| At present,with the continuous expansion of the scale of shrimp aquaculture,the impact of mariculture wastewater on environment becomes more and more serious,which makes aquaculture wastewater purification not allowed to ignore.Effectively reducing the discharge wastewater and the risk of polluting the surrounding environment would promote aquaculture to be more environmentally friendly and sustainable.The intensive zero-change aquaculture mode of shrimp based on Biofloc technology（BFT）has many advantages,such as saving water resources,reducing pollution,reducing the probability of disease of aquaculture organisms,and improving the utilization rate of bait.However,with the increasing amount of biofloc in breeding water,excessive biofloc and high concentration of nitrogen and phosphorous salt in water may make the culture system produce ultra-eutrophic block effect of the culture system,which is neither favourable for the healthy aquaculture nor for stable operation of the breeding system.In this study,GIFT Oreochromis niloticus used for feeding the biofloc particles and Spirulina platensis used for absorbing the inorganic nitrogen and phosphorus were introduced into the wastewater of shrimp biofloc culture,and the biological purification effect of tilapia and spirulina on wastewater under different conditions were analysed.The following results are obtained:1)Four salinities groups of 0‰,10‰,20‰and 30‰were set in the experiment.The initial body weight of tilapia is about 7g.Under normal feeding conditions comparing and analyzing the adaptability of GIFT Oreochromis niloticus and Spirulina platensis at different salinities,and the feeding situation of GIFT Oreochromis niloticus to Spirulina platensis.The results of the 16 day experiment showed that the body length and body weight of each group of tilapia before and after the experiment increased significantly（P<0.05）,and there was no difference between the groups（P>0.05）.The survival rates were 93.5%,88.9%,98.9%,98.9%,the weight growth rates were 30.0%,26.7%,44.4%,and 36.5%,respectively.There was no significant difference in the density of Spirulina cells before and after the 16day experiment at each salinity,which was 105 cell·m L-1.Microscopic examination found that algal cells were detected in the intestines,feces,and aquaculture water of the fish supplemented with Spirulina.And under different salinity,because tilapia ingested Spirulina,the algal cell concentration decreased significantly on the 2 day of the experiment（P<0.05）.The above results show that both the tilapia and the Spirulina used in the experiment can adapt to the high salinity seawater environment.2)Experiments on GIFT Oreochromis niloticus and Spirulina platensis were carried out in the tail water of biofloc shrimp culture with salinity of 30‰.The experimental groups were control group Ctrl,single algae group SP,fish and algae group SPGT,single fish group GT,the experimental time was 16 day without feeding the whole experimental period.To analyze the purification effect of fish and algae on the tail water of biofloc culture.The results showed that the concentration of algae in the SP group increased but not significantly before and after the experiment（P>0.05）.The body length and body weight of 16 day tilapia in SPGT group increased but not significantly（P>0.05）,and the cell density of Spirulina platensis decreased by two orders of magnitude and on the 4 day of experiment,the algal cells decreased significantly from 105 cell·m L-1 to 104 cell·m L-1（P<0.05）.The body length and body weight of the tilapia in the GT group did not change significantly,and the survival rate of the tilapia in two groups were 70%.The sedimentation volume of biofloc in SPGT and GT groups decreased by 42.6%and 54.9%,and the maximum removal rates of total suspended solids（TSS）in water were 32.2%and 46.5%,due to the feeding of tilapia.The removal rates of nitrate nitrogen and total inorganic nitrogen in SP group were 43.2%and27.7%.The above results show that in the tail water of marine biofloc shrimp culture,the effect of tilapia on the removal of suspended particulate matter in the tail water is obvious,and tilapia has a good feeding effect on Spirulina.3)In order to avoid instantaneous changes in the chemical forms of carbon,nitrogen,phosphorus and other compounds in the water during the monitoring process.There is a bias in evaluating the effect of tail water purification only from water quality indicators.Research and analysis the biofloc effect of tilapia on environmental carbon,nitrogen and phosphorus nutrients under different salinity conditions and no feeding in biofloc tailwater was studied.The results showed that the deposition of carbon,nitrogen and phosphorus in the environment by fish was different under different salinities.Among them,the total organic carbon（TOC）,total nitrogen（TN）and total phosphorus（TP）contents of tilapia per unit mass of dry matter in the salinity 20 group rate of increase were 9.1%,2.2%and 30.5%,and the growth rate of the TP content per unit mass of dry matter in the tilapia with salinity 30rate of increase was 20.9%.However,the TOC and TP in the salinity 0 group rate of descend were 19.8%and 35.4%.It indicated that the phosphorus deposition effect of tilapia was more favorable under the normal mariculture conditions.The body mass of tilapia in the SPGT and GT groups in the tail water environment of biofloc culture was not significantly different from its initial value（P<0.05）.However,the dry matter TOC,TN and TP per unit mass of fish in the SPGT group rate of increase were 5.9%,18.7%and 14.7%compared with the initial stage,and the TN and TP of the tilapia in the GT group rate of increase were 9.8%and 11.6%compared with the initial stage.It shows that the synergistic use of Gift Oreochromis niloticus and Spirulina platensis in the tail water of biofloc shrimp aquaculture improves the biological deposition of carbon,nitrogen and phosphorus in the water environment by tilapia and beneficial to the biological purification of nitrogen and phosphorus in biofloc tail water. |
