their temperature gradually increased to 20 °C over seven days and then maintained at 20 °C for 21 days. Sampling was conducted on day 1, after the temperature reached 20 °C, and the last day of high-temperature exposure on day 21. At each sampling point growth, hepatosomatic index and the fat content of the viscera were measured and/or calculated, and liver samples were taken for gene expression and metabolite analyses. Elevated temperature significantly increased the expression of genes related to cellular stress response (hsp70, hsp90 aa1, cat, and casp8) compared to controls. However, there was no significant difference in the expression of genes associated with lipid and carbohydrate metabolism (d5fad and pfkfb4). Furthermore, there was a decrease in energy storage indicated by a decrease in the hepatosomatic index, glycogen, triglycerides and ATP in the liver as well as the fat content of the viscera. Cellular energy status also significantly decreased, as indicated by the calculated adenylate energy charge. Physiologically, this culminated in suppression of growth in the treatment group after 21 days. This study shows that elevated temperature leads to significant trade-offs in brown trout, which may lead to ecological consequences over the long run. These findings offer critical insights into the physiological impacts of elevated temperature that help evaluate the species' acclimation to rising water temperature and inform the development of effective conservation strategies in a warming world.