Cortisol affects feed utilization, digestion and performance in juvenile rainbow trout (Oncorhynchus mykiss)

To quantify bioenergetic consequences of chronically or acutely elevated plasma cortisol levels, growth rates, feed conversion ratios and digestive performance was investigated in juvenile rainbow trout (Oncorhynchus mykiss) reared at 15 °C. Plasma cortisol levels were elevated using coconut oil based implants. A sham group received coconut oil without cortisol, (S), while a low (LC) and high (HC) cortisol group received implants containing 30 or 60 μg cortisol g⁻¹ fish. An additional group of fish was reared under moderate thermal stress (22 °C, TS), while acute increases in plasma cortisol were achieved by subjecting fish to a daily stressor by chasing (DS). An undisturbed treatment group, reared at 15 °C served as control (C). Although feed intake was not affected by treatments, cortisol treated fish showed reduced growth rate and feed conversion efficiency compared to all other treatments. This was caused, in part, by impaired digestibility of protein, lipids and nitrogen free extract in the HC treatment, leading to a decrease in available metabolizable energy. The observed decrease in lipid digestibility did not appear to be caused by a reduced digestive lipase activity in the pyloric caeca and anterior intestine as this was not significantly different between treatments 90 min after feeding. Chronically elevated cortisol levels led to atrophy of the digestive system revealed by a loss in tissue mass of the intestine and pyloric caeca in the high cortisol treatment, as well as an overall decrease in viscerosomatic index. In addition, the energetic cost of growth was higher in both cortisol treated groups compared to all other treatments. The chronic temperature elevation resulted in improved lipid digestion compared to the control treatment, while growth performance was impaired, however apparently not correlated with elevated plasma cortisol levels. A moderate random daily stress event did not result in any changes of the studied parameters. The results obtained suggest both an atrophic effect of chronically elevated plasma cortisol levels on digestive tissues resulting in impaired nutrient assimilation, less available metabolizable energy, and an increased routine energy expenditure, leading to a higher cost of growth.