Effect Of Salinity On Growth And Physiolgy,Biochemical Of Juvenile Epinephelus Coiodies Reared In The Seawater

In the natural environment,there is a periodic or sudden changes in the fish due to environmental factors such as water temperature,salinity,light,etc.,which often cause varying degrees of stress to the fish.After the remove of environmental stress,the stress response of fish may gradually disappear,return to a normal state,then fish show different degree of compensatory growth or no growth compensation at the individual level.So,in-depth study of whether fish have mechanism and ability of compensatory growth is the basis to understand the fish adapting to the changing environment,which has important significance to explore the fish growth and the change of population,and can provide theoretical support for the improvement of aquaculture production technology.In this study,the juvenile Epinephelus coiodies(grouper)was as experimental material.The growth performance,digestive enzyme activity,muscle nutrient composition and antioxidant defense system of juvenile grouper under different salinity of artivicial seawater(15-35‰)were researched.On this basis,the experiment of the salinity stress and compensatory growth.The main results were as follows:1.When the artivicial seawater salinity rang for 15-30‰,survival rate of grouper survival rate was 100%.When salinity was 35‰,survival rate of grouper was 92.5%.It showed that the grouper had high salinity tolerance,and growth of suitable salinity range was 20-30‰,the optimum salinity was 25‰.When grouper were not in the optimum salinity range,physiology performance was shown that the growth was inhibited,condition factor decreased,food conversion ratio increased and digestive enzyme activities were inhibited.The artivicial seawater with different salinity for juvenile grouper had no significant effect on muscle moisture,crude protein and crude ash,and significantly effect on crude lipid in muscle,components of amino acid and fatty acids.2.The activity of antioxidant enzyme and the content of malondialdehyde in different juvenile grouper tissues had different physiological response in artivicial seawater with different salinity.All kinds of activity of antioxidant enzymes in liver and the content of malondialdehyde were significantly higher than that in gills and brain.In optimum salinity group(salinity is 25‰),activity of antioxidant enzyme(except catalase)and the content of malondialdehyde were the lowest in juvenile liver,suggesting that juvenile grouper protect their body from oxidative damage by increasing their activity of antioxidant enzyme when under salinity stress,but the protection was a certain limit,as in 35‰ salinity group,the regulatory function of catalase in juvenile liver had emerged disorders.Long-term salinity stress might lead to oxidative damage of juvenile liver cells,a large number of aspertate aminotransferase and alanine transaminase in liver released into the bloodstream.3.Under the stress of low salinity(15‰)and high salinity(35‰),the growth of juvenile grouper was significantly inhibited.After 14 days,the weight were significantly lower than the control group(salinity is 25‰).After the remove of salinity stress,juvenile grouper in low salinity group(15‰)after 28 days growth could get full compensatory growth,and growth retardation caused by high salinity(35‰)failed to get full compensatory growth during the trial.4.Salinity stress significantly inhibited the activity of gastrointestinal digestive enzyme of juvenile grouper,and high salinity stress effects stronger.After release from the salinity stress,gastrointestinal digestive enzyme of juvenile grouper in low salinity treatment groups was significantly improved,while in high salinity treatment groups,recovery was slower.After 28 days the protease activity compensatory growth could not return to normal levels.5.Salinity stress induce juvenile grouper produced more malondialdehyde.At the same time,juvenile grouper removed excess free radicals from the body by activation of antioxidant defense system.After the remove of salinity stress,antioxidant enzyme activity in liver of juvenile grouper in high salinity treatment groups needed more time to return to normal levels than juvenile grouper in low salinity groups.High salinity stress induced metabolic disturbance of hepatic hydrogen peroxide,and failed to return to normal levels during the experiment.These results suggested that juvenile grouper had high salinity tolerance,optimum salinity wais 25‰.Although juveniles could protect themselves against external salinity stress through their osmotic regulation and antioxidant defense system,while long-term salt stress could bring certain oxidative damage to the body and the damage degree caused by high salinity stress was greater than the low salinity stress.