The ongoing load-shedding by Eskom raises questions about any system that depends on a constant source of electricity for survival. Is it possible to design intensive fish-rearing systems that do not rely on a 24/7 power supply, and if so, what is required?
Intensive concentrations of livestock are dependent on the prevailing environmental conditions to survive. If these parameters fall short of ideal, stress leads to immune system failure, illness and even mortality. Fish, which are immersed in their medium (water), are particularly dependent on these conditions being right, and those conditions are for the most part dependent on a constant electricity supply.
Through their movement, metabolism and feeding, fish are continually altering that environment: oxygen is extracted from the water and toxic gases such as ammonia are released into it. To keep the fish in a condition where they can thrive and grow, these gases must be either replaced or removed from the water. This requires technology that must operate on a continuous basis.
This is the use of ’beneficial’ bacteria that break down waste products like ammonia and nitrite, both of which are toxic at even low concentrations. Bio-filtration competes with the fish for oxygen. The bacteria are living organisms; if they are starved of oxygen they will die and produce toxic gases. Thus, systems to keep the flow of water running continuously are essential.
Water is heavy; it takes a lot of energy to both move and heat. Even small systems of 100 000l stocked at 28kg/m3 require two circulation pumps of 0,75kW each. Adequate oxygenation demands a third unit in the form of a blower. So, even such a small system consumes around 2kW to 3kW of electricity around the clock.
High-capacity, low-energy pumps are becoming available, but many have such low head potential that even the restriction of the distribution piping to the fish tanks reduces their advantages over conventional pumps. I have heard of 300w pumps that will move 30 000l/ hour, but at zero head. Side channel blowers are inefficient at saturating the water with oxygen. Injection of micro-bubbles under pressure seems an alternative, but the threat of nitrogen narcosis is always a possibility. (The fish can get ‘the bends’ like a diver.)
Solar energy can be used for both water heating and electricity production, but do not underestimate the amount of energy required. For example, a 180 000l grow-out system would need 69 x 12-tube vacuum-type solar heaters to warm it up
by 6°C. At around R7 000/unit, this becomes prohibitively expensive.
Battery systems and inverters may work well for small systems but only last a few years. In Burundi we used a 10kW inverter with 12 deep-cycle batteries which lasted for only two years. There is much potential for reduction in energy requirements in aquaculture, but reality must prevail in putting such a design ‘off the grid’. The capital costs are still high enough to more than double the start-up costs of building a new project. In the current scenario of endless, random blackouts, this seems like a damn good deal!
Nicholas James is an ichthyologist and hatchery owner.