Effects of High Rearing Densities and Loading Rates on the Growth and Stress Responses of Juvenile Rainbow Trout

Publisher Website
Google Scholar
Abstract
Growth and changes in serum concentrations of cortisol, glucose, and chloride in response to acute stress challenge tests were compared in juvenile rainbow trout Oncorhynchus mykiss (143-190 mm TL) reared at density indices of 2.8-9.3 g/(L . cm) and loading rates of 0.5 and 0.75 kg/(L . min). In experiment 1, there were no differences in growth or measures of physiological stress in fish reared for 4 weeks at loading rates of 0.5 or 0.75 kg/(L . min). Fish reared at density indices of 5.6 and 8.4 g/(L . cm) gained significantly less weight than fish at a density index of 2.8 g/(L . cm), and fish reared at 2.8 and 5.6 g/(L . cm) had better food conversion rates (weight of food fed/weight gained) than those at 8.4 g/(L . cm). Changes in the serum concentrations of cortisol, glucose, and chloride, however, suggested that the reduced growth at elevated densities was not due to physiological stress. In experiment 2, cortisol levels were higher in fish 3 d after transfer to net-pens than in fish 10 or 17 d after transfer. There were no differences in the cortisol levels of fish reared at density indices of 3.1, 6.2, and 9.3 g/(L . cm), suggesting that acclimation to the net-pen conditions was not density dependent. Elevations in serum glucose and reductions in serum chloride in fish reared at 3.1 g/(L . cm), however, suggested that fish at low density were stressed, possibly a result of stressful social interactions at low rearing densities. Our findings indicate that, at relatively high loading rates, rainbow trout reared at high densities exhibit decreased growth and food conversion rates. Additionally, this study provides evidence that high rearing density itself is probably not a chronic stressor in rainbow trout, because fish reared at high densities did not experience changes in physiological measures that would normally be expected under stressful conditions.