There is a need to study alternative systems of fish production that do not depend on purchased feeds and which make better use of available resources in farming systems that recycle organic wastes. The objectives are to reduce the present dependency on imported concentrate feeds and to encourage farmers to adopt farming methods that are more economically viable. In an integrated farming system nothing is wasted, the byproduct of one system becomes the input for another. By contrast, in the intensive system, the fish are given supplementary feeds the residues from which can be a source of pollution.
In the natural pond system the strategy is to fertilize the water to encourage the growth of phytoplankton which become the main source of feed. Supplements may also be given but usually these are in the form of water plants such as duckweed and water spinach, which can be grown locally. By contrast, in the intensive system the feed is provided in a concentrated form with a high protein content made from ingredients that are purchased from outside the farm.
The importance of the natural system has been realized of late and the scientific basis is being investigated to evolve appropriate technologies to get optimum productivity of the land, labour, waste and water. Better integration of the systems to involve crop and livestock production with recycling of wastes for aquaculture can help small-holder farmers to diversify their farm production, increase cash income, improve quality and quantity of food produced and exploitation of unutilized resources particularly labour and waste. More emphasis is needed on the integration of fish farming with agriculture and irrigation, livestock farming, sewage utilization and water pollution control not only to increase the productivity of land and water and improve the economic conditions of poor farmers but also to maintain health and hygiene of the rural poor and city dwellers alike.
The two main components of the natural system are: (i) the means to fertilize the growth of phytoplankton; and (ii) the choice of water plants that can be a direct feed source to the fish. Early research used animal manures as replacement for chemical fertilizers (Cruz and Shehadeh 1980). Later research evaluated the processing of the manure in biodigesters and the use of the biodigester effluents as fertilizer in the pond (Edwards et al 1988; Pich Sophin and Preston 2001; San Thy and Preston 2003). Biodigester technology has developed considerably in the last decade and the use of low-cost tubular polyethylene has enabled the process to be within the reach of poor farmers (Bui Xuan An et al 1997; Doung Nguyen Khang and Le Minh Tuan 2002). The recycling of waste gives additional value to both human and animal wastes through gas production, production of good quality fertilizer and the control of pathogens.
Duckweed has been used successfully as feed for mixed fish species in outdoor ponds fertilized with biodigester effluent (San Thy et al 2008; Sen Sorphea et al 2010). The leaves of Taro (Colocacia esculenta) have been shown to have high nutritive value for pigs (Chhay Ty et al 2010; Du Thanh Hang and Preston 2010; Manivanh Nouphone and Preston 2011) and ducks (Giang et al 2010). Recent observations indicate that the fresh whole leaves of Taro are readily consumed by Cachama (Colossoma macropomum) and Tilapia (Rodriguez Lylian, personal communication). Duckweed and Taro grow naturally in most villages in SE Asia.
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Juan Gonzalo Angel