MANAGING AQUACULTURE FOR SUSTAINABILITY IN TROPICAL LAKE-KARIBA, ZIMBABWE

In Lake Kariba, Zimbabwe, small-scale pond farming and experimental ca ge-culture of Tilapia fishes have been running for some years and ther e are now plans for large-scale aquaculture. As a basis for deciding o n how aquaculture could be developed to improve the chances for sustai nable resource use and long-term maximised fish production in the lake , we compare the potential ecological life-support demand of two alter native aquaculture methods. First, the economic and ecological resourc e demand, expressed in industrial and solar energy units, respectively , were estimated for semi-intensive pond farming and intensive cage fa rming. Next, the ecosystem areas appropriated by the two farms for pro duction of feed, oxygen, and phosphorus assimilation were estimated. O ur estimates indicate that intensive cage farming would require about 17 800 MJ solar energy (Gross Primary Production) to produce 1 kg of f ish. 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/k g). Intensive cage farming must be supported by ecosystem areas that a re all substantially larger than the area of the farm itself. The ecos ystem 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 mu st be located close to the farm. For semi-intensive pond farming, oxyg en production and nutrient assimilation could probably be provided wit hin the pond system, and no external life support from Lake Kariba wou ld be needed. At least from an ecological point of view, semi-intensiv e 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 L ake Kariba. Aquaculture has potential to become successful in Lake Kar iba, but only if it is developed within a linked economic, social, and ecological framework.