Chronic exposure to increased water temperature reveals few impacts on stress physiology and growth responses in juvenile Atlantic salmon

Fish are subjected to a variety of stressors under common cage aquaculture conditions. While short-term exposure to a stressor often results in an adaptive response to cope with stress, repeated and/or chronic exposure to stress can result in negative impacts on fish welfare and production. In fish, little is known about the impact of long-term exposure to stressors, including elevated water temperature. In this study we examined and developed temporal response profiles of physiological indicators of stress and growth in juvenile Atlantic salmon (Salmo salar) exposed to 12 °C, 16 °C, and 20 °C for 99 days. Five times throughout the study we quantified plasma cortisol, glucose and cholesterol levels, and growth. Fish body mass and fork length were not significantly different amongst temperatures after 99 days. Plasma cortisol was significantly elevated at 16 °C when comparing day 8 with 99, while at 12 °C plasma cortisol increased from day 1 to day 8, then returned to initial levels (day 1 and 8) after 99 days. Plasma glucose and cholesterol were not significantly different amongst the temperatures throughout the experiment. In addition, at the end of the experiment we quantified eye darkening, and identified the development of a bimodal growth distribution in all temperatures. Fish with a fork length ≤ 240 mm were categorised as lower mode (LM) and those with a fork length > 240 mm as in the upper mode (UM) of growth. Plasma cholesterol was significantly lower in the LM group in all three temperatures, but plasma cortisol and glucose levels did not differ between modes. Eye darkening also did not differ between modes, but increased significantly in the groups exposed to 16 °C and 20 °C when compared with 12 °C. This study showed a clear physiological stress response (elevated levels of cortisol) and eye darkening in fish maintained at 16 °C but not at 12 °C or 20 °C, suggesting that some aspects of the physiological responses available to deal with chronic stress are affected by temperature.