| The growth, reproduction and metabolism of fish are always affected by changes in water environment. Water environment is often influenced by the variety of factors. To gain higher yields, modern aquaculture always depends on intensive of feeds, drugs, or high stocking density. It results in strong stimulation to the fish, causing the fish physiological metabolism disorder, the drop of disease resistance ability, lower aquatic product quality, and even the death of fish. In this study, we investigated the effect of different stocking densities and different concentrations of nitrite on growth, metabolism, meat quality and non-specific immunity of blunt snout bream Megalobrama amblycephala, with the purpose of promoting healthy aquaculture.In the experiment of stocking density, five stocking density groups including 25, 50, 75, 100, 150 ind/ tank(26, 52, 78, 104, 156 g/m2)were designed. Each group was divided into three replicates and fed three times per day. After 90 days, the blood, liver, and whole body of fish were collected, then growth performance, physiological and biochemical indexes, antioxidant indexes, immune index and body composition were measured. In acute nitrite exposure experiment, four nitrite exposure groups(1 mg/L, 5 mg/L, 10 mg/L, 20 mg/L) and a control group with three replicates were set. The physiological and biochemical parameters of blood, liver metabolism and antioxidant parameters, and muscle nutritional parameters were measured at 12 h, 24 h, 48 h, and 96 h after nitrite exposure, respectively.1. Growth performance of juvenile blunt snout bream was significant affected by stocking density. When the growth trial completed, an inverse relationship was observed between body length/weight and stocking density. Higher stocking densities had a significant negative effect on both absolute and specific growth rate(SGR) at the end of the experiment(p<0.05). Although feed intake was higher in the lower stocking density groups, different stocking density did not have significant influence on feed conversion ratio(FCR), protein utilization rate(PRE) and viscera/body ratio(VSI). The contents of triglyceride(TG), cholesterol(CHOL) and lactic acid(LD) in serum were significantly increased with the increase of stocking density, but in the higher stocking density group, the content of blood glucose(GLU) decreased. In addition, activities of aspartate transaminase(AST) and alanine aminotransferase(ALT) in higher stocking density were significantly higher than others. For antioxidant system in serum, total antioxidant capacity(T-AOC) increased first and then decreased with the increase of stocking density, but content of MDA showed a significant positive correlation with the change of stocking density. In the high density group, the activities of superoxide dismutase(SOD) and glutathione peroxidase(GPX) increased significantly. The content of superoxide dismutase(SOD) and nitric oxide(NO) in liver decreased in higher stocking density, while the content of MDA increased. Besides in high stocking density groups, the activitives of acid phosphatase(ACP) increased, and the activitives of lysozyme(LZM) in serum decreased significantly. In addition, higher body composition moisture content occured instead of crude fat when the stocking density increased.2. In acute nitrite exposure experiment, the level of cortisol in blood increased significantly and was positively correlated with exposure concentration at 12 h, followed by a gradual decline to normal levels. And the level of glucose was significantly higher in 10 mg/L and 20 mg/L at 12 h and 24 h. Cholesterol levels in the 20 mg/L decreased at 12 h and increased significantly at 96 h. The activity of aspartate aminotransferase increased significantly at 12 h and 24 h in exposure group(20 mg/L).It also increased at 48 h in 5 mg/L, 10 mg/L and 20 mg/L groups significantly with the extension of time. And the activity of alanine aminotransferase in 20 mg/L exposure group significantly increased at 12 h. The activity of lactate dehydrogenase increased significantly during the exposure period, and was positively correlated with the exposure concentration. In the liver, the glycogen content decreased with the increase of the exposure concentration, and the difference was gradually reduced with the extension of the exposure time. The activity of alkaline phosphatase in the lower exposure group was higher than that in the high concentration group. Nitrite exposure stimulated the activation of antioxidant system of juvenile blunt snout bream, and the activity of superoxide dismutase and catalase increased significantly compared with control group. Also in muscle tissue, the content of muscle glycogen content decreased with the increase of exposure concentration. Besides, nitrite exposure also brought changes in muscle physical properties and nutritional value. Significant differences were observed in hardness, springiness, and resilience between the control and expose groups. Meanwhile, the content of polyunsaturated fatty acids, especially DHA and AA in muscle, decreased the same asthe content of proline.The results showed that high stocking density could suppress the growth performance of juvenile blunt snout bream, and induce the reduction of muscle fat content. Simultaneously, excessive consumption of energy and the activation of the antioxidant system occurred, and a certain inhibition for immune system has been observed. At the beginning of nitrite exposure, increased energy consumption of juvenile blunt snout bream appeared. Then, enhanced fatty acid metabolism and gluconeogenesis were used to resist the external stimuli with the extension of exposure time. To reduce the oxidative damage the antioxidant system was activated. But the accumulation of metabolic waste would, result in tissue damage, and reduce the nutritional value of muscle. |
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