62 SEPTEMBER 2012 the form of a pit resulting from the excavation of soil used to raise the level of the homestead floor above the high water mark during the rainy season. People without a river nearby also need a pond to supply water for household use. Most ponds connect with other water bodies during the rainy season, receiving runoff and floodwater. At this time, ponds become home to indigenous fish, which people catch during and after the rainy season. Many small ponds dry up completely during the dry season. Fish farmers have adopted a variety of production methods to suit the diversity of ponds. Rice-fish culture Rice-fish culture started in the 1990s in open water bodies, particularly with carp polyculture and tilapia-mixed species culture. Rice-fish culture follows two patterns: • Pond or ditch adjacent to rice field. A remarkable number of poor people are gradually adopting this culture system. The ponds of small farmers are usually dominated by such exotic fish as tilapia, sarpunti (Puntius sarana), and catfish, with lower numbers of carps. The major difficulties of this culture system are poor water quality and vulnerability to seasonal flooding, which allows fish to escape, leaving farmers with serious economic losses. Farmers see this culture system as second only to rice in terms of importance to their food, nutrition and income security. • Fish culture in seasonally flooded rice fields. Two rice-fish culture systems are practiced in seasonally flooded rice fields. One is concurrent culture of deepwater rice with stocked fish during the flood season, followed by dry-season (boro) rice in shallow flooded areas. The other is alternating culture, with boro rice followed by stocked fish in a closed area. Nursery culture During the past decade, private nurseries have expanded in periurban Dhaka because more farms are using improved methods based on traditional culture techniques. A large number of lower-income households are associated with periurban aquaculture in and around Dhaka. The increasing population and rapid urbanization is resulting in a considerable number of ponds and tanks in lowlying lands and abundant brick fields, now being used for fish farming. Moreover, production from all forms of aquaculture has increased greatly. The traditional extensive culture system has been improved and, in some upazilas, commercial fish farms are developing (Table 3). A large number of new ponds and water areas are available for aquaculture as a result of urbanization. Edwards and Gregory (1997) considered urban Dhaka to have only a limited potential for aquaculture development. Surface water quality is generally poor, and excessive organic loadings are linked to poor fish growth and mortality and possible contamination with pathogens, while land-tenure issues and rapid urbanization are constraints to development in slums. There are several lake and private ponds in Dhaka that are not producing many fish. No improved aquaculture is practiced in these water bodies. Assuming that the national average annual production in natural ponds and lakes is approximately 750 kg/ ha, total production from the urban area of Dhaka is about 695 t. Pond-based polycultures of carp, sometimes incorporating tilapia, and monoculture of catfish are practiced in and around Dhaka. Polyculture of carp accounts for the majority of production, although tilapia Oreochromis niloticus production is becoming more popular (Fig. 2). Catfish Pangasius hypophthalmus production in ponds is also increasing and monocultures are stocked at 15,00020,000 fingerlings/ha and the annual production rate achieved 0.71 t/ha, with harvested fish destined for affluent urban markets. A combination of inorganic and organic fertilizers and locally available feed inputs such as rice bran and mustard oil cake enhance production in ponds. Within Dhaka, public toilets constructed adjacent to ponds are used by more than 1,000 people daily, providing additional nutrient inputs to ponds (Islam et al. 2004). Sewage-fed fish culture systems practiced in some areas around Dhaka constitutes an important trend in periurban aquaculture in Bangladesh. There are even recent reports of illegal fish cultures at the outfalls of sewage disposal systems of the city. Cage and pen culture is rare in periurban Dhaka. However, cage and pen culture has emerged on the floodplains surrounding Dhaka, notably near Madartek and Manda to the east and FIGURE 1. Proportion (%) of production of different fish from open-water fisheries in periurban areas of Dhaka, Bangladesh. FIGURE 2. Proportion (%) of production of different fish from aquaculture in periurban areas of Dhaka, Bangladesh. FIGURE 3. Supply channel of fish in periurban areas of Dhaka, Bangladesh.
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