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Controlled use of natural resources
Polyculture is one means of developing aquaculture as a sustainable activity (Grant, 1999). Since external feeding is not required, bivalves constitute a primary product and serve as bio-filters in the integrated system. They can be managed as polyculture constituents with marine and brackishwater plants and animals that make use of the dissolved nutrients and organic matter (Negroni, 2000). No net addition of nitrogen and phosphorus to the environment as a result of external food consumption is involved. However, as in other coastal aquaculture systems such as the pen and cage culture, considerable accumulation on the sea floor of organic matter in excreted waste from farmed bivalves could cause deterioration of water quality and unfavourable biological changes around the aquaculture installations. It is estimated that a typical oyster rack having 460000 oysters could produce about 16tons of faecal matter in one season (Nunes and Parsons, 1998). On the positive side, bivalves are such highly efficient biofilters that an individual mussel for example can filter 2–5 litres of water per day. Further, bivalves can retain 35–40 per cent of seston ingested (Barg, 1992). In spite of constraints, polyculture remains an effective system of aquaculture production (Grant, 1999).
The prerequisite for sustainable development is the controlled use of natural resources on a renewable basis to meet food security of increasing populations and their economic growth. It is very likely that advancements in technology will accompany generational changes, but it is difficult to foresee the interdependent options available to future generations. One can devise environmental management techniques, but it is not likely that a generation can avoid using intensive production technology to feed increasing populations which in turn will cause greater environmental perturbation. Intensive farming often gives rise to occurrence of diseases, which reduce yields and consequently the returns on investments. Growth in aquatic farming did not result in an increase of sustainability on a long-term basis, and did not take into account natural resource assessment that incorporates environmental externalities in cost-benefit analyses. Without stakeholder involvement, the public image of aquaculture in many areas became damaged and led to opposition from media, politicians, and environmentalists.
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