An estimated 840 million people lack adequate access to food (FAO, 2002); of these about 25 percent are in sub-Saharan Africa (Pinstrup-Anderson, Pandya-Lorch and Rosegrant, 1999). As the population grows and puts more pressure on natural resources more people will probably become food insecure, lacking access to sufficient amounts of safe and nutritious food for normal growth, development and an active and healthy life (Pretty, 1999). The hungry are the poorest of the poor. Hence reducing hunger must be one of the first steps towards reducing poverty (NEPAD, 2003). Poverty in sub-Saharan Africa is becoming more widespread with nearly half the population living below the international poverty line (Clover, 2003). Although many local successes have taken place, the food situation in sub-Saharan Africa is still extremely unstable (Pinstrup-Anderson, Pandya-Lorch and Rosegrant, 1999). Agricultural production in the African continent is low, economic stagnation widespread, political instability persistent and environmental damage increasing (Pretty, 1999). The challenge is therefore to provide the poor and hungry with a low cost and readily available technology to increase food production using less land per caput and less water without further damage to the environment (Pretty, Morison and Hine, 2003).
Aquaculture, the farming of aquatic organisms, including fish, molluscs, crustaceans and aquatic plants, is often cited as one of the means of efficiently increasing food production. Fish provides a good source of protein and essential micronutrients and thus plays an important role in the prevention of many human diseases (Williams and Poh-Sze, 2003). About nine million people are employed in the aquaculture industry, which provides them with supplementary income during lean seasons (FAO, 2003a). Aquaculture could increase the availability of low-cost fish in local markets bringing poor households above poverty threshold levels relatively quickly (Edwards, 1999). Larger scale commercial aquaculture, practised in many developing countries, can enhance the production for domestic and export markets bringing much needed foreign exchange, revenue and employment, thereby contributing to food security (Ridler and Hishamunda, 2001; Subasinghe, 2003).
Although aquaculture could theoretically bring numerous benefits to the quality of life of millions throughout the world, until the effects of this technology can be measured and quantified, the true benefits cannot be fully understood. There is a need for a direct, simple and rigorous method to measure the potential contribution of aquaculture towards improving food security. Food security exists when all people, at all times, have physical, social and economic access to sufficient, safe and nutritious food that meets their dietary needs and food preferences for an active and healthy life (SPFS, 2003). This study examines some of the aspects that could be included in any such measure. Particular reference is made to fish farming in sub-Saharan Africa where the need to improve food security is greatest.
 The poverty line provides a
measure of the minimum income or consumption level necessary to meet basic
needs. Information is obtained through surveys and global comparisons are made
using a reference line, set at US$1 per day in Purchasing Power Parity (PPP)
terms (where PPP measures the relative purchasing power of currencies across