Effects Of Acute High Temperature Stress On Some Physiological Indexes And Study Of Thermal Tolerance In Juvenile Mandarin Fish(Siniperca Chuatsi)

Mandarin fish（siniperca chuatsi）,widely distributed in the lower reaches of the Yangtze River and other lakes in China,is one of the most aquaculture species in southern China.However,high temperatures in summer often cause diseases and death in the process of juvenile fish breeding.Temperature is one of the most important ecological factors in aquaculture.Change of water temperature will directly affect the feeding,growth,metabolism,behavior,etc.This thermal resistance experiment is helpful to understand the thermal physiology of Siniperca chuatsi and the ecological impact of high temperature stress on the natural environment.In order to explore the influence mechanism of high temperature on the thermal tolerance of juvenile mandarin fish,Dynamic method and Static method were used in this paper,and two-month-old juvenile mandarin fish were taken as the experimental subjects.The Critical Thermal Maximum（CTM）,Lethal Temperature Maximum（LTM）and 24hour Upper Incipient Lethal Temperature(24h-UILT₅₀)of mandarin fish under different basic water temperatures（22℃,26℃,30℃,34℃）were compared and studied.Acute high temperature stress experiment was carried out by the method of abrupt temperature rise.The samples were divided into normal temperature control group（26℃）and high temperature stress group（36℃）.After 0 h,6 h,12 h,24 h,48h and 96 h of stress,respectively,the liver,intestine,stomach and pyloric caecum tissues of juvenile mandarin fish were collected.Then the relative expression levels of antioxidant enzymes,digestive enzymes and heat shock protein genes of juvenile mandarin fish were measured,and the physiological response process of juvenile mandarin fish to high temperature stress was analyzed from two aspects of enzyme activity changes and gene expression changes.The main results of the test are as follows:1.Study on high temperature tolerance of juvenile mandarin fishBase temperature significantly affects the maximum critical temperature（CTM）of juvenile mandarin fish,and CTM was positively correlated with base water temperature.The CT_maxof mandarin fish at 22℃,26℃,30℃and 34℃was 36.91℃,37.73℃,38.37℃and 39.54℃,respectively.The maximum lethal temperature（LTM）of mandarin fish was significantly affected by the basal temperature,and LTM was positively correlated with the basal water temperature.The LT_maxof mandarin fish was 37.18℃,37.91℃,38.74℃and 39.86℃,respectively.Base temperature significantly affected the 24h high initial lethal temperature(24h-UILT₅₀)of mandarin fish,and the 24h-UILT₅₀was significantly positively correlated with the base water temperature.The 24h-UILT₅₀of mandarin fish was 35.63℃,36.65℃,37.60℃and39.44℃,respectively.In conclusion,the thermal resistance of juvenile mandarin fish increases significantly with the increase of basic water temperature.2.Effects of acute high temperature stress on digestive enzyme activities of juvenile mandarin fishHigh temperature has significant effects on the activities of pepsin,amylase and lipase in stomach,intestinal tract and pyloric caecum of juvenile mandarin fish.The activities of pepsin and amylase in the stomach and amylase in intestinal of mandarin fish showed a decreasing-increasing-decreasing trend with the prolongation of high temperature stress time.The activity of lipase in the stomach and intestine of mandarin fish showed a trend of increasing-decreasing-increasing with the prolongation of high temperature stress time.The activities of pepsin in the intestine and lipase in the pyloric caecum of mandarin fish were firstly increased and then decreased with the prolongation of high temperature stress time.The activities of pepsin and amylase in the pyloric caecum of mandarin fish were firstly decreased and then increased with the prolongation of high temperature stress time.The results showed that the mutant high temperature cause stress to juvenile mandarin fish,and the digestive system was inhibited in the early stage.Subsequently,fish body increased the activity of digestive enzymes in some tissues by inducing them to decompose glycogen,fat and protein to provide energy for body to fight high temperature stress.3.Effects of acute high temperature stress on antioxidant enzyme activity,energy metabolism and heat shock protein gene expression in liver of juvenile mandarin fishAcute high temperature stress has significant effects on liver antioxidant enzymes,and gene expression of heat shock protein.Juvenile S.chuatsi had poor tolerance to high temperature in a short time（6h）,the antioxidant system was suppressed after a sharp rise in temperature,and gradually recoverd after a short period of adaptation,however,it was inhibited again at the end of stress（96h）;In this study,the superoxide dismutase（SOD）activity,catalase（CAT）activity in the liver of the experimental group showed a decreasing-increasing-decreasing trend;The alanine aminotransferase（GPT）activity and malondialdehyde（MDA）content in the liver of the experimental group showed a increasing-decreasing-increasing trend;The glutathione（GSH）content in the liver of the experimental group showed a decreasing-increasing-decreasing-increasing trend.Compared with control group,the relative expression of HSP70 and HSP90 gene in the liver of juvenile S.chuatsi had a tendency of increasing firstly and then decreasing after high temperature stress,and was significantly increased after 12 h stress,and peaked at 12 h and 24 h after stress;And then continued to decrease,they were significantly higher than the control group at 96h.