Aquaculture is claimed to aid the production of large quantities of low-cost protein-rich food to help feed the world, and to diminish pressure on ocean fisheries. In our opinion, neither of these claims is justified.

First, except in some parts of Asia, the main purpose of aquaculture has been to produce a luxury product for those who can afford to pay high prices. Second, Naylor et al.1 analysed the consequences of aquaculture practices and indicated that the growth of global production of farmed fish and shellfish will not relieve pressure on ocean fisheries.

Naylor et al. also indicated that aquaculture can diminish world fisheries indirectly by habitat modification, collection of wild seedstock, food-web interaction, nutrient pollution and the introduction of exotic species. However, in this last factor the authors referred only to possible hybridization between farm escapees and wild populations of Atlantic salmon, and to the spreading of pathogens. We believe it is also necessary to consider other consequences of introducing non-indigenous organisms, such as the elimination of autochthonous species by altering food webs, competition, hybridization and so on.

For scientists in developing countries such as ourselves, this point is of great importance. We would like to call to the attention of international organizations such as the World Bank, the International Development Bank and the Food and Agriculture Organization (FAO) the need to stop promoting aquaculture as a means of obtaining income from exports, and to concentrate on producing food for poor people. These organizations should stop promoting technological packages using exotic species and instead help the development of culture technologies for native species with the potential for aquaculture.

To give some examples: the FAO approved a technical cooperation project in Venezuela to genetically improve red tilapia, an unnecessary waste of resources that was a total failure2. The International Development Bank is partially financing a programme3 to culture exotic species in Panama, including the scallops Argopecten purpuratus, the cachamas Colossoma macropomus and Piaractus brachipomus, the channel catfish Ictalurus punctatus, the Sergeant or Peacock bass cichlid Cichla ocellaris , the giant prawn Macrobrachium rosenbergii and the bullfrog Rana catesbiana.

The World Bank finances several programmes in Latin America (see introducing species such as Litopenaeus vannamei and L. stylirostris into countries where they do not occur naturally. In one Honduran project, the World Bank claims there is no risk of introducing alien species because the shrimps L. vannamei and L. stylirostris occur naturally there, in the Gulf of Fonseca. But it does not mention the risk of introducing alien species in a Venezuelan project which also uses L. vannamei, although they are not native to that region.

Aquaculture can make unique contributions to world nutrition, thanks to its extremely high productivity in many situations and to the fact that aquatic crops are primarily protein rather than starch. Certain aquatic organisms may be better at converting primary foods than ruminants, fowl or even pigs. Some, such as filter-feeding fishes and molluscs, feed on microscopic plankton that cannot be used directly for human food.

However, if the aquaculture industry is going to reduce the pressure on wild fish stocks and provide food for the world's growing population, substantial changes must be made by governments, the private sector and international funding agencies. They must protect coastal ecosystems; promote research and development of native species; and encourage farming of low-trophic-level fish — those low on the food chain. International technical funding agencies can exert great influence in changing practices. Otherwise, as Naylor et al. point out, an expanded aquaculture industry poses a threat, not only to ocean fisheries, but also to itself.