Effects Of Salinity,biofloc/Total Suspended Solids (TSS) Concentration And Stocking Density On Growth,Digestive Enzymes And Non-specific Immune Enzymes Of Lateolabrax Maculatus

The production of Lateolabrax maculatus（Japanese sea perch）,a common marine fish species farmed in China,has been on the rise in recent years.However,with the increase of environmental pressure,traditional pond and cage culture models are gradually eliminated.Thus,practitioners and aquaculturists urgently need a new aquaculture production system for this fish.Biofloc Technology（BFT）as a new type of aquaculture system for fish production is considered to be a better alternative for fish farming in the future.At present,breeding has been achieved on a number of breeding species,but there is no report on the breeding of L.maculatus.At the same time,L.maculatus,as a wide-salt fish,can survive at a salinity of 0-40.There have been many reports about the effects of salinity on L.maculatus but no report has found that focuses on the study of salinity on the muscle characteristics of L.maculatus.Therefore,this study attempted to determine the optimal level of salinity for the proper growth performance of L.maculatus.Also,the study investigated the effects of salinity on the muscle characteristics of L.maculatus,and at the same time studied the effects of different TSS concentrations and different stocking densities on the growth,physiological and biochemical characteristics of the L.maculatus cultured in a biofloc technology aquaculture system.The finding of this study will provide the practical basis and theoretical guidance for the use of bioflocs to breed L.maculatus.1.The effect of salinity on the growth performance,physiological,biochemical and muscle characteristics of L.maculatus.In order to study the effects of salinity on the growth performance,physiology,biochemistry and muscle characteristics of L.maculatus,the experiment was divided into5 salinity groups 0,10,20,30 and 40,and a 60-day experiment was carried out.The results showed:（1）The water quality was maintained in a good state throughout the breeding period,and there was no significant difference between the groups（P>0.05）（2）The final quality of seabass increased first and then decreased with the increase in salinity,and the highest salinity recorded was 47.38±2.34 g at 10 salinity group,which was significantly higher than the salinity 0,30 and 40 groups（P<0.05）;while the weight gain rate and specific growth rate of sea bass when the salinity was 10 were 1920.95±99.94%and 5.01±0.08%/d,which were significantly higher than in the other groups（P<0.05）.Also,the feed coefficient in this group（salinity 10）was the lowest 1.04±0.02（P<0.05）when compared to the other groups.（3）Digestive enzyme results showed that the amylase activity of the 10 salinity group was 0.140±0.03 U/mgprot,which was significantly lower than that of the other groups（P>0.05）.There was no significant difference in the activities of trypsin and lipase in each group（P>0.05）.Respectively,the results of antioxidant enzymes indicated:GSH-Px activity and T-SOD activity showed a trend of decreasing first and then increasing with the increase of salinity.Notably,GSH-Px activity and T-SOD activity were the highest in the salinity group 40,which were 327.00±23.45 and 98.39±3.76 U/mgprot,and these were significantly higher than the 10salinity group（P<0.05）.Respectively these values were not significantly different from other groups（P>0.05）,whiles in the highest salinity group,CAT was 21.38±0.95 U/ml which significantly higher than the other groups（P<0.05）.Importantly the Na⁺-K⁺-ATPase activity showed a trend of first decreasing and then increasing with the increase of salinity.The activity of Na⁺-K⁺-ATPase in the 10 salinity group showed the lowest activity of 10.21±0.11,which was not significantly different from the 20 salinity group（P>0.05）,and was significantly different from other groups（P<0.05）.（4）With the increase of salinity,the water content among the groups gradually decreased,and the difference between the salinity 0 group and the salinity 40 group was significant（P<0.05）,however there no significant differences were between the other groups（P>0.05）.There was no significant difference between the crude ash and crude fat among all the groups（P>0.05）;the crude protein content increased slowly with the increase of salinity,and there was no significant difference between the crude protein content of the 10 salinity group and the 30 salinity group（P<0.05）.Like wisethere no significant differences was observed among the other groups P>0.05).Notably,except for the significant difference in elasticity between the 40 salinity group and the other groups（P<0.05）,there were no significant differences in muscle hardness,chewiness,cohesion,restoring force and brittleness among the salinities（P>0.05）.The muscle quality of salinity group 10 and salinity group 40 was slightly higher than that of other groups.2.The effect of TSS concentration on the growth,digestion and antioxidant enzyme activity of L.maculatus.In order to study the effect of TSS concentration on the growth,digestion and antioxidant enzyme activity of L.maculatus,the experiment was divided into three groups:L group:TSS 200-300 mg/L,M group:TSS 400-600 mg/L and H group:TSS 600-800mg/L.Respectively,a 30-day experiment was carried out,and the results showed that:（1）Although the water quality of each group fluctuated during the breeding period,it was maintained at a low level.After the experiment,there was no significant difference between the groups（P>0.05）.（2）The final body weight,weight gain rate and specific growth rate showed a trend of increasing initially and then decreasing with the increase of TSS,but the difference between the groups was not significant（P>0.05）.At the same time,the survival rate,liver weight of each group,the difference in ratio,fullness and feed coefficient did not show any significant（P>0.05）differences,but the feed coefficient showed an upward trend with the increase of TSS concentration.（3）Digestive enzyme activity gradually decreases with the increase of TSS concentration,and starch.Importantly,the enzymes including lipase and trypsin activities were significantly different between group L and group H（P<0.05）However,that of group M was not significantly different from other groups（P>0.05）.（4）Antioxidant enzyme activity gradually increased with the increase of TSS concentration,and there was no significant difference in GSH-Px and CAT activity between the groups（P>0.05）;the difference between T-SOD in group L and group H was significant（P<0.05）,but the difference from group M were not significant（P>0.05）.3.The effect of breeding density on the growth and blood biochemical indexes of L.maculatus.3.The effect of stocking density on the growth and blood biochemical indexes of L.maculatus.In order to study the effect of breeding density on the growth and blood biochemical indicators of L.maculatus,3 experimental groups were set up（LD group:10 kg/m³,MD group:20 kg/m³ and HD group:30 kg/m³）which lasted for a 30-day breeding period.Respectively,the results showed that:（1）The nitrite in the three groups reached the peak on the fourth day,and the NO₂^--N of the three groups gradually decreased and stabilized during the experimental period.The TAN of the LD group and the MD group were reached the peak on the next day,then gradually decreased and stabilized.NO₃^--N and TN increased with the increase of the cultivation time during the entire cultivation period（P<0.05）.（2）The final body weight,weight gain rate and specific growth rate showed a trend of increasing initially and then decreasing with the increase of stocking density.The weight gain rate and specific growth rate of the MD group were significantly different from those of the HD group（P<0.05）,and there was no significant difference in the LD group（P>0.05）;the survival rate,liver body index,fullness and diet coefficient of the three groups were not significantly different（P>0.05）.（3）Alanine aminotransferase and aspartate aminotransferase gradually decreased with the increase of breeding density,and the difference between the groups was not significant（P>0.05）.The content of alkaline phosphatase and total protein in the LD group was the highest 30.45±2.70 U/L and34.67±1.62 g/L,respectively,but the difference between the groups was not significant（P>0.05）.Also,the content of triglyceride and glucose gradually decreased with the increase of stocking density,and the highest content of triglyceride in the LD group was8.87±0.82 mmol/L.Notably,the difference between MD and HD group in terms of levels of triglyceride was found to be significant（P<0.05）,however,the differences in terms of total cholesterol and glucose content among the three groups was not significant（P>0.05）.