Aquaponics, the rearing of fish and plants together in a mutually beneficial closed system, is a growing sector of the aquaculture industry. The only input, apart from water and energy, is the feed given to the fish. At the end of a cycle, both fish and plants are harvested… at least that’s the theory.
Many local designs fall short when it comes to the growing conditions for the fish; some even border on the unethical. The fundamentals of aquaponics dictate that the fish are fed and then produce waste that’s an ideal source of nutrient for the plants. The medium in which the plants grow – usually gravel in an aquaponics system – filters the water. Once cleaned of its nutrients, the water can be re-used for the fish. Again, this all sounds fine – in theory.
Fish require high-quality feed and water to grow optimally. Solid waste must be removed, and dissolved waste in the form of ammonia excreted by the fish is poisonous to them, even at low concentrations. In fact, the water can be crystal clear, yet highly toxic. Furthermore, fish need plenty of oxygen, especially with a high stocking rate, and oxygen content in the water is largely a function of surface area.
Many aquaponic systems being promoted provide inadequate living conditions for the fish. I have seen some where the fish are confined to what can only be called ‘dungeons’ – small drums recessed in the ground. Sometimes these are covered, leaving the fish in almost perpetual darkness, with a build-up of solid waste.
Does anyone really imagine that fish can thrive under such appalling conditions? A tilapia (blue or red breast kurper) can grow to at least 2kg. Even with optimal water quality there is no way a tilapia can grow to even 400g in a 200l drum of water. Confining these fish like this is both cruel and unproductive. The minimum size lattice-frame type tank for tilapia should be at least 1 500l.
A better way
My view of closed aquaponic systems is that they do not work. Tilapia need water temperatures of over 25°C to grow well, and this requires closed tunnels. Many plants struggle in such hot, humid systems, where the air temperature can reach more than 50°C. If ventilated, the cooling effect reduces fish growth, often to near zero.
Aquaponics can only really offer something if the two systems are divorced from each other. The fish should be contained in a closed tunnel with filtration to keep water quality and temperatures high. The effluent from this system – and there is plenty of solid effluent – can then be used for the plants in a separate system.
A daily flushing of all the solid waste from the primary stage of the filtration will provide more than enough nutrient for the plant-growing system, which can be either a ventilated greenhouse or under shade-cloth. In this way, the fish wastes are used by the plants in their own cooler system, and the fish grow and prosper under ideal conditions.
Nicholas James is an ichthyologist and hatchery owner.