Microbial Flocs Spare Protein In White Shrimp Diets
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The authors conducted a study to determine how reducing the protein content of feed affects shrimp growth in a biofloc system. Shrimp given feed with less than 25% crude protein performed similarly to shrimp raised under regular intensive culture with a 37%-protein feed. The biofloc system also delivered more consistent survival rates, especially at higher densities. Biofloc production increased over time in higher density tanks and helped support shrimp growth with lower protein feeds.
Microbial Flocs Spare Protein In White Shrimp Diets
- 1. 22 May/June 2010 global aquaculture advocate The Pacific white shrimp, Litopenaeus vannamei, is the most widely farmed shrimp worldwide today. Much of this achievement can be accredited to its pre- dominant omnivorous feeding habit, which can spare expensive nutrients in commercial feeds. Protein ingredients make up the bulk of the formula costs in shrimp diets. Although feeds with less than 25% crude protein can be used at densities of 10 ani- mals/m2 or less in growout, protein levels can rise as high as 40% under higher stocking densities. Since microbial floc production sys- tems are rapidly emerging in the com- mercial culture of L. vannamei, it becomes crucial to evaluate the role of high-protein diets under these condi- tions. Bioflocs, or microbial detritus, are rich in several essential nutrients that support and enhance the growth of L. vannamei under intensive culture. Experimental Design The authors conducted a study to determine how reducing the protein con- tent of a diet would affect the growth per- formance of L. vannamei reared in an experimental microbial floc culture system. Thirty circular tanks of 1,000-L vol- ume were used for the study. Each tank was covered with nets and equipped with four aeration stones 15 cm from the tank bottom. In addition, two floating PVC airlifts were placed in each tank for verti- cal water circulation. Juvenile L. vannamei weighing 3.53 ± 0.77 g each were stocked in rearing tanks at 50, 75 and 100 shrimp/m2 . Five repli- cate tanks were assigned for each stocking density, totaling 15 tanks under microbial floc culture (MFC) and 15 under regular intensive culture (RIC). Two diets were prepared. Diet RIC consisted of a commercial shrimp diet with 36.9% crude protein and 5.5% lipids that was ground, cooked and repelleted with lab manufacturing equipment. The MFC diet was formulated to contain less than 25% crude protein and a final carbon:nitrogen (C:N) ratio of 12:1. This lab-made diet was a combination of the RIC diet mixed with a low-fiber com- mercial poultry feed, dried molasses and a synthetic binder (Table 1). Shrimp were daily fed to satiation from feeding trays at 7 a.m. and 4 p.m. Animals under RIC received the com- mercial diet alone, whereas for MFC, dried molasses mixed with seawater was added once daily based on feed consump- tion and a targeted C:N ratio of 20:1 for the culture water. Water Preparation Prior to shrimp stocking, clean seawa- ter in each rearing tank was fertilized with sodium nitrate, sodium silicate and monoammonium phosphate. To boost phytoplankton growth, greenwater from six nursery tanks was collected, mixed and inoculated at 20 L/m3 . Rearing water was aerated for three more days before the shrimp were stocked. At the start of the trial period, only tanks intended to operate under the production of micro- bial flocs were treated with a commercial microbial mixture. No water exchange took place during culture in any of the tanks, but the tanks were filled whenever necessary with sea- water lost due to evaporation. No other Each rearing tank was covered with nets and equipped with four aeration stones and two floating airlifts for vertical water circulation. Microbial Flocs Spare Protein In White Shrimp Diets production Summary: The authors conducted a study to determine how reducing the protein content of a diet would af- fect the growth performance of L. vannamei reared in an experimen- tal microbial floc culture system. Shrimp given feed with less than 25% crude protein performed similarly to shrimp raised under regular intensive culture with a 37%-protein diet. The biofloc sys- tem also delivered more consistent survival rates, especially at higher density. Alberto J. P. Nunes, Ph.D. Instituto de Ciências do Mar Av. da Abolição, 3207 – Meireles Fortaleza, Ceará 60165-081 Brazil albertojpn@uol.com.br Leandro Fonseca Castro, M.S. Hassan Sabry-Neto, M.S. Instituto de Ciências do Mar
- 2. global aquaculture advocate May/June 2010 23 attempts to correct water quality were made during 76 days of culture, which included four days of acclimation. Biofloc formation was measured every three days from all tanks. Results Water salinity and temperature reached means of 36 ± 2.6 ppt and 31.6 ± 0.93° C, respectively. No significant vari- ation was observed for these parameters between culture systems or among stock- ing densities used (P > 0.05). In both types of systems, water pH decreased from over 8 at the onset of the study to less than 7 close to harvest. Within each system, pH also varied as a function of stocking density, particu- larly when culture water with 50 and 100 shrimp/m2 under regular intensive culture were compared (7.7 ± 0.51 and 7.4 ± 0.55, respectively). Daily oxygen mea- surements taken at 3 a.m., 4 p.m. and 11 p.m. never fell below 2.23 mg/L, with a mean value of 3.53 ± 0.77 mg/L. Despite the reduction in feed protein content, shrimp had very similar perfor- mance under both microbial floc culture and regular intensive culture. Some parameters varied as a function of stock- ing density and/or culture system. Final shrimp survival did not differ between the MFC and RIC treatments, but only the MFC system was able to support 100 shrimp/m2 without a deleterious effect on survival (P > 0.05). Shrimp survival in the RIC system decreased progressively as higher densi- ties were adopted, whereas the MFC showed more consistent survival rates. This suggested that without water exchange, the RIC system was not able to support a high shrimp biomass, either due to insufficient dissolved-oxygen lev- els or a failure in recycling excess nutri- ents from feed remains and shrimp feces. Shrimp weekly growth, final body weight and yield showed no differences between culture systems. Under both conditions, growth was acceptable and always above 1.3 g/week, but there was a trend toward slower growth and lower body weights at harvest with increasing stocking density. This became more pro- nounced under the MFC system, particu- larly when 50 shrimp/m2 was compared with 75 and 100 shrimp/m2 (P < 0.05). Shrimp yield for RIC did not surpass the 0.8 kg/m2 threshold, whereas for the MFC, it reached as high as 1.0 kg/m2 . Microbial Flocs Microbial floc material was formed in
- 3. 24 May/June 2010 global aquaculture advocate both the RIC and MFC systems, although molasses was not added to the RIC tanks to control C:N rations in the water. Tanks with 75 and 100 shrimp/m2 under MFC increased the amounts of floc material in the water throughout the culture period. However, a biofloc plateau seemed to be reached at five to six weeks of culture in tanks with 50 shrimp/m2 and in all stocking densities operating under RIC conditions. Under MFC con- ditions with 75 and 100 shrimp/m2 , bio- floc production continued to increase until shrimp harvest. Data collected in this study presented strong evidence that the biological per- formance of L. vannamei was not lost when lower-protein diets were used under intensive floc conditions. If these results can be replicated within a com- mercial culture setting, the method can give a significant competitive advantage to those operating or shifting to microbial floc systems. Ingredient Content (g/kg) Poultry feed1 Shrimp feed2 Molasses3 Synthetic binder 540.3 407.3 50.0 2.4 Proximate Analysis Content (g/kg) Crude Protein Fat Fiber Ash Moisture 235.2 425.0 43.8 98.8 79.9 Table 1. Ingredient composition and proximate analysis of experimental diet used for microbial floc culture. Table 2. Performance of L. vannamei farmed at different densities under microbial floc culture (MFC) or regular intensive culture (RIC). Lowercase and capital superscripts indicate significant differences among densities in each system (α = 0.05) Performance Variables System Shrimp Stocking Density P Value 50 Shrimp/ m2 75 Shrimp/ m2 100 Shrimp/ m2 Initial body weight (g) RIC 3.99 ± 0.35a 3.28 ± 0.22b 3.58 ± 0.14ab < 0.05 MFC 3.70 ± 0.36A 3.26 ± 0.75B 3.31 ± 0.25B N.S. Final body weight (g) RIC 21.22 ± 1.10a 18.57 ± 1.26b 17.27 ± 1.29b < 0.05 MFC 20.22 ± 0.43A 17.99 ± 1.67B 16.95 ± 0.35B < 0.05 Weekly growth (g) RIC 1.68 ± 0.12a 1.49 ± 0.11ab 1.33 ± 0.12c < 0.05 MFC 1.61 ± 0.04A 1.33 ± 0.05B 1.48 ± 0.16B < 0.05 Final survival (%) RIC 92.8 ± 7.6a 72.7 ± 10.7ab 67.2 ± 21.7b < 0.05 MFC 81.6 ± 15.6 85.1 ± 10.4 80.0 ± 15.7 N.S. Final yield (g/m2 ) RIC 771 ± 116 751 ± 158 766 ± 308 N.S. MFC 629 ± 167A 883 ± 152AB 1.002 ± 225B < 0.05 1 147.3 g/kg crude protein, 37.1 g/kg fat, 61.1 g/kg fiber, 65.3 g/kg ash 2 368.9 g/kg, 55.0 g/kg fat, 28.0 g/kg fiber, 100.6 g/kg ash 3 53.4 g/kg, 0.4 g/kg fat, 1.8 g/kg fiber, 191.6 g/kg ash With higher stocking densities, biofloc production continued to increase until shrimp harvest.