Polyculture of bivalves and seaweeds in marine environments
The majority of rice/fish culture is done in fresh waters though rice/fish is practised in brackish water in the delta areas of major rivers in Asia. The role played in sustainable development by the ecologically balanced integrated aquaculture conventionally practised in Asia is gaining more recognition (Costa-Pierce, 2002). The need for sustainable mariculture practices has given an impetus to the formulation of bivalve polyculture in marine environments. Feed is the major source of pollution, and no external feeding is required. They constitute a primary product, do not need the addition of nitrogen, and serve as biofilters in integrated systems, feeding on existing plankton and enhancing water quality through feeding. They can be maintained as a polyculture system with marine and brackish-water plants that make use of dissolved nutrients (Negroni, 2000). No net addition of nitrogen to the environment is involved, as they feed on existing food items.
There are some potential constraints, such as food limitations, carrying capacity of the environment and competition of fouling organisms. Production of faeces and pseudofaeces is a major source of organic loading, but there is no net addition of organic matter to the environment since bivalves feed on material already present in the ambient water. Permanent extensive culturing (Tenore, 1973) may bring about changes in benthic communities. In spite of all these constraints, polyculture is an effective means of achieving sustainability (Grant, 1996).
It contributes to the dual goal of maintaining the environment and increasing the ability to produce protein. However, Canzonier (1998) points out that bivalves can concentrate and accumulate pathogenic micro-organisms and chemical substances in polluted waters.
Integrated seaweed culture in marine environments has been used as a biofilter for regulating water quality and disposing of dissolved nutrients from aquatic farms more frequently in recent years as a means of building up sustainable culture.
Plant (seaweeds) and animal crop (finfishes, crustaceans and molluscs) integration in aquaculture has been reviewed in FAO/NACA (1996), which concluded that this important area has not been researched well. However, there are some positive studies culturing seaweed as an ancillary crop along with shrimp, concurrently reclaiming the shrimp pond effluents. Use of Gracilaria spp., in reclaiming shrimp pond effluents has been reported from Thailand (Chandrakrachang et al., 1991;
Chaiyakam, 1996) and polyculture of oysters with shrimps from Hawaii (Wang, 1990). Several enterprising farmers and feed companies in Thailand have set up semi-closed recirculation culture systems with shrimp as the main crop, using planktonivorous finfishes (mullets and milkfish), molluscs (mussels and oysters), and seaweeds as biological filters in the recirculating water.
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