Effects Of Low Salinity On Growth And Physiology Of Juvenile Orange-spotted Grouper (Epinephelus Coioides)

This thesis took juvenile orange-spotted grouper (Epinephelus coioides) as the centralobject. During the experiments, ecology, physiology and immunology methods were takento study the effects of low salinity on growth, body composition, serum biochemicalindexes and antioxidant defense system of fish by gradually descending of salinity. Wehoped that it could provide scientific data for diluting culture. In our study,720tail ofhealthy juvenile orange-spotted grouper (44.17±0.19g) were cultured in six groups (fourparallel) with different salinities which were3,5,8,12,15and salinity31±0.5(naturalseawater) as control group. We sampled and tested at0,14,28,42and56d during theculturing period, respectively. The results were listed below:1. Effects of low salinity on growth and body composition of juvenile orange-spottedgrouperThe survival rate of juvenile orange-spotted grouper at low salinity were all highercompared to control group (P<0.05), both of salinity of8and12group were100%. Thespecific growth rates had an increasing trend, the group with salinity12increased thehighest, in the43-56days got its maxima to3.51%. The feeding rate was the highest atsalinity of12, reaching1.03%, but there was not significantly difference in feed conversionrate compared to other groups (P>0.05). There was no significant difference in liver indexand condition factor among all the groups (P>0.05). Among body compositions, the crudeprotein values in the muscle was not significantly affected (P>0.05), but moisture, crudefat, crude ash and energy were significantly affected (P＜0.05), crude fat in the muscle oflow salinity groups decreased. In conclusion, the juvenile orange-spotted grouper couldadapt to low salinity, and got best growth at salinity of12.2Effects of low salinity on serum biochemical indexes of juvenile orange-spotted grouperThe change within the control group was the lowest. At the salinity of3,5,8and12groups, the GLU, TP, ALB and CHOL were significantly higher than the control group(P<0.05). At salinity of3and15groups, TG increased and then decreased and this wasopposite to salinity of5,8and12groups. At low salt groups HBDH, ALP, LDH and ASTactivities were significantly higher than control group (P<0.05). At the salinity of3,5,8, 12and15groups, the contents of Ca firstly decreased and then increased, got theirminimum at the14thday. At salinity of3, the CRE increased and this was opposite tosalinity of15; the salinity of5,8and12groups were firstly decreased and then increased.At salinity of3, the BUN firstly increased, then decreased, peaked at14d, as (9.08±0.76)U/L; the salinity of5,8and12group firstly decreased, then increased; it dropped atsalinity of15. At salinity of3and12groups, the UA increased firstly and then decreased,got their peaks at the42thd and14thd separately; the salinity of5and8groups firstlydeclined and then increased; at salinity of15it dropped and disappeared at the42thd; therewas no UA in the control group. The experiment showed that the stress of juvenile grouperreared at salinity of15was smaller. As the salinity decreasing, the stress increased and itcaused a certain degree of damage.3. Effects of low salinity on antioxidant defense system of juvenile orange-spotted grouperThe activities of SOD, CAT and GSH-PX were the highest in liver, the same wasMDA content. The SOD activity in liver and kidney of each experiment group was firstlydecreased and then increased, the salinity of5,8and12groups were significantly differentform the control group(P<0.05). The SOD activity in muscle firstly increased and thendecreased; the salinity of3group was significantly different with the salinity of8and15group (P<0.05). At salinity of3,5and8groups, the GSH-PX activity in liver increasedfirstly and then decreased, but it had an increasing trend in the muscle. At salinity of12and15groups, the GSH-PX activity in the liver showed decreasing trend; it decreasedfirstly and then increased at salinity of12and15groups; the results of kidney and liverwere almost opposite. At the low salinity groups, the change of CAT activity in liver andkidneys was similar with SOD; CAT activity was not detected in the muscle. MDAcontents decreased firstly and then increased in different experimental groups and tissues.The experimental results showed that juvenile orange-spotted grouper had strongendurance in low-salt environment. With changing salinity, it could alter the activities ofantioxidant enzymes to eliminate the free radicals produced by oxidative stress andmaintain a relative stable environment in the body.