Inundated rice fields always have a small population of fish which gain access with the water, and this probably gave rise to the practice of deliberate stocking and harvesting. The trapping of shrimp larvae in fallow rice fields and growing to market size is an age-old practice and still exists in parts of India. It is believed that ricefield fish culture was introduced into Southeast Asia from India about 1500 years ago (Tamura, 1961). It seems to have started in the 19th century in Indonesia, where it became an important peasant activity. Scarcity of food during the Second World War impelled farmers to devote greater attention to this type of integrated farming in countries like Japan. As will be discussed later in this section, improvements in rice farming techniques led to its general decline, but efforts are now being made to revive the practice.
Objectives and types of rice-field aquaculture
Rice-field aquaculture has been practised mainly to improve the income of the farmers and to make available an essential item in the diet of rural people in areas where ‘rice and fish’ form the staple food. In its traditional form, it required only very little extra input and provided off-season employment to the farmers and farm labour. Even though complete evaluations have not always been made, observations do indicate that the combination of rice and fish farming is mutually beneficial. The fish feed on organisms which grow in the fields and on many of the noxious insects and their larval stages, thus promoting better rice production. When a strongly herbivorous fish is cultured, the weed growths can be controlled to a considerable extent, as the fish will feed on them. When there is proper water management it is possible to control the growth of molluscs and the breeding of mosquitoes, thus reducing public health hazards. The movements of fish in inundated rice fields cause increased tillering, which can result in higher rice production. The greater depth of water maintained in the fields is reported to prevent pests like rats digging holes in the bunds, and will also flood any holes that exist. Despite these benefits, the combination of these cultures entails additional costs for the farmer, particularly in management and labour, which will have to be offset by income from fish production.
Three major types of rice-field aquaculture are practised. The first and probably the simplest form consists of using flooded rice fields after harvest, to raise one or more crops of fish or shrimps. The second is growing fish along with the rice and harvesting the rice and fish at the end of the rice-growing season. The third and more complicated system, which ensures a prolonged period of fish culture, involves transferring the stock to specially prepared ditches, channels or pools at the time of the rice harvest, and restocking them in the field for a further growing period. By this system, the fish are grown to a larger size than is possible in the short duration of one rice crop.
In rice/fish farming the main crop is rice and therefore fish farming techniques have to be modified to make them compatible with rice farming. It may become necessary in certain cases to reinforce and increase the height of bunds to prevent escape of fish, but this will not affect rice farming. The construction of ditches and canals will reduce the area available for rice planting, as they may occupy 5–10 per cent of the land. Higher levels of water have to be maintained (10–25cm) for growing fish together with rice. In areas where the water supply is limited, this may prove to be a major handicap. Also the short-stemmed, high-yielding varieties presently used by farmers may tolerate only moderate water depths, even when the water supply is not a constraint. The duration of cultivation of such varieties is shorter (105–125 days) and may not be long enough to grow fish to a marketable size. Deep water (floating) rice will be more suited for combined farming with fish. Fields with a high soil percolation may be unsuitable for rice/fish culture. The fertility of the soil is equally important to rice farming and to fish culture. The small additional fertilization that may be necessary to stimulate adequate growth of fish food may not affect production costs very much. The water quality in the fields has to be maintained at a level which is suitable for the fish and its food organisms. The very serious problem which has affected the combined culture of rice and fish, and contributed to its decline in many countries, is the intensive use of pesticides which create lethal conditions for fish life. There is also the risk of accumulation of pesticides in the fish and their effect on consumers.
The recommended dose of insecticides used in rice fields depends on the stage of rice growth, severity of attack, method of application and the pest species. Foliar sprays and the broadcast of granules are the most common methods of application. Because of the practical problems in penetrating the rice canopy and the short duration of the effectiveness of these methods capsule application in the root zone has been developed. This has provided higher efficiency and residual action. Carbofuran is a typical systemic insecticide which can be used in gelatine capsules for root-zone application, but capsule production is costly and its application is too laborious for general acceptance. Because of this, a method of application of carbofuran using a liquid band injector has been devised. If this method is used by farmers or if pesticides are incorporated otherwise into the soil, one of the major constraints to rice/fish culture can be overcome. Experimental work has shown that if fish are stocked in fields treated with carbofuran (by broadcasting, rootzone application or spraying of 15–18kg Furadan, 3G with a basal fertilizer) after seven days, no mortality of fish will occur. The use of pestresistant rice varieties will also reduce the need for insecticide application. Seiber and Argente (1976a,b) reported that carbofuran is not accumulated in the fatty tissues of tilapia and so tilapia grown in fields treated by this pesticide is safe for human consumption. Carbofuran appears to be completely converted to water-soluble metabolites.