| The cassava residue was selected as carbon source to study the effect of biofloc technique (BFT) on water quality control, growth performance and survival rate of shrimp. The trails were carried out in litopenaeus vannamei culture system of water zero-exchange. The study included four parts:the optimization of cultivating biological floccules; the effect of different diets on biofloc nursery system; the effect of different C/N rates on biofloc nursery and culture system. Experimental design and the main research were as follows.(1) The optimization of cultivating biological floccules. Design the fermentation time of cassava residue and the bacterium composition. The result showed that the removal rate of NH4+-N and NO2--N on 6 h fermentation time reached 82.81±0.02% and 76.35±0.03%; the group of lactobacillus:bacillus subtilis:nitrobacteria=0.4 mL/L:0.04 g/L:0.2 mL/L could wipe off NH4+-N and NO2--N on optimum level.(2) The effect of different C/N rates on biofloc nursery system. Improve the C/N rate of nursery system by adding pretreated cassava residue. The BFA groups designed graded level of C/N (15/1,20/1 and 25/1) were compared to the control groups on three parallel samples. At the end of the nursery trial of 15 days, NH4+-N concentration of the graded level of C/N significantly decreased 10.7±1.4%,14.4±0.81%,46.8±1.4% to the control (p<0.05); NO2--N concentration distinctly decreased 20.4 ±1.20%,45.3±0.24%, 68.5±1.2%(p<0.05); seedling emergence obviously increased 80.08±0.49%, 56.74±0.19%,149.71±0.29%(p<0.05) and total body weight obviously increased 175.07±0.82%,184.49±0.35%,195.99±0.35%(p<0.05). Therefore, in a shrimp nursery system selecting cassava residue as carbon source and keeping C/N rates equal to 25/1 was made for ideal water quality and improving seedling emergence and Promoting the growth of prawn.(3) The effect of different diets on biofloc nursery system. Design the Chaetoceros muelleri Lemmerman group (CL) and Spirulina platensis group (SP) compared to each other in litopenaeus vannamei nursery system of 40 L culture water. Cassava residue was added to Aquaculture water 10 min after deeding. The result showed that NH4+-N and NO2-N concentration of SP group were respectively inferior to 0.5 mg/L and 0.4 mg/L; and were respectively under 0.9 mg/L and 1.9 mg/L in CL group. The seedling emergence of SP group and CL group were respectively 18.5±0.2% and 12.2±0.5%, and average body weight of both were respectively 0.309±0.32 mg/tail and 0.258±0.17 mg/tail. The Spirulina platensis as a kind of tiny vegetable feed could obviously reduce NH4+-N and NO2--N concentration and raised seedling emergence and body weight (p<0.05). And both of the microalgae did not distinguish on controling vibrio.(4) The effect of different C/N rates on biofloc culture system. Improve the C/N rate of cultivation system by adding pretreated cassava residue. The BFA groups designed graded level of C/N rate (10/1,15/1,20/1 and 25/1) were compared to the control on three parallel samples. Cultivation period was 30 days. The result showed that NH4+-N and NO2--N were obviously reduced in biofloc cultivation system of which C/N rate were 10/1,20/1 and 25/1. The ability of biological floccules assimilating NH4+-N and NO2--N distinctly negatively related with C/N rate in certain extent. The weight gain rate of shrimp presented the positive correlation with C/N level. The utilizable proportion of bait presented negatively related with C/N level. The biological floccules could effectively restrain vibrio and which is associated with the C/N ratio. |
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