Effects Of Stocking Density On Growth And Physiology Of Micropterus Salmoides Reared In In-pond Raceway Culture Systems

In-pond raceway system(IPRS)is a new type of aquaculture model,which is characterized by small area for fish culture and large area for water purification.Compared with traditional pond culture,IPRS achieves a sustainable development model of environmental protection,fresh meat and high yield because water resources are greatly saved by recycling treated water,and the flowing water is conducive to the movement of fish.Stocking density is considered to be one of the important actors determining fish growth and profitability in IPRS.In order to obtain high farming profits,increasing stocking density is an inevitable choice for farmers.However,excessive stocking density could cause stress to fish and bring about a series of negative effects,such as growth retardation,metabolic dysfunction,and decreased immune function.Largemouth bass,native to freshwater lakes and rivers in North America has become an important freshwater aquaculture species in China and has been widely cultivated nationwide because of its strong adaptability,rapid growth,and delicious meat,In order to provide scientific support for the popularization of IPRS,a 120-day feeding experiment was performed to study the effects of stocking densities on the growth and physiology of largemouth bass reared in In-pond raceway culture systems(IPRS),fish were divided into 3 density groups(SD1,0.2 kg/m~3,SD2,0.4 kg/m~3,SD3,0.6 kg/m~3).Each density was tested in triplicate.1.The growth index of largemouth bass was significantly affected by the stocking density.The ultimate body weight decreased significantly with the increase of stocking density(P<0.05).There were decline in weight gain rate,specific growth rate,condition factor,appetite of Micropterus salmoides with rising stocking density,so the lowest body weight,weight gain rate,specific growth rate and daily food intake was seen in the SD3 group at the end of the experiment.In this study,no significant difference were observed on the body water,ash and protein of the largemouth bass in each density group(P>0.05)while the crude lipid content of the SD3 group was significantly lower than that of the SD1 and SD2 groups(P<0.05)at the end of the test,so the conclusion that we came to was that fish preferred to use fat in response to energy requirements under stress.2.The contents of serum cortisol,glucose,triglyceride were significantly affected by stocking density.The cortisol and blood glucose levels in each group all showed increasing firstly then decreasing latter trend.By the end of the study,the contents of serum cortisol,glucose,triglyceride in the SD3 group were significantly higher than that in the SD1 group(P<0.05),but there was no significant difference in total cholesterol and total protein in the serum of largemouth bass in different stocking density groups during the breeding period(P>0.05).Meanwhile,the result that lysozyme content in SD1 group was significantly higher than that in SD3 group at day60-120 indicated high density inhibited the immune performance of largemouth bass.3.No significant differences were found in the activities of SOD and CAT in liver among all treatments on day 90(P>0.05),however,fish reared in the SD1 group had significant higher values than those reared in the SD3 group on day 120(P<0.05).These results showed the stress induced by high culture density inhibited the antioxidant capacity of largemouth bass.The fish reared in the SD3 group had significantly higher MDA values than those reared in the SD1 group on days 90 and120(P<0.05),suggesting high stocking density increased oxygen free radicals and enhanced lipid peroxidation in fish.In addition,there was no significant difference in ALT,AST,ALP,T-AOC and GSH-PX in the three groups.Observation of tissue sections showed that the liver of largemouth bass was not harmed and the structure of hepatocytes in the fish liver of SD1,SD2 and SD3 group was normal,but vacuoles in the SD3 group was relatively large.4.There was no significant difference in intestinal amylase and lipase of largemouth bass in each density group during the 120-day culture period(P>0.05);The length and thickness of the intestinal villi of largemouth bass in different culture density groups were not significantly different(P>0.05).Compared with SD1group,the number or size of goblet cells in SD3 group was reduced,which indicated although the stocking density in this experiment had a negative effect on the number and size of intestinal goblet cells,it did not affect the intestinal digestion and absorption function.5.The relative expression levels of brain GH and liver IGF-I genes of Micropterus salmoides in SD2 group were significantly higher than those in SD1group(P<0.05)at days 30,however,the relative expression levels of brain GH genes in SD3 group were significantly higher than SD1 Group(P<0.05),while the relative expression of liver IGF-I gene in SD1 group was significantly higher than that in SD3group(P<0.05),but there was no significant difference in the expression levels of brain GH and liver IGF-I gene m RNA between density groups at 120 days(P>0.05).The liver HSP70 m RNA expression level of the largemouth bass in the SD3group was significantly enhanced at 90 and 120 days,indicating that the density stress induced the expression of HSP70 in the liver,and the expression of SOD in the liver of the SD1 group was significantly higher than that of the other two groups at days 90and 120.In summary,the stocking density influenced the growth,serum biochemical indicators,immune indicators,and liver antioxidant enzyme activities of Micropterus salmoides reared in IPRS.The growth rate of largemouth bass in the culture group of0.4 kg/m~3was better than that of the 0.2 kg/m~3and 0.6 kg/m~3group before days 30.After that,the growth and immunity of the largemouth bass were reduced when the stocking density increased.Therefore,the antioxidant and immune functions of the fish in the high culture density group were suppressed,and the liver and intestinal structures were slightly damaged.The physiological status of the largemouth bass in the 0.2 kg/m3 density group was the best.In addition,based on the results of this experiment,it is recommended that the stocking density of largemouth bass in IPRS should be controlled between 0.2 and 0.4kg/m~3from a practical point of view.The regulatory role the relationship between the GH and IGF-I under stress need to be further studied.