Abstract:This study aimed to determine the optimal stocking density for juvenile Megalobrama pellegrini. Meanwhile, enzymology and molecular biology analyses were performed to analyze the effects of stocking density on the intestinal antioxidant enzyme activities and the expression of stress-related genes in this fish species. 5 density treatments groups, including 0.15 kg/m3 (50 tails/barrel), 0.24 kg/m3 (80 tails/barrel), 0.34 kg/m3 (110 tails/barrel), 0.42 kg/m3 (140 tails) /Barrel), 0.50 kg/m3 (170 tails/barrel) were set up in triplicate. The experimental period was 42 days. The results showed that fish growth performance (weight gain rate and specific growth rate) increased as the stocking density increased from 0.15 kg/m3 to 0.34 kg/m3, but no significant differences were found among the groups (P>0.05). When stocking density continued to increase, fish growth performance decreased, and significantly lower growth was found in fish reared at a stocking density of 0.50 kg/m3 compared with the 0.34 kg/m3 group (P<0.05). Meanwhile, the highest density group (0.50 kg/m3) had a significantly higher feed conversion ratio compared with the low and medium stocking density groups (0.15 kg/m3, 0.24 kg/m3, 0.34 kg/m3) (P<0.05), which indicated that the higher stocking density had side effects on fish growth and feed utilization. The results also showed that increasing the stocking density did not significantly affect the survival rate of M. Pellegrini (P>0.05), and the survival rate of each group was very high (>97%). The crude protein and crude fat content of the whole-body in the fish of the 0.34 kg/m3 group were significantly higher than those in other experimental groups (P<0.05). The intestinal antioxidant enzymes and other related indexes of fish were significantly affected by stocking density. Specifically, the 0.24 kg/m3 and 0.34 kg/m3 groups had significantly higher total antioxidant capacity than the lowest-density group (0.15 kg/m3) and the higher-density groups (0.42 kg/m3, 0.50 kg/m3) (P<0.05); the activities of intestinal superoxide dismutase and catalase were lowest at 0.50 kg/m3, while content of malondialdehyde was the highest (P<0.05); the highest value of glutathione content appeared in the 0.34 kg/m3 group and was significantly higher than that of other experimental groups (P<0.05). The lowest density experimental group, 0.15 kg/m3, had the lowest intestinal CYP1 A gene expression, but a significant difference was only found between the 0.42 kg/m3 experimental group (P<0.05). Fish in the 0.50 kg/m3 group had significantly higher relative expression levels of the Nrf2 gene and significantly lower relative expression levels of MnSOD compared with the 0.34 kg/m3 density treatment group (P<0.05), which indicated that the intestinal oxidative stress response was induced when the stocking density was too high. The results of this study indicated that M. Pellegrini at the weight of 0.45~1.36 g could be cultured at a density of 0.34 kg/m3, which will provide a theoretical basis for improving breeding efficiency and promoting the restoration of the resources of M. Pellegrini.
