Agri Kultuur September 2014 - Page 39

needs of the fish and the microorganisms in the water, good aeration and agitation is fundamental. The placement of aerators, usually paddle-wheels, within the pond must be thought through to prevent the flocs from settling in an un -agitated area of the pond. The design of the tank or pond must also allow you to effectively drain the excess heavy sludge which does settle out on the bottom. As the Biofloc starts to develop in the system, the water colour starts off by turning green as an algae bloom is induced, and although algae does play an important role in a Biofloc system, it is heterotrophic bacteria which you want to dominate the microbial community. Heterotrophic bacteria are usually limited by the supply of Carbon within a pond, and works best off a C:N ratio of 10-20:1. By adding a carbon source to the water, you effectively allow the heterotrophic bacteria to multiply and dominate the system, which becomes characterised by the water turning brown. In brief, heterotrophic bacteria uses the excess nitrogen in the system which is supplied by your fertilizer at first, and then from the uneaten feed and excreta from the fish. It removes these potentially toxic nitrogen compounds from the water, as a biofilter does in a normal Recirculating Aquaculture System (RAS). Together with the carbon source, the bacteria then incorporates the nitrogen into microbial protein in the form of the flocs, which filter feeding fish such as Tilapia and Shrimps are able to consume. The flocs typically have a dry-weight protein content of 30-45%. So in effect, you enhance the use of heterotrophic bacteria to control your nitrogen levels and produce a supplementary microbial food source which can allow you to save up to 40% of your feed costs by simply recycling the nutrients. The key is to provide a favourable environment for the heterotrophic bacteria through a well mixed, well aerated water body with an additional carbon source and of temperature from 20-35 degrees Celsius. Monitoring the key water parameters is an important aspect of a Biofloc system. Nitrate, nitrite, ammonia, turbidity and total suspended solids (TSS) should be measured at least once every 2 weeks with dissolved oxygen, temperature and pH monitored daily. The level of the biofloc can be measured as TSS and through the use of an Imhoff cone which is a glorified rain gauge with a different scale on the side. After 1 A well mixed, well aerated system. litre of water is given 20 minutes to settle in the cone, the level of the biofloc can be measured and should be around 50ml/l for Tilapia. The stocking density of a biofloc pond is generally around 15-30kg fish / m³ at harvest, and should be worked back to calculate how many fingerlings are needed to initially stock your pond, given its size and the size at which you plan to harvest the fish. Biofloc Technology offers clear opportunities as a Tilapia industry will begin to develop in South Africa. Setup and running costs are low as no external filtration, pumping of water; better heat conservation as well as savings on feed becomes a reality. The low environmental impact further punts this green technology to play a role in providing a good, cheap source of protein to our hunger stricken continent. Please feel free to contact me with any questions or where I may be able to assist you. I would recommend reading Biofloc Technology, A Practical Guide Book by Yoram Avnimelech to anyone interested. Scott Day is an Aquaculture Masters Student, Stellenbosch University. e-mail: A biofloc pond within a tunnel