Managing aquaculture for sustainability in tropical Lake Kariba, Zimbabwe

In Lake Kariba, Zimbabwe, small-scale pond farming and experimental cage-culture of Tilapia fishes have been running for some years and there are now plans for large-scale aquaculture. As a basis for deciding on how aquaculture could be developed to improve the chances for sustainable resource use and long-term maximised fish production in the lake, we compare the potential ecological life-support demand of two alternative aquaculture methods. First, the economic and ecological resource demand, expressed in industrial and solar energy units, respectively, were estimated for semi-intensive pond farming and intensive cage farming. Next, the ecosystem areas appropriated by the two farms for production of feed, oxygen, and phosphorus assimilation were estimated. Our estimates indicate that intensive cage farming would require about 17 800 MJ solar energy (Gross Primary Production) to produce 1 kg of fish. The industrial energy input would be more than 1.5-times higher (about 85 MJ/kg) compared to semi-intensive pond farming (about 50 MJ/kg). Intensive cage farming must be supported by ecosystem areas that are all substantially larger than the area of the farm itself. The ecosystem area for feed production is the largest (21000 m(2) m(-2)), but the areas required for oxygen production (160 m(2) m(-2)) and nutrient assimilation (115 m(2) m(-2)) are of special importance since they must be located close to the farm. For semi-intensive pond farming, oxygen production and nutrient assimilation could probably be provided within the pond system, and no external life support from Lake Kariba would be needed. At least from an ecological point of view, semi-intensive pond farming is more sustainable than intensive cage farming because it needs a smaller input of external resources to survive. However, a moderate level of intensive cage farming should not be ruled out in Lake Kariba. Aquaculture has potential to become successful in Lake Kariba, but only if it is developed within a linked economic, social, and ecological framework.