48 JUNE 2017 • WORLD AQUACULTURE • WWW.WAS.ORG culture and ranching of mud crab. The availability of suitable water resources, human resources, keen interest, and export demand expedite its expansion. In addition, mud crab is less susceptible to disease, easier to culture, more resistant to adverse environmental conditions, and is even able to live without water for a certain time (Salam et al. 2012). Its short production cycle, soaring demand and price in international market, and low investment requirement make this sector a rapidly growing enterprise in the country. In recent years, the Bangladesh government has earned considerable foreign currency through crab exports to international markets, mostly to China (92 percent), Taiwan, Malaysia, Singapore, North America and European countries. Foreign currency earnings are increasing every year, US$ 6.7 million in 2010-2011 and US$ 21.1 million in 2013-2014 (DoF 2015). The sector is growing toward a potential industry for the marginalized coastal communities and the country’s economy. Mud crab fattening and culture are sustainable and promising for poor coastal communities in many Southeast Asian countries (Patterson and Samuel 2005). Crab Farming Initially crabs were harvested from wild stocks, especially those in mangrove forest channels, tidal rivers and coastal creeks. Now, many brackishwater ponds and shrimp ghers are being used for crab fattening and grow-out to some extent. Coastal people are busy with crab culture activities year-round. In March and April, farmers prepare for culture by draining and drying ponds and ghers, removing bottom mud, repairing dikes, liming, fertilizing, setting enclosures (pata, a bamboo-framed structure with nets), and filling with water. Then the practice of fattening and grow-out continues until the year’s end. In the morning, farmers feed crab and perform other pond management activities like liming, fertilizing, and water exchange, if needed. Some farmers collect feeds (snails, low-value fish and shrimp) from natural sources. Farmers harvest crab from ponds and ghers, tie them up, wash in clean water and store in baskets. Hooks and crab traps or pots baited with tilapia or eel are used to catch mud crabs. It takes about 3-4 hours. In the late afternoon, they carry harvested crabs to local buyers in baskets or porous bags. Farmers purchase feeds (trash fish, minced tilapia, some marine fish, eels, maize, wheat, snails) from the market, process it and feed to the crab at night. They purchase crab juveniles from the depots too. The crab farming system involves following three different practices: fattening, grow-out and soft-shell production. Fattening Crab fattening was the earliest attempt after collection of crabs from natural stocks for export. In Bangladesh, crab fattening in ponds started in the early 1990s (Kamal 2003), while culture of mud crab in bamboo cages, pens and pots started in the early 2000s at an experimental level (Khatun 2007) but is now practiced widely among coastal fishers and farmers. For fattening, farmers generally collect small, undersized, underweight male juveniles with average individual size of about 100 g (SM grade), any sized female crab with undeveloped gonad, and both male and female soft-shell crabs. Most farmers purchase seed crabs from depots, while some collect directly from crabbers exploiting wild stocks. Seed crabs are stocked in ponds and ghers in monoculture or polyculture with other finfish (mostly tilapia) and shrimp for a certain period to attain market size. A stocking density of around 5,00015,000/ha may give the highest productivity and economic benefit in earthen ponds, depending on culture intensity. The suitable sex ratio for stocking is four females for every male. Fattening farmers usually feed crabs with low-value fresh fish and snails twice daily in the morning and evening. Ponds are fertilized with organic fertilizers (cattle, poultry manure, molasses) and inorganic fertilizers (urea, TSP, ammonium phosphate). Also, liming and water exchange are done as pond management practices. Crabs grow by molting its exoskeleton. Cannibalism is a big problem in this critical period of the life cycle. An adequate supply of feeds and provision of three-dimensional shelters, including PVC pipes, straw-sheafs, leaf fronds, bamboo pieces, unused nets, coconut leaves and mangrove twigs, can reduce cannibalism during molting. About 8-10 days after stocking, farmers start to check the stock and harvest if the desired grade or size is attained (Fig. 2). The duration of the culture period varies and depends on the sized stocked, amount of supplied feeds and market price. From 15 to 45 days are required to complete the production cycle, allowing 8-10 production cycles per year. One of the common features of mud crab FIGURE 2. Harvesting mud crabs from a fattening pond in Rampal, Bagerhat, Bangladesh. Photo: Md. Mojibar Rahman. FIGURE 3. Small crablets for grow-out in coastal areas, Bangladesh. Photo: Md. Mojibar Rahman.
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