Recirculating aquaculture systems (RAS) are the only ones currently legal in South Africa for the commercial culture of Nile tilapia. This is in stark contrast to the rest of the world, where pond culture dominates.
Due to the need to utilise these complicated systems, the spotlight is placed squarely on filtration design.
Many high-investment systems, many of which are complicated, hi-tech designs, do not work efficiently.
RAS filtration may include components such as swirl or drum mechanical filters, fluidised bed biological filters, static gravity-based sediment traps, serial bio-filters, revolving bio-drums, and even bio-floc filtration.
Additional equipment includes ultraviolet sterilisers, oxygen generators, carbon dioxide degassers, ozone generators and aerators. Most of these designs are copied from sophisticated US and European examples, and many are from research institutions or high-investment companies.
But why are so many systems failing? Maintaining very high water quality is essential to RAS production, and many apparently well-designed systems are failing to do this.
Because fish are reared at high stocking levels in a very small area and the operation has to be economically viable, the demands on the system to break down waste products and provide sufficient oxygen are extremely high.
There have been some notable large-scale business failures in the sector. However, with smaller systems there is some evidence that with the right market, system design and cost of operation, RAS filtration can be viable. However, one must be aware of the basic technical and economic risks involved.
Keep it simple
Hi-tech systems are totally dependent on spares back-up, duplication of components, technical support and staff who can cope with emergency breakdowns.
Instead of throwing this sophisticated technology at a system, perhaps a better solution, especially in our unique African situation, is to focus on higher volume and less intricacy.
This approach satisfies the need for high oxygen content, removal of ammonia and nitrate, stability of pH and temperature, and resilience.
In a highly stressed, high-risk design, reliant on (for example) drum separators, component failure will almost immediately result in smothering of the bio-filter, de-oxygenation and filter crash. At stocking levels of >30kg/m3, this can result in immediate heavy stock loss.
If the volume being filtered is enormous, but high-tech units are replaced by gravity-operated sediment tanks, and high-volume serial filtration does not rely on mechanical devices requiring parts or energy to operate, the risks are reduced.
The system has the capacity to accommodate water quality fluctuations, over-feeding events, or staff mistakes.
Surely this is a no-brainer?
Nicholas James is an ichthyologist and hatchery owner.