Aquaculture

Aquaculture involves the rearing in water of animals or plants in a process in which at least one phase of growth is controlled or enhanced by human action. The animals used are generally finfishes, molluscs and crustaceans although a number of other groups are also cultured in small quantities (eg. sea-squirts phylum Tunicata, sponges phylum Porifera, frogs and marine turtles). Aquaculture of plants largely involves various seaweeds.

Today aquaculture production counts for a third of the total world fish production.

The most convenient place for aquaculture to develop is close to the coast where sources of original stock and plentiful seawater are nearby. Usually fish and crustaceans are raised in shallow holding ponds connected to rivers and estuaries or in partitioned areas of mangrove habitats. This can mean an alteration to the natural flow of rivers and natural habitat loss, especially when aquaculture is developed on a large scale that involves wholesale cutting of mangrove forests. The fine mesh holding pens used in aquaculture ventures can stop the natural flow of sediments downstream and cause river blockages. Also non- target fish caught in these meshes are often discarded leading to reductions in viable local fish populations.

Pens for rearing fish in Thailand. Photo taken by R. Faidutti courtesy of FAO.

Waste disposal back into the river, estuary, or coastal ecosystem is the most prominent problem involving the sea. Effluent from farms that do not use fertiliser or feeds have much less impact on the coastal environment compared to those that do.

• In Mexico 80% of shrimp farms are semi-intensive ie. apply fertiliser and add feed. Another 17% are considered intensive, combining high stock density with major feeding rates. The run off from these activities increases the level of pollution in coastal waters already under pressure from municipal effluents. India has a similar situation with shrimp farms.
• India is the largest exporter of wild caught shrimp; Shrimp farming seems an obvious progression extending this industry. Making a start on the barren lands along the coast, the industry soon engulfed mangrove forests, casuarina windbreaks, saltpans, and paddy fields. As is typical, fine mesh nettings collected sediments and fish caught in nettings were often disregarded. In 1996 the Supreme Court ruled that practising non-traditional farming methods within coastal regulation zones was illegal and had to cease.

Aquaculture in Latin America is mainly export oriented, with shrimp and salmon as the main products. Ecuador: special nets are used to fish shrimp larvae. Photo taken by G. Bizzarri courtesy of FAO.

Disease, infection, and parasites will spread very quickly through a confined population. Many aquaculture facilities combat this by incorporating drugs into food to combat common infestations. The majority of this food is never eaten, it drifts into the surrounding environment then both the added nutrient load from the food itself and the drug are marine pollution problems. Fish excretion and its decomposition, that causes oxygen depletion in water, are the two major problems to overcome when managing aquaculture. Fish faeces is high in ammonia, nitrite and nitrate. Nitrite binds with blood haemoglobin in the fish and can be fatal if water is not properly aerated.

Global production of farmed fish and shellfish has more than doubled in the past 15 years. In 1999, 32.9 million tonnes of fish were produced through aquaculture. This could be an important factor contributing to increased nutrients in coastal waters.