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.

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.

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

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.

Chambo Fisheries

Chambo Fisheries is purportedly the world’s largest tilapia biofloc technology (BFT) tank farm and the largest such tank farm in Africa, located on the outskirts of Blantyre, Malawi (Fig. 3). The farm became operational in 2013 based on the mandatory production of Mozambique tilapia Oreochromis mossambicus and Shiranus tilapia O. shiranus due to restrictions that forbid the importation and culture of farmed breeds of Nile tilapia O. niloticus in the country.

Despite the slow growth of Mozambique and Shiranus tilapias, compared to the best farmed breeds of Nile tilapia, good flavor quality without purging, low feed conversion ratios (FCRs), year-round production potential (upon completion of a shallow solar pond supplemental heating system) and favorable economic factors has Chambo Fisheries poised for expansion regionally. The system design was developed by the author of this article in his capacity as Chief Technical Officer of SustAqua Fish Farms (Pty) Ltd. (SAFF), a South African concern, who developed the farm blueprints, production schedules, management systems and executed start-up, monitoring and management training.

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.

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.

Read the rest of this article in the June 2017 issue of World Aquaculture Magazine here