Effect Of Culture Density On Litopenaeus Vannamei In Biofloc Culture System

In the process of high density culture,high feed input and large amount of water exchange are one of the most commonly used measures to maintain good water quality.The discharge of these wastewater will have a negative impact on the environment,which is a major challenge for the ecological sustainable culture of shrimp.Biological floc system is a kind of environment-friendly and economical aquaculture model.In this study,the biological floc comes from the nitrification type biological floc mainly constructed by the laboratory and mainly composed of the flora in the water body of the area.The experiment set five culture densities of 100,300,500,700,and 900 tails/m~3.The effects of the culture density on the Litopenaeus vannamei in the biological floc system were discussed and the following results were obtained.1.The concentration of harmful nitrogen in each treatment group was not high,which indicated that biological flocs could effectively control the concentration of ammonia nitrogen and nitrite nitrogen in cultured water body,but the higher the culture density,the worse the stability of water quality,and the peak value of ammonia nitrogen and nitrite in high density(E group)was 0.90 mg/L?1.19mg/L respectively.The concentration of nitrate nitrogen increased continuously,the accumulation of high density group was the largest.The total alkalinity and p H of high culture density group decreased obviously,and the greater the density,the greater the volume of flocs in each treatment group changed,the maximum value did not exceed 25 ml/L.In addition,biological flocs have inhibitory effect on Vibrio,and the number of Vibrio in each treatment group is low,however,the higher the culture density,the more obvious the fluctuation of the abundance of the total heterogeneous bacteria and Vibrio in the early stage of culture.2.Proper increase of culture density can increase the activity of digestive enzyme and immune enzyme and promote the growth of prawn.B(300 tails/m~3)group grew fastest with mean body length(5.8±0.91)cm and body mass(2.28±0.79)g,the highest amylase activity in group C,the highest lipase activity in group B,the highest SOD activity in group D,the highest CAT activity in group B,and the highest AKP and ACP activity in group C.At the stocking density of E,the growth and digestive enzyme activity of Litopenaeus vannamei were significantly inhibited,such immune enzyme activity as CAT,AKP,ACP,etc.were the lowest,and the antioxidant capacity was weakened.Hence,under the biological floc culture mode,it is recommended that the culture density of vannamei is about 300-500 tails/m~3.3.The most abundant Phylum of biological flocs in each treatment group were Proteobacteria,Bacteroidetes,Actinobacteria,Chloroflexi,Crenarchaeota,Cyanobacteria,Planctomycetes.Nitrosopumilus,which has a transformation effect on nitrogen in the water body,is the most abundant genus in 15 samples.The high-density taxonomic unit is the least,and the biological floc bacteria between the low-density groups gather closer.Significant changes occurred in the abundance of Actinobacteria,Bacteroidetes,Cyanobacteria,Firmicutes and Tenericutes in the intestine of Litopenaeus vannamei.The abundance of Bacteriovorax,Marivita,shimia,Tenacibaculum and Pseudoalteromonas in the low density group(A,B,C)was higher,but the abundance of the genus Bdellovibrio,Rhodobaca and Propionigenium in the high density group(D,E)was higher.4.Using high-throughput sequencing technology to analyze the differences in the transcription levels of shrimp under different density conditions,a total of 24389unigenes were obtained,with an average length of 1025 bp and an N50 length of 1848bp.Among them,Nr,KEGG,Swissport,and KOG databases were annotated to 10957,10061,8968 and 8104 unigene.The differential gene expression was most obvious in the high-density group E,which significantly changed the muscle cell genes of about1195 shrimp,of which about 706 genes were up-regulated and 489 genes were down-regulated.The differentially expressed genes were mostly enriched in phagosomes,Glycolysis/gluconeogenesis,proteasome,Salmonella infection,influenza A,Rap1signaling pathway,platelet activation,and tight junction pathways.The results showed that the response to high density stress of L.vannamei was mainly related to immune and disease-related pathways in biological floc culture system.The above research results provide some data basis for the healthy culture of L.vannamei under the biological floc culture model.