62 September 2009 growing and healthy grass carp, a voracious grass eater, which consumes an amount of grass equal to its body weight every day. As a result, dense macrophyte populations growing in the trench be controlled. Another system of fish culture may take placeusing the rice-Azolla-fish system. It is a common cultivation practice with high potential in the Indian DWR context. Azolla is used as a biofertilizer to enhance the soil fertility. Anabaena azollae, a Cyanophycean algae associated with Azolla, an aquatic fern, can trap atmospheric N2. This action makes Azolla important to the addition of extra N into soil substrata, which then becomes fertile afterwards. In a closed DWR system, the presence of Azolla can bring an additional benefit to both rice and fish. Rice plants require N for growth and it is necessary in fish habitat as well. Dykes of the closed DWR field are found to have substantial areas used for vegetable cultivation in addition to being utilized by farmers to access their fields and to better manage their crops. Many vegetables/pulses, which creep/trail around the substratum, may be grown on fallow dykes. On the edges of dykes, supports made of bamboo branches, locally known as Macha/Vera, are placed tilted at about 45º angles toward the trench. Along the side of these support vegetables/pulses (Moong bean and Biri bean) and sometimes relay vegetable crops (ridge gourd, cowpea and kidney bean) are sown and found to be growing well. These dykes appear fertile and ideal for plant growth. Capturing Fish, Shrimp and Other Species Usually, closed DWR fields are less than 0.1 ha and are suitable for capturing wild fish and freshwater shrimp using indigenously bamboo fish tras, locally called Atols/ Ghuni/ Mugri. These are of rectangular or triangular shapes 0.5 m high. Their shape and size vary from place to place and depend on the quantity as well as individual size of fish to be trapped. These are made from the processed sticks of bamboo in such a way that fish can enter, but cannot exit. Also, farmers dig a few sumps/ditches in the field with raised levees wherein wild fish/catfish/shrimp are attracted to congregate during sunny days. These fish are then harvested periodically when required. There are a considerable number of molluscs found in closed DWR rice areas. The major species among molluscs with respect to abundance and edible value are Pila globosa, Bellamya spp. and Gabbia orcula, which are collected by local farmers and sold in the market. Other aquatic animals, such as crabs, are well known decapods in this ecosystem and are habitually harvested by local people. The edible and common species are Paratelphusa hydrodromus and P. spinigera. Locally designed and made bamboo traps are used to collect large numbers of crabs from the fields. They place the traps at night and recover them the next morning; on average 30-40 crabs/trap can be collected daily. The crabs are commonly eaten by the poor, because they are a cheap source of protein and easily available. The presence of fish may control weeds and nuisance algae that are serious competitors to rice. They consume rice pests in considerable amounts as well as harmful insects and their larvae, stir the soil minerals that are then deposited on the upper layer and, therefore, available to the rice plants and cause aeration of rice fields so that they are suitable for tilling of rice plant. Beneficial Sides of Closed DWR Areas Benefits include: • The reduction of rodent infestations in the field because of continuous submergence; • The formation of organic manure from litter deposition and detritus accumulation; • The holding of field water for 5-6 months, a major growing period of the rice; • The use of the submerged part of the rice plant by periphyton, which are essential food organisms for fish; and • Less amounts of herbicides and pesticides are required. Adopted Technologies for Improved Yield of Rice and Fish With these multi-faceted advantages, the closed DWR areas were utilized in different districts of West Bengal for harvest of its maximum benefits by testing suitable methods of fish culture. Culture of fish under different experimental conditions, plot and fish (control), DWR plus fish (no feed), DWR and fish (supplementary feed) showed these significant differences among those three experiments: superior performance of DWR plus fish (supplementary feed) was found over DWR plus fish (no feed), which, in turn, showed superiority in terms of yield production over plots + fish (control). It has been reported that average fish yield increased by 60.7 percent in the DWR plus fish (supplementary feed) and by 45.7 percent in DWR plus fish (no feed) as compared to plot plus fish (control) condiA typical deepwater rice-fish farming field during monsoon in West Bengal, India.
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