World Aquaculture 61 sential benefits pertaining to their life cycle; they become healthy as a result of the fresh environment, grow well, increase their population and are nourished. This area, by and large, contains a high DO, almost neutral pH and a temperature fluctuating between 20-30ºC. The soil of this area is rich in organic carbon, along with highly variable phosphate content and high potassium. Phytoplankton is estimated to predominate (811.0 units/L) among the total plankton populations. For these ecological components, the closed DWR field is fertile with respect to the natural ecosystem, which is a suitable situation for adoption of fish culture along with rice cultivation. Utilization of Closed DWR for Better Fish with Rice Production: A Trial The DWR area utilized for rice with fish was a traditional capture fishery, where farmers virtually neither provided any input to increase their yields nor were they interested in increasing the yields of rice and fish. They considered captured fish from DWR fields as an additional benefit. To motivate them to change that attitude, a system of culture fisheries was developed in DWR areas in addition to the normal captured fishes. Initially, the production of both yields along with captured fishes was not achievable; later, when selective culture of fish along with rice was adopted, a much better result was achieved, which prompted economic development among the DWR producing farmers. In the course of time, the interest for rice with fish culture among the DWR farmers gained momentum and a scientific approach blended with sound technologies to utilize fish culture in DWR fields was adopted. The culture system was then applied to two types: simultaneous culture - rice and fish cultured together, where rice was the main crop and fish became secondary crop and rotational crop - rice followed by fish by rotation and in this system both were equally important. In the eighties, the International Rice Research Institute (IRRI) urged attention to the improvement of total productivity of neglected agro-ecosystem areas by instituting various programs, such as multiple cropping, mixed farming and rice with aquaculture. Some experiments were conducted by incorporating new technologies at on-station and onfarm research in the natural environment of West Bengal. Those experiments achieved some noteworthy findings: selection of suitable carp species in the closed DWR system, evaluation of the yield potential of both rice and fish in different environments and development of technologies for culturing carp species along with rice in a closed type of DWR. Suitability of Fish Production with Rice in Closed DWR Areas The program is a system of culture fisheries within closed DWR areas to produce more fish. The chief source of daily dietary protein from the rice field may be developed among the farmers of some low lying rice areas, in addition to normal collection of indigenous fish. The interest may get momentum following development of some pest tolerant rice varieties, less toxic pesticides having no residual effect on fish. On the other hand, high-level water stagnation and various biotic and abiotic factors can enhance the yield of DWR. Input of fertilizer and pesticides are usually low, which has been evident from the survey in West Bengal. The most important and positive criteria are water depth and duration of water stagnation, which allow experts to predict that the environment in rice fields would be quite suitable for raising a fish crop, inasmuch as a higher water depth is more suitable for rice-fish culture. Initially, the rice field is prepared by digging peripheral trenches with 1 m wide x 1m deep with 10 percent of the total rice field, sometimes 50 cm wide x 50 deep is found, along the building dykes of 25 cm high by placing bamboo pipes and screens at the inlet and outlet. These trenches serve as refuges for the fish and a passageway for their ease of movement around the paddy. In DWR areas, inasmuch as the water depth is favorable for fish in comparison to other rice ecosystems, it is used as the harvest basin at the time of fish capture. So, adoption of a lateral trench (1m deep x 1 m wide) would, perhaps, be a reasonably good trench design for deep-water rice fish culture in closed DWR environment. In other cases, the ditches are 1.2-1.8m wide and 60 cm deep, constituting 3-7 percent of the area of rice field. Each field may have 3-4 catch basins, each with 3.3m2 x 45-60 cm deep below the field level across the rice field. Rice yield may increase 7-30 percent by integration with fish culture in those DWR areas, which have a constructed trench/ditch. It is reported that rice yields increase when integrated with fish. Apart from that, the trench/ditch provides an essential shelter for fish when field water levels go down because of prolonged dry weather. When the trench/ditch becomes full of macrophytes, grass carp (Ctenopharyngodon idella) of 200 gm each may be released. This will give extra benefit to the farmers, because without putting out any effort they can harvest fast A farmer with his haul of crabs in the trap from a deep-water rice field
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