Anthropogenic climate change is increasing the magnitude and frequency of marine heatwaves, challenging the physiological limits of ectothermic animals, such as fish, and negatively affecting human activities that depend on them, such as aquaculture. Therefore, there is an urgent need to understand the physiological capacity of fish to cope with rising temperatures and provide solutions to increase the resilience of hatchery fish to heatwaves. Previous studies have suggested that thermal tolerance in fish is strongly related to cardiac function. In this thesis, I studied how phenotypic plasticity of cardiac function can help fish to cope with high temperatures. In study I, I investigated whether acclimation to a mild level of hypoxia can induce functional and molecular adjustments similar to those induced by high temperature acclimation, and whether hypoxia acclimation can induce cross-tolerance protection to high temperatures. By measuring the thermal performance curve for maximal heart rate in European sea ass, I have determined that European sea bass do not show evidence of cross-tolerance protection between hypoxia and high temperature. In studies II, III, and IV, I investigated whether improvements in cardiac performance induced by aerobic exercise training could provide cross-tolerance protection to cope with high temperatures in hatchery-reared rainbow trout and brown trout. By training fish with aerobic exercise training, I demonstrated that trained fish have significantly higher cardiac performance and cardiac thermal tolerance, which might help them to face heatwaves. However, the improvement in thermal tolerance was highly dependent on the intensity of the training. Furthermore, in Study IV, the transgenerational effect of maternal exercise training on the thermal tolerance, survival, and growth rate of the offspring were investigated. The results of this study demonstrated that maternal exercise can improve the survival of embryos and growth of offspring without affecting their thermal tolerance and cardiac performance. This thesis provides deeper insights into the cardiac plasticity of fish and its relationship with thermal tolerance. Taken together, these results suggest that exercise training improves cardiac thermal tolerance in hatchery fish and potentially enhances their ability to cope with heatwaves. In the context of global warming, these findings have significant implications for aquaculture sector and conservation programs, providing a valid aquaculture practice to enhance the resilience of fish to climate warming.