World Aquaculture 59 Table 1. Showing rice farming areas in Asia Country Rice field areas (‘000 ha) Total Irrigated Rain fed1 Bangladesh 10,229 1,227 9,002 China 32,798 30,902 2,296 India 40,991 14,349 26,644 Indonesia 9,889 6,230 3,659 Korea 1,229 1,118 111 Malaysia 647 427 220 Philippines 3,426 1,473 1,953 Thailand 9,378 1,313 8,065 Vietnam 5,691 2,276 3,415 Lightfoot et al. 1992a 1Deepwater rice farming fall under this group The DWR areas in India with particular reference to West Bengal fall under the last two categories. They are located under the geographical range between Latitude 21.30ºN -27.40ºN and Longitude 85.50ºE-89.50ºE along the range of flood plains of the river Ganga and its number of tributaries and distributaries. All these DWR areas are commonly known as Jala, Bil, Jhee’, Daha and Doba distributed throughout 13of the 19 districts in West Bengal. The agro-ecosystem of DWR areas in those districts is favorable for fish growth and culture because the ecological components prevalent in DWR areas pertaining to fish growth, such as dissolved oxygen (DO), pH, specific conductivity (SC), water temperature, physico-chemical characteristics of soil and rainfall pattern are ideally suited for fish. Biotic components, which play a major role in fish growth, are comprised of plankton, benthos, periphyton and other biotic components of sufficient quantity. Usually, during the flood period, the DWR areas are reeled under environmental conditions that make them ideal for completion of the greater part of the fish life cycle: the average value DO is 11.00 mg/L, rainfall ranges from 134157 mm/week, water temperature ranges between 28-31ºC, pH ranges between 6.4-7.8 and light intensity ranges from 17.1x 103-43.5x 103 lux, along with water depth 86-126.0 cm. With the approach of the flood and after the accumulation of water in the flood zone, the DWR areas contain low specific conductivity, about 188.5 μmhos/cm, and shows the increasing trends of various inorganic and organic solids deposited through runoff water from the catchment areas, river water, decaying vegetation and animal matter. Consequently, the field water during the flood period contains more anions, such as carbonate, chloride, sulphate, phosphate and nitrate; metallic cations such as Ca, Na, K, Mg and Fe; and organics of P, N, sugars, acids and vitamins, along with increasing levels of organic carbon. On the other hand, biotic communities, which favor fish growth, increase during the flood period ad are utilized as live food organisms. All these physical and chemical parameters have cumulative effects that enhance fish growth (Das et al. 1991). This diversified agro-ecosystem of DWR areas in West Bengal is categorized into two major types (1) closed and (2) open. The closed type refers to the DWR area enclosed by arTable 2. Showing deepwater rice area (million ha) in Eastern India States Deepwater (>50 – 100cm) Very deepwater (>100cm) Coastal saline low land Total Assam 0.4 0.1 - 0.5 Bihar 0.4 0.7 - 1.1 Orissa 0.4 0.1 0.4 0.9 Uttar Pradesh 0.2 0.5 - 0.7 West Bengal 0.2 0.7 0.82 1.52 Total 1.6 2.1 1.22 3.7 (Bhakta 1989, Kar et al. 2005) Harvest of a crop of fish - a common sight at the end of rice growing season.
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