An Effective Method of Prompting Juvenile Rainbow Trout (Oncorhynchus mykiss) to Cope with Heat Stress

In the present study, juvenile rainbow trout (Oncorhynchus mykiss) were acclimated at 5 temperatures. Fish in the control group (C-0) were reared at a constant temperature (16 degrees C); trout in four other treatments (A(0), A(3), A(6), and A(9)) were acclimated to a high temperature (22 degrees C) for 0, 3, 6, and 9 d, respectively, and then returned to normal temperature (16 degrees C) for 7 d. The temperature was then raised to 20 degrees C and the fish were cultured for 40 d. The results showed that the growth rates of A(3), A(6), and A(9)) were higher than that of A(0) but lower than that of C-0. The growth rate of A(9) was the highest among the 4 acclimation groups. The activities of serum transaminase and liver antioxidant enzyme significantly increased during acclimation. At the beginning of growth, the activities of enzymes were lower in A(3), A(6), and A(9) with the lowest in A(9). In addition, the transcript abundance of heat shock protein (HSP) 60 gene in A(9) was not significantly different from that of C-0 during growth. HSP70 in A(9) significantly increased at the beginning and returned to that of C-0 at the end. Our findings indicated that pre-acclimation improved the high temperature tolerance with the best effectiveness observed at 22 degrees C for 9 d. A possible mechanism underlining such phenomenon is the improvement of antioxidant defense system.