Large-Scale Biofloc Tank Culture of Tilapia in Malawi – a Technical Success Story

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Malawi — a Fish Eating Nation

Although Malawi is endowed with the ninth largest lake in the world and the third largest and second deepest lake in Africa, overfishing resulted in the collapse of the tilapia fishery around 1990- 1991. Tilapia, known locally as chambo, is the country’s favorite fish and now costs from US$ 4.00-8.00/kg (R55-110/ kg) for whole or live fish.

This is a result of market scarcity which is unfortunate considering the massive demand for the fish and its abundant availability at very affordable prices (< US$1.50/kg) prior to 1990. Population growth at nearly 3 percent per annum in Malawi since 1990 has put the anticipated supply shortfall of fish in the country at around 80,000 t for 2017 based on sustaining 1990 per capita supply levels.

Tilapias are the preferred fish and the national dish in the country. A substantial market exists for producers capable of providing whole fish more competitively than dressed broiler chickens, less than a cost of US$2.40/kg (R33.00/kg) in Malawi. Under favorable economies of scale and scope, tilapia production using biofloc technology represents an attractive investment proposition if that selling price can be matched.

Figure 1. Imhoff cones are used to measure biofloc volume in BFT tanks at Chambo Fisheries.


Biofloc Technology

Biofloc Technology (BFT) is a relatively new and potentially revolutionary technology that is especially productive for tilapia and shrimp aquaculture. BFT is a sustainable and environmentally friendly method of aquaculture that controls water quality and harmful pathogens along with providing value-added production of microbial protein feed for the aquatic farm system. Bioflocs are clustered aggregations of microbial communities such as phytoplankton, bacteria, and living and dead particulate organic matter (Fig. 1). Shrimp and tilapia especially benefit from BFT due to their ability to filter-feed on floc in the water column, thereby reducing feed costs by improving feed conversion.

The beauty of BFT is in the mechanisms for ammonia removal from water. Using feeds with a carbon to nitrogen (C/N) ratio greater than 15 results in the dominance of heterotrophic bacteria as the major pathway for the removal of toxic nitrogenous compounds via assimilation into new bacterial cell biomass. BFT simultaneously provides an abundant source of “bacterial plankton” and a rich source of good quality protein and nutrients for filter-feeding fish and shrimp — BFT is then rather like killing two birds with one stone. Figure 2 provides a schematic of the process of Biofloc Technology (BFT) to promote nitrogen uptake by heterotrophic bacteria which then becomes a food source for tilapia and shrimp.

Experience raising tilapia in BFT, where feeding rates per unit area are at least 4 to 5 orders of magnitude greater than shrimp BFT systems, is limited. Knowledge gaps remain about BFT engineering economics, tilapia feeding systems and bioenergetics, cost factors and the economics of this new technology relative to conventional tilapia aquaculture systems. Insightful experience gained at Chambo Fisheries fills many of these knowledge gaps.

Figure 2. A schematic of the process of biofloc technology to promote nitrogen uptake via heterotrophic bacteria that becomes a food source for tilapia and shrimp.


Figure 3. A battery of eight 766-m3 multi-cohort sequential SAFF-BFT grow-out tanks at Chambo Fisheries near Blantyre, Malawi.


Vertically Integrated Farm Design

Chambo Fisheries operates a vertically integrated farming operation that includes a quarantine facility, broodstock pairing tanks, an artificial incubation room for hatching eggs collected from female brooders, a dedicated nursery system, purging tanks, a moist feed milling plant, an ice plant and cold-storage facilities apart from the Biofloc Technology (BFT) grow-out tanks. Figure 4 details the farmed production cycle of Shiranus tilapia at Chambo Fisheries. 

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.


The farm has eight large round-ended (R-ended) grow-out tanks, each holding 780 m3 of water and capable of producing up to 100 t of tilapia per tank in a year, or up to 130 kg/m3 of effective rearing volume per year via a multi-cohort sequential production schedule, although rearing densities average only around 20 kg/ m3. Because of the cool climate near Blantyre at 1130 m above sea level, all production facilities require placement within greenhouse enclosures. Supplemental heat is sourced from shallow solar ponds (SSP) coupled to a hydronic heating system that includes stainless steel heat exchangers built into the tank floor that are regulated by thermostatically actuated heat exchange pumps.

The design of the R-ended BFT grow-out tanks by SAFF at Chambo Fisheries is novel in many respects. The tanks include a built-in lamella separator for solids capture and removal. Control over floc concentration in the water column and the retention time of fecal and organic material is achieved by regulating water flow through the lamella separator from a full-width floor drain in the main tank. Water is pulled through the lamella separator through development of a water head differential at the far end of the central channel by using a multiple-pod airlift pump that permits flexible control over water pumping rate.

Every aspect of the BFT R-ended tank design aims to minimize capital and operating costs, taking full advantage of the superior hydraulic environment created by the integrated R-ended tank design, creating a beneficial streaming effect while improving the driving concentration gradient for oxygen transfer by carefully selected and positioned aeration devices.

Forced moderate exercise has been shown to induce muscle hypertrophy, improve growth rates and reduce the energetic costs of protein accretion. Fish that are fed under continuous moderate exercise exhibit a shift in metabolism to derive energy for swimming activity largely from carbohydrates and lipids rather than protein, a survival mechanism to spare protein loss from the muscle. This results in fish at harvest with a lower fat content while FCRs are reduced, growth is enhanced, flesh texture firmness is improved, and fillet yields are elevated marginally (more plump fish relative to their body lengths). Horizontal water velocity control in the range 15-30 cm/sec is achieved by adjusting the submergence depth of the paddles on paddlewheel aerators.


Multi-cohort Production

The farm design has been tailored to take advantage of the benefits of continuous sequential production where each grow-out tank is stocked and harvested every three weeks. This production strategy is enabled by using screened compartments (Fig. 5) where fish are moved in a conveyer fashion every three weeks to a larger compartment using simple crowding screens. This management technique results in production output yields of 4.6-5.8 t every three weeks. This elevates the production to capacity (P:C) ratio to 5.5-6.2, indicating the high rate of crop turnover relative to system carrying capacity.

Figure 5. A single multi-cohort sequential 766-m3 SAFF-BFT grow-out tank at Chambo Fisheries


The use of a multi-sequential, multi-cohort production system, in essence, doubles output production and halves input power costs in comparison to batch production, which yields an effective P:C ratio of only 2.4. Economic viability is greatly enhanced due to the doubling of production output based on the same investment in equipment and infrastructure compared to a batch production system. This unique innovation by SAFF, called the SAFF One-Tank Husbandry Approach, was first pioneered by the company in the Middle East on two RAS farms and a third RAS farm in Malawi.

Shiranus tilapia reach an average marketable weight of 218 g in 189 days from hatch at a temperature range of 27-29 C. Although purging fish to improve flavor quality is practiced, it is unnecessary in well-managed BFT systems because the fish carry no off-flavors or flavor taints. Fish are sold whole on ice and no processing takes place on site.


Specialized Feeds Produced On Site

Feeds — including broodfish diets, starter feeds and specialized C/N ratio feeds — are produced on-site using a low-cost moist-pellet milling plant that produces sinking pellets of medium to low water stability. The major components of the Chambo Fisheries feed plant include a hammer mill and a combination grinder, mixer and moist feed pelletizer, and a horizontal dryer and grading sieves for starter feed production on-site.

Feeds for fish >5 g are all animal protein-free and are based 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 custom formulate 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.


Water Use Efficiency

Another big advantage of BFT is the massively reduced water requirement compared to conventional tilapia aquaculture systems. Currently, Chambo Fisheries uses around 150 L of water to produce 1 kg fish, which compares well against greenwater pond culture that requires 2500-5000 L/kg of fish. The high annual fish yields per unit tank surface area and volume and reduced water use in BFT opens up great possibilities for applications in greenhouse enclosures on the highlands of Africa and on the periphery of major cities, reducing transport logistics to urban markets. The results obtained by Chambo Fisheries clearly highlight the merits of BFT as a competitive and sustainable alternative low-cost intensive feedlot technology for tilapia aquaculture.


Environmental Sustainability

BFT tilapia along with RAS farms, particularly when incorporating an aquaponics component, with or without duckweed (Lemna spp.) nutrient recovery lagoons designed to achieve zero effluent status, represents exemplary Best Management Practice (BMP). Direct loading of organic solids and dissolved nitrogen and phosphorus emissions to receiving surface water bodies can be eliminated in BFT and RAS farms, thereby comparing favorably to lake-cage culture operations that are characterized by high N and P emissions and solids loadings into sensitive lake ecosystems such as Lake Malawi

Four selected tilapia grow-out technologies were compared (Table 2) on the basis of the following sustainability indicators:

  • Feed conversion ratio (kg feed/kg edible weight),
  • Whole carcass protein efficiency (%),
  • Nitrogen emission (kg/t protein produced),
  • Phosphorus emission (kg/t protein produced),
  • Land use (t edible product/ha), and
  • Consumptive freshwater use (m3/t)

By these measures, the SAFF-BFT system performs best, followed by the SAFF-RAS. Greenwater pond farming has poor input:output nitrogen and phosphorus efficiency ratios, likely caused by high N losses through volatilization and denitrification to the atmosphere and P losses to the sediment, despite being a low-cost production technology. Clearly, the lake cage culture model represents a relatively less sustainable form of intensive tilapia aquaculture (Table 2) where no opportunity exists to recover neither dissolved and fecal-bound nutrients or organic matter that can overload the nearby benthic environment of lake ecosystems.


The Future of SAFF and Chambo Fisheries

BFT promises to revolutionize tilapia and shrimp aquaculture industries globally. Some leading scientists believe that harnessing the flow of microbes to fish represents the next revolution in food production. Chambo Fisheries aims to expand operations into warmer regions of Malawi and internationally using the same design templates with minor improvements (a second generation BFT system) to be supplied by SAFF, based on technical successes at the less-than-ideal site on the outskirts of Blantyre, Malawi.

SAFF has moved operations from the Middle East and Sub-Sahara African countries to focus on South Africa in support of various initiatives, including BFT farms that include an aquaponics component and multitrophic land-based marine RAS farming of several high-value finfish species on the East Cape coast and Atlantic salmon farming on the West Cape coast.

This article was originally published in World Aquaculture, June 2017.
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