To study the effects of acute ammonia stress on the poisoning reaction of the hybrid grouper (♀ Epinephelus fuscoguttatus × ♂ E. lanceolatu), 300 healthy grouper juveniles with an initial body weight of 51.4 ± 2.57 g were selected for an acute ammonia stress experiment using a half-lethal concentration of ammonia for 24, 48, 72, and 96 h with triplicate. The results show that (1) The half-lethal concentrations of ammonia for a hybrid grouper were 39.5 mg/L for 24 h, 27.3 mg/L for 48 h, 26.5 mg/L for 72 h, and 25.0 mg/L for 96 h, and the safe concentration was 2.50 mg/L. (2) The biochemical indices of the serum and livers of the groupers fluctuated under acute ammonia stress (P < 0.05), and the contents of serum cortisol and lactate reached the maximum value in 72 h (P < 0.05). (3) 24 h ammonia stress increased the activities of alkaline phosphatase, acid phosphatase, catalase, glutathione peroxidase, superoxide dismutase, lysozyme, and the content of immunoglobulin M in the liver (P < 0.05). Meanwhile, all ammonia-stressed fish groups had an increased amount of malondialdehyde in the liver (P < 0.05). (4) The ammonia poisoning caused significantly up-regulation of antioxidant genes, inflammatory cytokines, and apoptosis genes (P < 0.05), and the expression of inflammatory cytokines and apoptosis genes were the highest in 24 h ammonia stress group. (5) The ammonia content in the water changed the abundance and evenness of intestinal microbes in grouper, mainly in that ammonia stress significantly increased the relative abundance of fusobacteria, but significantly decreased the relative abundance of gemmatimonadetes (P < 0.05). It was speculated that apoptosis induced by ammonia poisoning in grouper may be related to oxidative stress and the activation of inflammatory factors, and excessive inflammatory stress may be one of the causes of ammonia poisoning in the hybrid grouper.
An 8‐week feeding trial was conducted to determine the requirement of protein for large‐size grouper Epinephelus coioides (initial body weight: 275.07 ± 1.56 g). Six iso‐lipidic (124 g/kg) diets were formulated containing graded levels of protein (350, 400, 450, 500, 550 and 600 g/kg). Grouper was hand‐fed twice daily to apparent satiation with triplicate. The results showed that significantly high weight gain, specific growth rate and significantly low feed conversion ratio were observed in fish fed 450 g/kg protein group. High‐protein level diets significantly increased protein content and significantly decreased lipid content of fish body and muscle. Total protein and cholesterol content in serum of 600 g/kg group were significantly higher than those of 350 g/kg group. However, serum glucose and triglyceride contents of fish fed low‐protein diets were significantly higher than those of fish fed high‐protein diets. Meanwhile, liver glutamic‐pyruvic transaminase and glutamic‐oxaloacetic transaminase in high‐protein diet groups were significantly higher than those of low‐protein diet groups. The intestinal protease activity in high‐protein diet groups was significantly higher that of low‐protein diet groups, but lipase and amylase showed opposite trend. With the increasing of dietary protein level, the activities of alkaline phosphatase, superoxide dismutase and lysozyme in liver of grouper increased significantly compared with 350 g/kg group, while the activities of acid phosphatase decreased significantly. With specific growth rate as the evaluation index, the optimum dietary protein level of large‐size grouper Epinephelus coioides was 438.39 g/kg by fitting the broken‐line regression analysis.
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