CRAB
Bangladesh Crabs Culture Implies growing or rearing of edible crabs to obtain a better crop or improve productivity. Worldwide crustacean aquaculture mainly represents the culture of shrimp and lobsters. crab culture particularly the culture of mangrove mud crab has received the most attention because of its market demand and decreasing natural production.
Bangladesh’s commercial-scale crab culture appears to be mainly limited to the mud crab, though Japan has been producing seeds of blue crab Neptunus pelagicus in hatcheries for open water stocking as cannibalism is the major problem in grow-out ponds. Larval rearing and spawning of Portunus trituberculatus and king crab Paralithodes camtschatica have also been successful.
Seed production is a critical issue in crab culture. Sources of mud crab seeds are the hatcheries and nature. Although there are reports of hatcheries seed production in Taiwan, China, Malaysia, Japan, and India, most of the work done is experimental in nature. In Bangladesh, seed crabs are collected entirely from natural habitats. Usually, mud crabs mature at a carapace width of 10 cm at the age of about six months and spawning takes place within 10 days after eyestalk ablation. Hatching takes place after about 17 days of spawning at temperatures between 23° and 25°C. A 250g female may produce up to 1.5 million first zoea (Z1) larvae which take 2-3 days for each intermoult to reach the megalopa stage. The megalopa in 7-8 days metamorphoses to the first crab (juvenile) stage.
The hatching rate may be close to 100 percent, but mortality is heaviest during changes between zoea, megalopa and juveniles. Various feeds including the rotifer (Brachionus chlorella), the brine shrimp (Artemia) and copepods are used to feed the larvae. High salinity sea water (25-53 ppt) is best for larval rearing. Mud crab farmers still depend largely on natural wild seed. So, seed is a limiting factor for the expansion of crab culture in Bangladesh. Most farmers collect crab seeds from the tidal rivers and seacoast by using different nets. Crab fry may be available year round but the peak seasons are summer (May-August) and winter (December-February).
The mud crab is widely distributed in Indo-Pacific including the Bay of Bengal. The culture techniques of this crab include (i) rearing of young crabs in large farms, and (ii) fattening of postmoult ‘water crabs’ as well as immature crabs to marketable sizes.
Crab seeds are usually stocked in large ponds, and are reared until they reach the marketable size and have well-developed gonads. They are reared as a part of polyculture system along with shrimp, milkfish, and seaweeds in Asian countries including Bangladesh where traditional shrimp farms, the ghers are the “growout” ponds. In such farms young crabs of 10-100 g are held up to 6-8 months until they reach the marketable size (usually 300-500g).
Fattening of crab is the monoculture of crab to produce ‘egg crab’ (female with ripe ovaries) and hard-shelled ‘meat crab’ (preferably a male with a hard body and large intact claws) in the ponds. Gravid female (egg crab) is the most valuable of all hard-shelled crabs in Asian markets.
The fattening is essentially the rearing of recently moulted ‘water crab’, immature female crabs and crabs with broken legs to hard-shelled gonadal mature crabs. They are usually fed molluscs (preferably snails), trash fish and other special food to promote maturation.
The culture is usually done in small earthen ponds fenced with bamboo screens and nylon nets to prevent them from escaping. The fattening cycle is short and generally last for 1-2 months. Stocking density is about 3 crabs (7-12 cm carapace width) per m2 water surface. Feeding rate is about 5 per cent of body weight daily, fed at dark. Fattening is also done in pens and cages.
In Bangladesh crab fattening is now very popular in Pikegachha, Mongla, Satkhira, Kaliganj and the Munshiganj areas of greater Khulna. It is done in small ponds (60-600m2) with facilities of water change, cleaning and liming for water quality management. The optimum temperature and salinity ranges recommended are 12-25’C and 15-30 ppt respectively.
Traditionally, mud crab culture in Asia has been a small-scale activity to provide better management and integration in terms of the coastal environment. In Bangladesh, mostly shrimp ponds abandoned due to white spot virus diseases are now being used as fattening ponds. Traditional ghers used for shrimp farming as well as integrated paddy-cum shrimp culture are also used as crab growing farms with shrimp as the main crop.
The major problem in crab farming in Bangladesh is the scarcity of seed. The natural populations of mud crab are declining throughout Southeast Asia due to over-exploitation, loss of natural mangrove habitat, and coastal environmental degradation. Obviously, much of the research efforts are now being diverted to commercial seed production in hatcheries and nurseries. With greater success in commercial seed production.