Effects Of Hypoxia And Culture Density On Physiological And Biochemical Indexes And Tissue Structure Of Megalobrama Amblycephala

Megalobrama amblyocephala Yin,commonly known as Wuchang fish,belongs to Cypriniformes,Cyprinidae and Megalobrama.As a good herbivorous fish species,blunt snout bream has been vigorously promoted throughout the country due to its wide feeding habits,fast growth,high survival rate,easy fishing and moderate specifications.Its aquaculture yield shows an increasing trend as a whole,with an output value of more than 10 billion yuan.Compared with other fishes,the tolerance to external stimuli of blunt snout bream is poor.When it is stimulated by external stimuli in the process of cultivation,production or transportation,the stress response is strong,and it is easy to cause injury or even death.Aquaculture density and hypoxia are common stress factors in production practice.In this study,two experiments were designed to study the effects of hypoxia stress and culture density on blunt snout bream.In the previous studies,there have been studies on the serum physiology and biochemistry,enzyme activity system,immune system and growth and development of blunt snout bream,but few studies have been done on intestinal microorganisms.The effects of different stocking densities on growth performance,serum physiology,antioxidant enzyme activity,digestive enzyme activity and intestinal flora of juvenile blunt snout bream were studied in this study.We try to solve the problem from a new perspective.The juveniles of the same origin and batch with the same specifications were randomly divided into groups.Four density groups were set up: 30 F group(30 fish/ m3;control group),60 F group(60 fish/m3),90 F group(90 fish/ m3)and 120 F group(120 fish/ m3).In this experiment,cage culture was used to make different water environmental factors(such as acidity,alkalinity,salinity,dissolved oxygen,water temperature,environmental microorganisms)consistent.The results showed that higher stocking density had negative effects on growth performance.With the increase of stocking density,viscera-body ratio,liver-body ratio and survival rate decreased significantly.With the increase of feeding density,serum cortisol,triglyceride,alanine aminotransferase,glutamic oxalate aminotransferase and alkaline phosphatase increased significantly,while blood sugar and total cholesterol decreased significantly.With the increase of feeding density,the activities of lipase and protease in intestine decreased significantly,the activities of glutathione peroxidase and superoxide dismutase in liver also decreased significantly,while the content of malondialdehyde increased significantly.There were significant differences in intestinal flora structure between groups.Among the flora closely related to aquatic products,with the increase of feeding density,the number of Acinetobacter,Aeromonas,Pseudomonas and Vibrio increased significantly,while the number of Clostridium,Bacteroides,Lactococcus,Lactobacillus and Bacillus decreased significantly.Slices showed that the lamina propria of intestine was damaged and goblet cells were significantly increased in 90 F and 120 F groups,suggesting that intestinal inflammation might occur.Because of the cluster effect,when the stocking density was less than 60 fish/ m3,the specific growth rate and weight gain rate increased significantly with the stocking density,but decreased significantly when the stocking density exceeded the tail/ m3.The second experiment was designed to study the effects of acute hypoxic stress on serum physiology and biochemistry,liver antioxidant enzyme activity and gill,liver and kidney microstructures of blunt snout bream.Because of different definitions and experimental conditions,the asphyxiation points in the past studies are quite different,and the reference value of extreme hypoxia to actual production is limited.This study simulated the acute hypoxia in practical production and explored the 24-hour semi-lethal dissolved oxygen concentration of blunt snout bream.Previous hypoxic studies have focused on the liver and rarely on the kidney.This study supplements this data.In this experiment,the same batch and the same specifications of blunt snout bream were randomly divided into groups.In the experimental group,hypoxia was maintained continuously(1.2+0.2 mg/L).Samples were taken 5 times at 0 h,3 h,6 h,12 h and 24 h,then oxygen was opened and reoxygenated for 24 h,that is,the sixth time at 48 h after the beginning of the experiment.The results showed that the contents of cortisol,lactate dehydrogenase,blood sugar,alanine aminotransferase,glutamic oxalate aminotransferase and alkaline phosphatase in serum increased significantly with the prolongation of hypoxia.After 24 hours of reoxygenation,cortisol,lactate dehydrogenase and blood sugar decreased significantly.Under continuous hypoxia,catalase and superoxide dismutase in liver decreased significantly with time,while glutathione peroxidase and malondialdehyde increased significantly.Glutathione peroxidase activity and malondialdehyde content decreased significantly 24 hours after reoxygenation.From the light microscopic section of the gill,the structure of the gill changed after hypoxia stress,the gill patches appeared congestion,the end curled and deformed,and the gill patches fused seriously.Epithelial cells showed marked edema,unclear cell outline,necrosis and exfoliation.After 24 hours of reoxygenation,the injury was not completely repaired.On liver slices,after hypoxia treatment,the cell boundaries were blurred,the hepatic sinuses and chordae were widened,and macrophages were enlarged.Fat droplets in hepatocytes became larger,and vacuolation and nuclear migration of hepatocytes were observed.The results showed that hepatic hyperemia,steatosis of hepatocytes and changes of metabolic pathway of fish body under hypoxia occurred.After 24 hours of reoxygenation,the microstructures did not improve significantly.Hypoxic stress has a negative impact on the kidney.It can be seen that the glomerular atrophy,enlargement of renal capsule cavity,shrinkage of renal tubular lumen,enlargement,necrosis,vacuolation,water-like degeneration of renal tubular epithelial cells,and even glomerular necrosis,tubular necrosis.After 24 hours of reoxygenation,there was no significant improvement in the histological structure.Based on practical production,this study explored the effects of culture density and hypoxia stress on blunt snout bream,which is of great reference significance for reducing the stress of blunt snout bream in the process of breeding and breeding of anti-stress varieties of blunt snout bream.