In certain areas of the world (states of W. Bengal and Kerala, in India - most populous areas in the country) tidal swamps in the coastal areas have been converted as rice fields. Rice culture operations here usually coincide with the rainy season (monsoons), when the coastal inundated areas become less saline. The paddy cultivated, as we have referred to, are long-stemmed (deepwater or floating) and salinity resistant varieties.
Culture of brackish-water prawn and fishes in Bengal is described by Pillay and Bose (1957) and Jhingran (1975). Paddy fields lying near irrigation canals are made use of for the culture of prawns and fishes. The canal water is maintained about 30 cm below that of the paddy field and at this time, before the onset of the South West Monsoon, the fields are manured and prepared and paddy seedlings planted. With the onset of the monsoon, the water level in the canal would increase with rainwater and salinity will be lowered. At this time the bunds surrounding the paddy fields are cut at selected places and fish and prawns fry, are allowed to enter the paddy fields and grown during the paddy cultivation period. The fish are cropped just before harvesting paddy. The fish in the canals are also harvested. In the brackish water paddy fields the fish productions works out to 100 – 200 kg/ha/year. Species cultured here are: Finfish - Mugil parsia, M. tade, Rhinomugil corsula, Lates calecrifer and Mystus gulio; Prawns - Palaemon carcinus, Macrobrachium rude, Metapenaeus monoceros, M. brevicornis and Penaeus semisulcatus.
Fish culture in brackish water rice fields in Kerala (S. W. Coast of India) is different. The low lying paddy fields are called ‘Pokkali’ and the culture of fish here has been described by several authors cited by Jhingran (1975). Raman (1968) describes an experiment on prawn-cum-tilapia culture in the brackish water paddy field, which is of special interest to us. The ‘Pokkali’ fields of Kerala are usually single crop paddy fields, extending to 10,000 acres and yielding and annual production of 5,000 tons. The paddy fields after paddy crop are usually used to trap through sluices high tide water along with prawns mainly, and then the water is let out through the filters during low tide (water filtered through net screen) and therefore is known as the paddy-field prawn filtration. A typical filtration field, in which the effect of size and number of sluice gates and area of the field on prawn catch has been studied, is shown in Raman and Menon (1963,
Paddy is usually cultivated in these fields during July - September (S.W. Monsoon active) when the brackishwater surrounding the paddy fields are low in salinity. In several cases prawns are trapped and caught (capture fishery) without allowing a growing phase for prawns, but in many cases the trapped prawns and fishes are allowed to grow as a culture fishery. The latter has been shown to yield better. After harvest of paddy when prawns are to be stocked the bunds provided with slices are strengthened, and during high tide the incoming water brings in plenty of prawn fry. For every subsequent high tide during the autumn the prawn fry are trapped - it is taken that more fry are trapped in the high tide, and often a kerosine oil lamp is hung on the sluice gate to ‘attract’ prawn fry. At neap tides the prawns are retained inside the pond, by letting the water go out through the sluice, across a conical bag net having a rectangular frame, in the sluice, thus literally serving as a filter. The stocked prawns grow in the ponds until December; they are harvested from December till April (multiple harvest - 7 or 8 nights distributed over the full and new moon days).
Prawns constitute about 80% of the catches. The species caught are Penaeus indicus, P. semisuldatus, Metapanaeus monoceros, M. dobsoni, Macrobrachium spp., Palaemon styliferus, Caridina gracilirostris and Acetes sp. Mullets (Mugil parsia, M. tade, M. cephalus) and pearl spot and chromide (Etroplus suratensis; E. maculatus - cihlids) are caught among fishes. Pillay (1967) reports that in the 4,400 ha of paddy fields 785 – 2,135 kg/ha/year are caught.
The brackishwater culture of rice and fish are of special interest, because large areas of coastal regions of developing world are now unutilised. If improved strains of rice and proper fish/prawn combinations can be used, the coastal swamps can be made very productive. Raman (1968) cultured Tilapia mossambica (36 – 80 mm fingerlings stocked at 4,800/ha) along with brackishwater prawns without adding fertilizer or feed in fallow paddy field (salinity range during experiment 6 – 21 ppt) and harvested 100 kg/ha of T. mossambica and 200 kg/ha of prawns in 4 months (January - April). Certainly better production can be achieved with feed and fertilizer added. Further coastal swamps are relatively free of biocides, commonly used for paddy, and fish production here can be augmented.
