| Chronic stress caused by high stocking density is a common stress factor in aquaculture.It usually has a negative impact on the growth performance of fish and damages fish welfare.As an important nutrient for fish,protein is also an essential material basis for the maintenance of vital functions of the body.Its metabolism is greatly affected by environmental stress.Skeletal muscle accounts for 65%to 75%of the total dryness of the fish,and its growth and development determines the growth rate of the fish.Muscle growth depends mainly on its protein metabolism efficiency,and the equilibrium between protein synthesis and degradation is an important basis for maintaining muscle growth.In this study,we investigated growth factor,body composition,blood physiological,biochemical indexes,UPS activity,protein metabolism,THs indicatorsetc and key components of IGF system in gibel carp(carassius auratus gibelio var.Ⅲ),intended to provide a basic scientific basis for aquaculture environmental control.Three stocking densities were designed in the experiment,which were low stocking density control group(LSD),15 tails/tank(1.21 kg m-2);medium stocking density group(MSD),35 tails/tank(3.78 kg m-2);High stocking density group(HSD),75 tails/tank(8.84 kg m-2),three replicates for each experimental group.The experinmental period was 60 days.Feeding at 3%of body weight.At the end of the experiment,the fish growth index was measured.The blood,muscle,liver tissue and thyroid of the gibel carp were collected for testing blood biochemical indicators,nutritional indicators,tissue sections,ubiquitinated protein content,free amino acid concentration and relative expression of genes.The results are as follows1.The chronic stress had significant effects on gibel carp growth index,serum biochemical index and body nutrient composition.At the end of the experinemnt,the final weight,weight gain rate(WGR),specific growth rate(SGR)and fatness of LSD were significantly higher than those in MSD and HSD.The feed conversion efficiency was significantly lower than that of HSD.In addition,there was no significant difference in terminal body length,feeding rate and survival rate among different stocking density groups.After 8 weeks of crowding stress,compared with LSD and MSD,the serum alanine aminotransferase(ALT)and aspartate trasaminase(AST)levels in LSD were significantly increased,and there were no significant differences between LSD and MSD At the same time,the serum cortisol levels in MSD and HSD were significantly higher than those in LSD.In addition,triglyceride(TG),blood glucose(GLU)and total cholesterol(CHO)decreased significantly with increasing stocking density.There was no significant difference between total protein and alkaline phosphatase(ALP)in each group.The water content of the back muscles in LSD was significantly lower than that of HSD.In addition,with the increase of stocking density,the crude fat content of back muscle decreased significantly.There was no significant differences in crude ash and crude protein content among three stocking densities.There was no significant differences in the diameter of the back muscle fibers among the different breeding densities2.In HSD,hsp70,hsp90 and chip expression levels were significantly higher than those in LSD.Compared to LSD,the expression levels of myf-5 and mrf4 were consistent with myog,and all significantly decreased in MSD and HSD.However,the expression level of igf-1 was significantly increased in HSD,while the igf-lr was significantly decreased.At the same time,there was no significant differences in the expression of myod and colla-2 genes.The expression levels of ub,e2 and psrma2 in the back muscle of HSD were significantly increased.There was no significant difference between el and psmcl in each group.The content of ubiquitinated-protein in back muscle was significantly higher in MSD and HSD than LSD group.In the middle and high density groups,the expression levels of murfl was significantly higher than LSD group.The mRNA level of the protein synthesis gene mtor was significantly lower in HSD,and there was no significant differences between LSD and MSD.S6kl did not change significantly amongeach group.In MSD and HSD groups,the content of tyrosine(Tyr)was significantly higher than that of LSD.There was no significant differences in the content of 3-MH among each group3.After 60 days of crowding stress,no significant difference in thyroid follicular tissue was observed between different stocking densities.Compared with LSD,the T3 content in HSD was significantly decreased.In addition,the T3/T4 ratio was significantly higher in LSD.There were no significant differences in FT3,T4 and FT4 levels among each group.The activity of ID1 was significantly decreased with the increase of culture density.There was no significant difference in the activity of ID2 and ID3 among the various culture densities groups.Overall,these data suggested that chronic crowding stress could impede fish growth by inhibiting skeletal growth.At the same time,stimulating the THs metabolism and down-regulates the IGF system,as well as enhancing UPS proteolytic pathways. |
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