Talking Point

The mud crab, Scylla serrata, also known as Mangrove Crab is a portunid crab, that is, it is a member of a group of swimming crabs which has the last pair of legs flattened for swimming. Scylla serrata is the largest portunid crabs. Mud crabs are large crabs with a smooth, broad carapace.  They have nine even sized teeth on each side of their eyes.  In the most common form, the colour varies from very dark brown to mottled green.  The other, generally smaller form has a deeper body and is reddish brown.

In India, the mud crabs have come into prominence since early eighties with the commencement of live crab export to the South East Asian countries which has created a renewed interest in the exploitation as well as in the production of mud crabs through aquaculture. The importance of live mud crabs as an export commodity has opened up great opportunities for crab farming. It has high demand and price in the export market.  

Technical parameters involved in crab fattening: Crab fattening is essentially an operation during which post moult or water crabs are kept for short periods until they fatten or immature female crabs are held until their gonads develop and fill the mantel cavity. The techno-economic parameters of crab fattening are as under.

Pond construction: Crab fattening is carried out in ponds, cages or pens. The ponds used for this purpose are normally smaller i.e. 0.05 to 0.1 ha in size with water holding capacity of 1 to 1.5 m depth. The ponds area varies from 0.05 to 0.1 ha with a water depth of 1 to1.25 m. Bunds have a minimum width of 1.0 m at the top to prevent crabs from escaping by borrowing through the bunds. Crabs are capable of climbing over the bunds, which is prevented by fixing overhanging fences on dykes. Fencing of height between 0.5 to 1.0 m over the dyke is done with materials like bamboo matting fixed on casuarina poles and knotless nets used as overhanging.

Generally the available water area is partitioned to create at least two ponds for stocking the males and females separately. As the crabs are highly cannibalistic especially on freshly moulted animals, ‘refugee cages’ made out of hollow bamboo pieces, cement pipes or stones are placed inside the pond to minimise mortality.

Soil quality: The soil suitable for crab fattening is sandy or sandy clay. A sand bottom inhibits burrowing.

Water quality: There should be availability of abundant and good quality water (Table 1). Mud crabs are highly tolerant to varying salinity conditions, so brackish water would be ideal for crab fattening operation.

Water supply and drainage: Water exchange is through tidal water. Sluice gates must be used to regulate the inflow and outflow of the tidal water. The sluice gates are fitted with bamboo screens to prevent the escape of crabs. In regions where tidal influence is less, sea water is pumped in and hence provision for pump is made.

Pond management: The pond is prepared by draining out the water. The pond is then allowed to dry or bake in sunlight before the liming is done. The water is let in during the high tide or seawater is pumped in to a height of 1.25 m. The water is treated with lime applied at the rate of 200kg par ha. In case of algal bloom, alum is applied to the pond water @10kg par ha. In some cases Zeolite is applied after a few crops are taken.

Stocking: Soft-shelled crabs weighing 200 to 500gm are stocked in the density of 1-3 crab/m². The female crabs weighing 200 to 300gm are generally stocked at the rate of 2 crabs/ m². The density is lesser in case the stocking size is bigger as in the case of male crabs, weighing 300 to 400gm stocked at the rate of 1.5 crab/m² in the scheme area under consideration (Table 2). The fattening period extends for 20 to 30 days by which time they flesh out fully.

Feeding: Crabs are fed with bivalve meat or trash fish. Feeding is done daily at the rate of 5 to 10% of body weight. The duration of fattening is 20 to 30 days. The feed cost using the locally available fish and prawn head, which is discarded by the fish processing units works out to Rs.10 /kg.

Harvesting: The crabs are harvested after the shell becomes sufficiently hardened and before next moulting in a selective manner according to the condition of the animal. The harvesting is done by draining the pond and using scoop nets and ring nets with baits. Harvesting should be done in the early morning hours or evening to prevent mortality of crabs due to overheating of water at noontime. The harvested crab is next tied in a fashion so as to immobilise the claws. The crabs are then put in net cages or baskets for transporting it to the local agents of wholesalers called ‘arats’.

Production: In a year 8 to 10 cycles of fattening can be done using a pond. The production from 0.1ha pond is approximately 500 kg. The incremental production may be marginal or in some case may be negative. However the economics of the operation is based on the price difference between the water crab and the hardened crab.

Marketing: In India, the importance of live mud crabs as an export commodity has opened up great opportunities for crab farming / fattening. At present crab has good market and in the future, crab is poised to be the next potential seafood in the world market among the edible marine crustaceans after shrimp and lobster. The harvested live crabs reach the exporters through a net work of agents procuring the harvest from the growers and supplying to ‘arats’ from where it finds its way to the wholesale markets from where the stock is picked up by the exporter using its transport.  

The mud crab, Scylla serrata, also known as Mangrove Crab is a portunid crab, that is, it is a member of a group of swimming crabs which has the last pair of legs flattened for swimming. Scylla serrata is the largest portunid crabs. Mud crabs are large crabs with a smooth, broad carapace.  They have nine even sized teeth on each side of their eyes.  In the most common form, the colour varies from very dark brown to mottled green.  The other, generally smaller form has a deeper body and is reddish brown.

In India, the mud crabs have come into prominence since early eighties with the commencement of live crab export to the South East Asian countries which has created a renewed interest in the exploitation as well as in the production of mud crabs through aquaculture. The importance of live mud crabs as an export commodity has opened up great opportunities for crab farming. It has high demand and price in the export market.  

Availability of natural seed: Crab seed is available in the nature at all sizes. Juvenile crabs can be collected from estuaries, lakes, backwaters, creeks, mangroves and salt water lagoons by using bamboo traps, lift nets or scissor nets. A hatchery is being set up at the Central Marine Fisheries Research Institute(CMFRI), Kochi for commercial production of crab seed.  Mud Crabs are caught in wire mesh pots baited with meat or fish.  Mud crabs are also a by-catch of the coastal set gillnet fishery. Live mud crabs are sold in the local markets and also exported.

Farming operations: There are three types of farming operation in practice:

1. Grow-out: Stocking crablets and grow them to market size (over 200g in weight) either in earthen pond or in cages put in ponds/backwaters.
2. Fattening: Collect the wild caught water crab and grow them in short period of time and harvest for sale when they are full of meat.
3. Female crab fattening: Similar to the fattening, but only mature female are stocked and grown to the stage where the ovary is fully developed before harvesting for sale.

These aquaculture practices have been reported by CMFRI, Kochi, involving crab rearing. The first method known as farming is done by stocking young crabs (juveniles) in earthen ponds and reared for 5 to 6 months. In some cases farming is also practiced by stocking the crabs in cages. The second method known as fattening involves stocking of large size (8-12cm CW) adult crabs which are soft shelled newly molted crabs known as ‘water crab’ and reared for 20 to 30 days during which they ‘flesh out’ fully. Commercial hatchery and nursery technology is not yet established. The common species involved are Scylla serrata, S. oceanica & S. tranquebarica.

The activity: Crab fattening is essentially an operation during which post moult or water crabs are kept for short periods until they fatten or immature female crabs are held until their gonads develop and fill the mantel cavity. Mud crabs of genus Scylla, also known as green crabs or mangrove crabs constitute an important secondary crop in the traditional prawn or fish culture systems in the Asian countries. In India the mud crabs have come into prominence since early eighties with the commencement of live crab export to the South East Asian countries which has created a renewed interest in the exploitation as well as in the production of mud crabs through aquaculture. The importance of live mud crabs as an export commodity has opened up great opportunities for crab farming. It has high demand and price in the export market. Traditional crab farming is an important economic activity in the Sandeskhali I & II blocks of 24-Parganas (North) district of West Bengal. Fattening involves stocking of large size (8-12cm CW) adult crabs which are soft shelled newly molted crabs known as ‘water crab’ and reared for 20 to 30 days during which they ‘flesh out’ fully.

The location: The activity is in practice in the Sandeskhali I & II blocks of 24-Parganas (North) district of West Bengal on a large scale. The study was conducted in the Sandeshkhali II block with total land area of 197.28 sq. km. and tidal riverine area of 77.0 sq.km. The block has 3200 ha. under brackish water and 1500 ha. under fresh water fish farming. There are 517 fish farms of which 315 farms are engaged in crab farming. Of the total population 90% is dependant on fishery related activities, of which, traditional crab farming appears to be the main activity of this deltaic block spreading over 2600 ha. There are 102 crab fattening units operating in Sadeshkhali block II alone. These units are supplied with live crabs by the crabbers numbering 300. These crabbers spend their time catching crabs mostly using hook and line. There are more than 600 crabbers in 24-Parganas Nort district engaged in crab catching. The soft crabs are collected by crab collectors from the crabbers and supplied to the fattening units. The fattening units sale the produce to the local agents of ‘aratdars’, who in turn sale the produce to the wholesalers. The exporters from Kolkata lift the consignments for export from the wholesale markets. There are 110 ‘arats’ supplying to 7 wholesale fish markets in the block.

To be continued …