Effects of high rearing densities and loading rates on the growth and stress responses of juvenile rainbow trout

Growth and changes in serum concentrations of cortisol, glucose, and chlori de in response to acute stress challenge tests were compared in juvenile ra inbow trout Oncorhynchus mykiss (143-190 mm TL) reared at density indices o f 2.8-9.3 g/(L . cm) and loading rates of 0.5 and 0.75 kg/(L . min). In exp eriment 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 signi ficantly less weight than fish at a density index of 2.8 g/(L . cm), and fi sh reared at 2.8 and 5.6 g/(L . cm) had better food conversion rates (weigh t of food fed/weight gained) than those at 8.4 g/(L . cm). Changes in the s erum 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 tran sfer to net-pens than in fish 10 or 17 d after transfer. There were no diff erences 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 condition s 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 fi sh at low density were stressed, possibly a result of stressful social inte ractions at low rearing densities. Our findings indicate that, at relativel y high loading rates, rainbow trout reared at high densities exhibit decrea sed growth and food conversion rates. Additionally, this study provides evi dence that high rearing density itself is probably not a chronic stressor i n rainbow trout, because fish reared at high densities did not experience c hanges in physiological measures that would normally be expected under stre ssful conditions.