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Introduction
Scylla serrata is the common mudcrab occurring in the estuarine and mangrove areas and is commonly called as “red crab” and it prefers to live in low saline waters. Male crabs of S. serrata grow to 700 to 800 gm at the maximum The export size of the crab is 500 g and above for males and 250 g and above for females.
Crab fattening is widely practiced in Thailand, Taiwan, Malaysia, Singapore and Indonesia. Gravid female mud crabs with full orange-red egg masses are in great demand in seafood restaurants of South East Asian countries. Due to its high price, people started to hold immature female crabs in some kind of enclosures and fed them until the gonads developed and filled the mantle cavity. This is how crab “fattening” spread, initially, throughout South East Asian countries. Subsequently, the practice of holding post-moult “water” crab of market size, in some enclosures, for short period of time and feeding them until they completely “flesh out” for getting quick returns also became popular. Cages, pens and small ponds with net are being used for holding crabs for a short period of 3-4 weeks.
The mud crab resource is a natural bounty for our country, which has a potential to change the socio-economic status of the coastal communities. The coastal poor fishermen and educated unemployed youths should realize this fact and take up crab culture or fattening in eco-friendly way to raise their economic status.
This is the process of stocking juvenile crabs (10 g to 250 g) and allowing them to moult and grow. Harvest is done after 3-8 months or once the crab reaches 400 g to 500 g size. Mud crab fattening is the most suitable method for small-scale aquaculture because:
- Turnover is fast, hence, the period between investment and returns is short.
- Fattened crabs can be stocked at higher densities (15 crabs/sq m) compared to grow-out systems (1 crab/sq m) as no molting occurs and therefore losses due to cannibalism are dramatically reduced.
- Short production time reduces the risk of losing crabs to disease, thus, rendering a higher survival rate for fattening (>90%) compared to grow-out systems (40%).
Different Methods of Crab Culture
Four methods of oyster culture are practised in the Philippines; broadcast (sabong), stake (tulos), lattice and hanging (bitin, sampayan, horizontal, and tray) methods.
Pond Culture. Pond size of ½ to 1 acre is most suitable for crab culture. However, large size ponds of more than one acre can also be used for this purpose. Sandy soils with a mixture of 50% clay are ideal for culture of these crabs. A water inlet system and an outlet system to drain out water during water exchange should be constructed as in the case of shrimps. The pond should be constructed in such a way that it should hold 3 ½ to 4 feet of water towards the inlet and 4 ½ to 5 feet towards the outlet. A flow through mechanism of water exchange should be there in order to remove any left over organic food material and also to efficiently remove excretory material. A fencing of nylon net used for fishing can be placed on the dike to prevent the escape of the crabs during nighttime. In addition, about 1000 numbers of stone ware, pipes of 6 inch diameter and 1 ½ feet length, worn- out tyres, etc., should be kept at the bottom of the pond through out the dike. The nylon screen fencing should be supported with split bamboos of 1.5-meter height around the pond periphery for preventing the escape of the crabs from climbing over the bunds. The maximum stocking density should be 1crab per sq. metre.
Pen Culture in Ponds. Several units of pens of 4 X 4 X 2.5 m could be made inside the ponds using bamboo strips which are driven 1-1.5 m deep into the soil to prevent the escape of the crabs by burrowing. The pens could be made nearer to the dykes for easy stocking and monitoring.