Production of tilapia, for home or local consumption and for export, has been raised tremendously in the last few decades. The tonnage of world wide tilapia production (in 2010, about 3 million tons) is second, among fish, only to carps. Global production of tilapia was estimated to be 2.5 billion US$ in 2010. The present trends indicate a continuous growth of production and expanded penetration of that fish to a variety of markets, from expensive restaurants to local households all around the world.
Higher production levels are needed and anticipated; however, increasing aquaculture production is limited, globally, by the severe limitations of water and availability of suitable land. The only feasible and environmentally acceptable way to raise aquaculture production is by the use of intensive systems. The choice of suitable intensive systems to produce commodity fish is limited due to the need to produce the fish at a cost lower than the market price. One of the systems that enable intensification at a relatively reasonable investment and running costs is biofloc technology.
Biofloc technology is based upon the running of the pond using minimal water exchange, subsequent development of dense microbial population and managing the microbial population through the adjustment of the C/N ratio so that it controls inorganic nitrogen concentration in the water. The bacteria, forming bioflocs, assimilate TAN, produce microbial proteins and enable to recycle the unused feed protein. BFT systems are widely used for shrimp production world wide. (For more details: Yoram Avnimelech, Biofloc Technology, A Practical Handbook, World Aquaculture Soc. 2010).
Tilapia is ideally adapted to BFT systems. It is herbivore, essentially a filter-feeder adapted to the harvest of bioflocs suspended in the water, it can grow and flourish in dense systems and is overall a strong and stable fish. Using BFT systems for tilapia production is an obvious choice. An essential feature of BFT tilapia production systems, especially as compared to shrimp systems, is the very high biomass. In our experience, tilapia biomass can reach 20-30 kg/m3 (200-300 ton/ha), as compared to shrimp biomass of about 2 kg/m3 (20 tons/ha) in very good ponds. This difference is a very significant feature for minimal water exchange systems.
The daily TAN release, if untreated and left in the water is high enough to lead to fish mortality. Two microbial mediated processes are acting in BFT systems to control TAN concentrations.
One microbial process taking place is nitrification that converts the toxic ammonia and nitrite to nitrate. Another process quite specific to BFT systems is the assimilation of TAN by heterotrophic bacteria into microbial protein. In systems with high levels of available carbon as compared to nitrogen (C/N ratio > 15), bacteria utilize the carbon as a building stone of new cell material, yet, since microbial cells are made of protein, they need nitrogen and take up ammonium from the water. Both experience and theory demonstrates that when C/N is higher than 15 (15-20), TAN concentration is kept low.
It is important to notice that both processes can take place only if the proper microbial consortia are present in sufficient levels in the water. The heterotrophic consortia develop rather fast following the build up of organic matter in the water. Nitrifying community develops slowly and it takes about 4 weeks before it reaches its capacity, unless proper inoculum is applied. Microbial assimilation of nitrogen has a high capacity to control nitrogen levels in the water. Microbial protein is produced concomitantly, and may serve as a high quality feed source to fish. In dense microbial systems (Bacterial density in BFT systems is 107 - 10 cells/ml); the bacteria stick together with many other organisms and organic particles, forming bioflocs that range in size from 0.1 to a few mm. Such particles are easily harvested and assimilated by tilapia.
The amounts of protein stored in the bioflocs are very significant. In a typical pond, even if only 50% of the excreted TAN (i.e. 7 mg N/l) is assimilated and available as a fish feed, this process adds, in any given day, 45 mg protein/l, an amount equivalent to feeding with 30% protein pellets at a daily ration of 150 mg/l or 150 g/m3. This is a significant contribution to the feed. Moreover, unlike the applied feed, the bioflocs are harvested and utilized by the fish continuously all day long. Observing feeding behavior of tilapia growing in BFT pond with tilapia in equivalent control ponds, it could be seen that fish in the control ponds were very hungry and rushed wildly to the feed pellets that were applied twice a day, while tilapia growing at the BFT ponds ate quietly, showing that they were not starved before feeding. It is expected that the semi continuous feeding through the harvest of the bioflocs will help the smaller fish that hardly compete with larger fish in regular ponds, and thus higher uniformity is expected In BFT ponds.
Total suspended solids (TSS) accumulate in the pond at a fast rate when fish biomass is high. As to be discussed later, TSS or biofloc volume has to be monitored. The desired microbial community and reserves of feed are associated with the TSS. Thus we should not release it carelessly out of the pond. However, excessive levels of TSS are not favorable since it adds to oxygen consumption and at very high levels may clog the gills of the fish. In addition, if water mixing is not well controlled, or when TSS concentration exceeds the mixing capacity of the system, solid particles settle down and may accumulate and create anaerobic layer or pockets. The existence of anaerobic sites in the pond bottom may lead to the production of toxic reduced compounds and eventually severely hamper fish growth. TSS levels may be controlled by drainage of sludge, proper mixing of the water and good design of pond bottom. This is one of the essential controls in BFT tilapia production systems.
Feeding is an important control means. Proper feeding enables one to get the proper C/N ratio (>15) that will promote the uptake of ammonium from the water. In addition, proper feed strategy is required to utilize the recycled microbial protein, to reduce costs and to minimize residues. There is a need for more research in order to get the right feed composition and ration. Some questions are still open.
a. The recommended C/N ratio can be obtained by either feeding with pellets of low protein percentage or by augmenting the feed pellets through the application of carbonaceous material (molasses, cassava, wheat or other flour, etc.). The first option may save labor. However we rely upon the passage and excretion of the added carbohydrates through the fish to be used by the bacteria. This assumption may not hold.
b. Feed rations can be lower than the ones used in conventional tilapia ponds. With shrimp in tanks it was found that feed ration can be reduced by 30% as compared to conventional systems. It was estimated, but not proven, that feed ration in tilapia BFT systems can be lowered be at least 20% as compared with conventional systems.
Oxygen consumption in intensive BFT tilapia culture is rather high, both due to the respiration of the dense fish biomass as well as due to respiration of the microbial community that metabolize the organic residues. Oxygen consumption was estimated or modeled by several scientists however, there are a number of critical assumptions depended on specific pond conditions. The range of required aeration is 10-20 hp for a pond of 1000m2. The exact aeration rate needed for a given pond under given conditions should be adjusted following the daily determination of oxygen in the pond, normally setting a minimal level of 4 mgO2/l. One should adjust aerator usage to the size of the fish and pond's biomass. Usually, lower aeration can be applied at the start of the cycle when fish biomass is low, though it is recommended to utilize the capacity of the pond by stocking large number of fingerlings and maintain a relatively constant biomass by appropriate transfers.
Proper placement of aerators is very important. Most pond aeration deployment is made in a way to obtain a circular movement of water so as to concentrate the settled particles as close as possible to the center drain. However, there are conflicting demands in this matter. We want to be able to effectively drain out the excessive sludge, yet we want to keep bioflocs suspended in the water. To prevent a fast sedimentation of particles near the center drain, it is advised to place an aspirator type aerator or air-lifts to resuspend particles sedimenting at the center. By properly adjusting the location of these units, we can approach an optimum of resuspending the less dense bioflocs while sedimenting and eventually draining the heavier particles.
An important role of the aeration system is to properly mix the water and to prevent build up of sludge piles in locations were it is not effectively drained out. In case one finds such accumulation, aerators deployment has to be adjusted as soon as possible. Placement of aerators is still an art, we lack models and we do not have appropriate aerators in the market.
A very important demand is to have a sensitive and reliable monitoring and backup system. A fault in the aeration when the fish biomass is so high may be critical if no backup is activated within less than an hour. Intensive tilapia BFT ponds are rather small, 100 -1000m2, mostly due to the difficulty in the perfect mixing of a large water body. Most ponds are round or square with rounded corners, the floor of the pond slopes toward the center to facilitate sludge concentration in the center. A central drain is located in the center, operated using a stand pipe or a valve. The drain is opened usually twice daily, letting the dark sludge to drain out, till a point when clear pond water is exiting.
Though the BFT tilapia ponds are rather simple to operate, the system demands a careful monitoring and a fast response to defects, when ever detected. It has to be remembered that the pond is highly loaded and that any fault not responded to, may develop and become critical. Normal aquaculture monitoring is certainly needed. Of special importance are the following parameters:
Oxygen, if oxygen is high, you can reduce number of applied aerators to save electricity. However, if O2 is less than 4 mg/l, add aerators.
TAN. Low TAN concentration (<0.5 mg/l) means that the system works fine. You may consider lowering carbon addition. If TAN increases respond quickly by raising carbon addition.
NO2. Nitrite may negatively affect tilapia, yet the effect is limited in salty water. However, an increase in NO2 may be an indication of the build up of anaerobic sites. In case of an increase of nitrite one should carefully check the presence of sludge piles in the pond and if found change aerators deployment.
Floc volume (FV) determination using Imhoff cones is easy and cheap. FV should be in the range of 5-50 ml/l. If it is too low add carbohydrates and in cases it is higher than 50 raise sludge removal.
Biofloc systems enable to intensify tilapia production. The fish is adaptable to conditions in BFT systems, grows well, harvest the bioflocs and utilize them as a feed source. The recycling of feed and minimization of water exchange are important contribution to the economy of tilapia production. Understanding the system, monitoring and fast response to negative developments are essential to the success of the culture.
Figure 1. Scheme of recirculating Aquaculture System