Oysters, clams, mussels, and scallops feed on phytoplankton and suspended detritus while seaweeds feed on nutrients dissolved in seawater. These species can be raised without supplemental feed, a practice called extractive aquaculture. This method of farming entails "planting" mollusc larvae or "seed" directly on the bottom of tidal flats or suspending the larvae from floating rafts, trays or mesh bags. Molluscs can also be hung in racks or placed on platforms anchored by sticks or posts. Nearly 12 million tonnes of molluscs were produced in 2002, comprising 23 per cent of global animal aquaculture products (FAO 2004).

Extractive aquaculture is extensively conducted in coastal waters, particularly off the shores of Asian, North American and European countries. Molluscs are often farmed in waters near urban areas, where discharge of nitrogen and other nutrients fuels the production of plankton. This, in turn, provides a source of food for molluscs, which also can help control eutrophication.

Environmental consequences
Molluscs could reduce nitrogen and other nutrients in the oceans - thus improving water quality. However, densely planted farms can result in the accumulation of faeces on bottom sediment below farming rafts and ropes, leading to oxygen-starved dead zones (Furuya 2003). Sediment can also change seafloor habitat and encourage growth of pollutiontolerant species (World Fisheries Trust 2002). Mollusc farming can also introduce exotic species. For example, the widely cultured Japanese or Pacific oyster is now common in the wild on almost all Northern Hemisphere coasts. Non-native molluscs become competitors, predators, disease vectors, and parasites of wild native species (Naylor and others 2001). Clams growing inside a cage, feed on suspended detritus before being harvested.
Source: WorldFish Center 69 EMERGING CHALLENGES - NEW FINDINGS

Toward best practices
Some environmental degradation can be avoided with sound planning. Farms should be sited in areas with adequate circulation along the ocean bottom to prevent dead zone formation. The density of molluscs "planted" also should be determined according to the environmental carrying capacity, so as to avoid robbing the food supply of other plankton-consuming species. It is possible to establish sustainable mariculture systems by integrating finfish or shrimp production with mollusc and seaweed production, since molluscs and seaweeds remove food particles and dissolved nutrients produced by the fish or shrimp (Naylor and others 2000, McVey and others 2002, Neori and others 2004) (Box 1).