The global interest in aquaponics has increased since the industry became a mainstream farming technology in 2010. Several universities are involved in aquaponics research and a host of companies offer products for sale or provide training courses.
As a result, the farming of fish and soil-less plant crops in a closed, circular system has been refined and is now well understood; the commercial case for aquaponics is clear.
There are different forms of aquaponics, but in the simplest type, water circulates continuously between fish tanks, bacterial beds and plants at a rate of four cycles per hour.
Fish convert their feed into growth and by-products, a primary component of which is ammonia, which is converted by the bacteria to nitrate. Plants utilise the nitrate, and allied macro- and micronutrients, for their metabolic and growth requirements, leaving the water clean to again return to the fish.
Such a system will operate continuously for many years without the water needing to be exchanged as the filters and plants maintain excellent water quality for the fish. Water lost to evaporation, transpiration and harvested crops is replaced with new water.
Nutrients from fish wastes
Depending on the composition of the source water and crops being farmed, supplementation with iron, calcium and potassium may be required, but all other nutrients are provided via the fish wastes.
This results in the situation where the waste from feeding the fish produces a secondary crop, usually more valuable than the fish themselves, with minimal additional supplements.
Plants at different stages of their growth cycle require varying proportions of macronutrients, such that seedlings require a higher proportion of nitrogen (N) than phosphate (P) or potassium (K), but as the crop produces fruit, the nitrogen requirement drops off in favour of a greater demand for phosphate and potassium.
Within hydroponics and soil farming the ratio of N:P:K is varied as the crop develops to ensure optimal availability according to the age of the plant, whilst in aquaponics we find this is not required in order to achieve excellent growth, as well as fruiting simultaneously within the same system.
Again, it appears that this is due to the very natural form of fertiliser in aquaponics that has been converted by bacteria from the fish feed and is optimally available for selective uptake by the plants according to their requirement.
As mentioned, there are different types of aquaponics, with the selection of fish and crops being one factor influencing the decision of which sort to install, and the size of the business being another.
Very few crops cannot be grown aquaponically, but although tree fruits such as mango, papaya and banana do thrive, the small quantity of fruit they produce and/or seasonal harvests do not make an economic case for cultivating them.
The most profitable crop to be farmed using aquaponics is usually lettuce, primarily because it has a short growth period, can be stocked at high densities and attracts a fair market price.
Tilapia is widely used as the fish species of choice due to their toughness and the simplicity of farming them. Early-stage aquaponics systems held the fish and plant crops in a single greenhouse tunnel and, given that many of the plants require temperatures around the mid to upper 20°C, this was also a good fit for tilapia.
However, as the industry matured and reached scale, the fish and plants are now held separately, each under ideal conditions, and linked via a mineralisation unit.
Slow daily exchange of water between the components does not significantly impact water temperatures, allowing farmers to produce crops that have high temperature requirements with trout (which prefer cold water).
This arrangement improves the economics as the market price for trout is significantly higher than that of tilapia. Naturally this is best done where the ambient conditions are cool for most of the year to simplify providing the <20°C required by trout.
An interesting side effect is that the trout wastes (because they are carnivores living on a diet that has a higher protein and fat content than the feed fed to tilapia) also results in better growth of the plant crops.
Numerous additional costs, such as the cost of capital, use of land, stable power supply, and so on, need to be brought into the equation, but done properly, aquaponics has the ability to generate strong returns.
The fact that it is polyculture means farmers have two simultaneous crops and therefore two income streams. Being largely weed free, using minimal water and having a low pest load, this really is an environmental, ergonomic and economically attractive farming technique.
Leslie Ter Morshuizen designs and builds fish farms across sub-Saharan Africa, trains farmers to manage them optimally and has run his own operations. He is the founder of Aquaculture Solutions.
Call him on 083 406 0208, or email leslie@aquaculturesolutions.org.
