WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2017 27 solely on oilseed meals and maize plus vitamin and mineral premixes. In addition to premixes, additives include choline chloride, vitamin C, monocalcium phosphate, an organic acid, molasses used as a nutritive binder and most importantly the use of mycotoxin absorbent which is mandatory under BFT conditions in Malawi due to the high prevalence of aflatoxin and other mycotoxins in locally sourced maize. Best results were achieved with a 20.2 percent protein feed that provides an input C/N ratio of about 15.5 based on the use of bioenergetic feeding rate models constructed by SAFF where FCRs average about 1. Meal intervals were spaced four hours apart and the day’s first meal size was the largest at 50 percent of the daily Feed Allotment (dFA). Tests conducted using feeding trays indicated that all feed is consumed at the specified meal intervals, meal percentage of dFA and feed applied within five minutes. The ability to customformulate feeds on site allows for adjustments to the formulation with ease. During the start-up of BFT, over the first 45-60 days, feeds require additional vitamin C fortification and sometimes the addition of crushed garlic to improve the immune response in fish until the biofloc becomes mature. After three months of continuous operation, it is necessary to reduce mineral inclusion levels in BFT tanks, particularly metals such as copper, iron and manganese as these tend to accumulate and are recycled through the biofloc. An iterative metal homeostasis model has been constructed that allows for the optimization of customized diets for BFT by SAFF given response data derived from actual systems. Bioenergetics and Feeding Strategy Feed often makes up 55-65 percent of the final farm-gate production cost in conventional tilapia culture systems, such as greenwater ponds, lake-cage culture, and recirculating aquaculture systems (RAS). Chambo Fisheries has been able to routinely achieve FCRs that average around 1 feeding a 20.2 percent protein feed (equal to a C/N ratio of 15.5) raising Mozambique and Shiranus tilapias. Such huge shortfalls in the supply of protein in formulated feed means that fish must obtain the balance of their needed protein — ranging from 55-60 percent of total required protein intake — through filter feeding on biofloc. This is not surprising considering the high carbon conversion efficiency via the “heterotrophic microbial loop” of around 40-60 percent into heterotrophic bacterial cell biomass and the very short trophic pathway (lower trophic energy losses) of microbial aquaculture systems. The basic pathway is dissolved C + N → C + N in microbial biomass → C + N in farmed organisms. Table 1 illustrates the advantage of biofloc-raised tilapia at Chambo Fisheries, achieving impressive performance metrics of 36.6 percent Net Protein Retention (NPR) and 20.9 percent Net Energy Retention (NER) on an edible meat yield basis. Based on results achieved at Chambo Fisheries, bioenergetic feeding rate models include the contribution of biofloc harvested by filter feeding tilapia, ranging from 20 to 25 percent of the Digestible Energy (DE) requirements of live-weight fish. Bioenergetic feeding rate models have not been properly applied to biofloc tank culture of tilapia and process optimization studies by the global research community are needed. The work at Chambo Fisheries by SAFF represents first attempts to optimize feeding rates, considering the contribution of biofloc grazing towards meeting a portion of the daily DE requirements of the fish reared. For NPR, biofloc tilapia production is more than 100 percent more efficient than tilapia production in a RAS system and 162 percent more efficient than lake cage culture (Table 1). These results suggest that properly managed biofloc tank culture of tilapia is potentially the most efficient form of feedlot animal production, outperforming lamb, broiler chickens, pigs and beef steers as well as feedlot aquaculture systems raising Atlantic salmon in net-pens and tilapia under typical lake cage culture, greenwater pond farming and RAS conditions in terms of protein recovery on an edible yield basis (Table 1). From an economic perspective, results at Chambo Fisheries represents a 50 percent feed cost saving when compared to feeding fish on conventional 32 percent protein feeds raised in a RAS. The merits of BFT include a significant reduction in the final farm-gate production cost of raising tilapias to around US$1.30/kg (about R17.70/ kg) in Malawi in 2016. A comprehensive economic study based on data gathered at Chambo Fisheries shows BFT farms to potentially produce tilapia at about 60 percent lower cost than large-scale cage culture, 34 percent less than RAS and 8.5 percent less than greenwater pond farming, assuming all farms are located in or near Lake Malawi. (CONTINUED ON PAGE 28) FIGURE 4. Farmed production cycle of Shiranus tilapia from a four-tank biofloc technology module producing up to 400 t/yr of 218-g fish year-round. FIGURE 5. A single multi-cohort sequential 766-m3 SAFF-BFT grow-out tank at Chambo Fisheries.
RkJQdWJsaXNoZXIy MjExNDY=