Shrimp are raised, fed and harvested in shallow coastal ponds, mainly in tropical countries such as Thailand, Ecuador and Indonesia. Shrimp farming has boomed as an industry. In 2000, global production totalled 1.3 million tonnes with a market value of over US$7 billion (FAO 2003). Approximately 80 per cent of farmed shrimp is produced in Asian countries - China alone produced 493 000 tonnes in 2003 (FAO 2005b). Much of the shrimp is exported to developed nations. According to the degree of intervention, shrimp farming is classified as extensive, semiintensive or intensive. Intensive farming implies increased stocking density, higher inputs of antibiotics, nutritional additives and probiotics, as a means of achieving extremely high productivity, all of these leading to greater generation of wastes. (Naylor and others 2000).

Environmental consequences
It has been estimated that a shrimp's assimilating efficiency of nitrogen from feed is around 22 per cent (Briggs and Funge-Smith 1994), while the remaining nitrogen is discharged into the water. The enormous quantity of faeces and feed waste resulting from intensive shrimp farming has resulted in the pollution of canal water, eutrophication of coastal areas, and the spread of human disease.

Shrimp farming has degraded salt marshes and freshwater wetlands and threatened coral reefs and seagrass beds. Pond construction has also damaged mangrove forests, one of the world's most threatened habitats. Shrimp aquaculture development is a major cause of recent mangrove loss in countries such as Thailand, and it has been estimated that it may be responsible for as much as 38 per cent of mangrove loss globally (Environmental Justice Foundation 2004).

Destruction of mangroves has left coastal areas exposed to erosion and flooding, altered natural drainage patterns, increased salt intrusion and removed critical habitats for many aquatic and terrestrial species (Environmental Justice Foundation 2004).

Toward best practices
Many Asian countries are rapidly developing policies to preserve mangrove forests while enabling sustainable shrimp culture. For example, the Southeast Asian Fisheries Development Center has developed guidelines that suggest limiting ponds' proximity to the coastline and reducing the amount of mangrove forests used for farming (SEAFDEC 2004).

Other attempts are being made to reduce pollution from shrimp farms. In Thailand, the government supports a project where effluent is treated extensively by removing sludge and using biological filtration before being returned to the ocean.

Additionally, pathogen-free and pathogen-resistant shrimp larvae and broodstock have been developed in an effort to reduce the use of drugs and other chemicals in shrimp ponds (Moss and others 2005).