1. Introduction
Mud crab constitute an important secondary crop in the traditional prawn or fish culture systems in some of the coastal states & Union Terrotories and has become increasingly popular by virtue of its meat quality and large size
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
Two species of mud crabs, namely Scylla tranquebarica and Scylla serrata are found in the inshore seas, estuaries, backwaters, coastal lakes and mangrove swamps of all maritime states on the main land and the creeks and bays of Andaman and Nicobar Islands. Both the species co-exist in the inshore sea as well as in the inland brackish waters preferring muddy or sandy bottom.
Among the edible species, Scylla serrata, commonly called the mud crab or green crab, is important. It is popular because of its size, meat quality, high price and export potential. Mud crab is locally called 'njandu' in Kerala and Tamil Nadu, 'petha'in Andhra Pradesh, 'kankada' in Orissa and 'kankra' in West Bengal. The ability of mud crab to grow fast and its suitability for culture in brackish water and estuarine areas make it attractive to develop mud crab culture/fattening programmes in India to meet the increasing demand for export and domestic market.
2. Market
The export as well as domestic market of crab is met through crab landings from the natural collection / harvest from the sea as also from the brackish water areas. Frozen crab meat and live crabs are exported from India to south East Asian countries.
Scylla serrata is much in demand in the domestic market and fetches a good price, compared to other species of crab. Medium and large crabs of more than 14 cm carapace width and weighing more than 400 g. are collected exclusively for export purposes from West Bengal, Orissa, Andhra Pradesh, Tamil Nadu, Kerala, Karnataka, Goa, Maharashtra and Gujarat.
3. Biological features
Mud crabs grow to a very large size of about 22 cm in carapace length and about 2 kg in weight. The crabs belonging to the species S tranquebarica is free swimming and grows to a large size with carapace width of 22 cm and those of species S. serrata have burrowing habit and grow to about 14.7 cm in carapace width. Mud crabs are omnivorous and they feed on a wide variety of food items and other crustaceans such as shrimps& small crabs, bivalve molluscs and fish.
The females reach sexual maturity at a size of about 12 cm in S. tranquebarica and 8.5 cm in S.serrata in the brackish water regime. Both the species are continuous breeders with peak breeding seasons varying from place to place. The peak seasons of seed abundance is May to October along the southwest coast, December to May in Tamil Nadu coast and March to June in Chilka Lake. Each crab spawns once in two months. The number of eggs carried by S. tranquebarica are about 1.1 to 7.0 million and by S. serrata are 0.5 to 0.9 million. The berried females migrate from estuarine areas to the inshore sea. The eggs hatch out in the sea and undergo metamorphosis later they migrate to brackish water areas and spread to different parts of the estuarine systems.
4. Mud crab culture practices
There are two basic forms of land-based mud crab aquaculture: fattening of crabs with low flesh content/ water crabs, and grow-out of juveniles to market size.
Grow-out systems for mud crabs offers more scope and production can be high. Grow-out systems are usually pond-based, with or without mangroves, although intertidal pens can also be used. Pond size of 0.5 to 1 acre is suitable for crab culture.The maximum stocking density may be 1 crab/sq.m.The period of culture extends from 3 to 6 months. The water quality need to be maintained at salinity of 10-35 ppt ,temperature 28-32 C , dissolved oxygen 5-7 ppm, pH 7.8 to 8.5, water depth 0.5 - 1.0 m However, this type of culture is not very popular in India and not practiced due to the non availability of juvenile/seed crabs in sufficient quantity and the lack of good quality artificial feed.
Crab fattening is essentially an operation during which post-moult or water crabs are kept in captivity for a short period of 20- 30 days until they 'flesh out' or immature female crabs are held until their gonads develop and fill the mantle cavity. This is very popular throughout the Asian countries due to increasing demand for gravid females and large size hard shelled crabs in the seafood restaurants. Fattening of mud crab is popularly undertaken in the states of Andhra Pradesh, Tamil Nadu, Kerala, Karnataka, Orissa and West Bengal.
5. Technical Parameters of Crab fattening
Crab seed are available in the nature in 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.Mud crab culture depends mainly on the natural sources for seed supply, and therefore a major limiting factor for culture of crabs.
The techno-economic parameters required for crab fattening are briefly described below.
5.1. Soil quality
The soil suitable for crab fattening is sandy or sandy clay as a sandy bottom discourages burrowing.
5.2. Water quality
Abundant and good quality water should be available. Mud crabs are highly tolerant to varying salinity conditions, so brackish water would be ideal for crab fattening operations.
Salinity - 15 to 30ppt. not less than 10ppt
pH -7.8 to 8.5.
Temperature - 28 to 30 degree centigrade.
Dissolved oxygen - should be more than 5 ppm.
5.3. Size of pond and Pond construction
Crab fattening is carried out in ponds, cages or pens. Small tidal ponds ranging from 0.025 to 0.1 ha in size with a water depth of 0.5-1.0 m are generally used for the purpose. The pond should preferably have a sandy bottom to discourage burrowing. Bunds should have a minimum width of 1.0 m at the top to prevent crabs from escaping by burrowing through the bunds. Crabs are capable of climbing over the bunds, which can be prevented by fixing overhanging fences on dykes.
Fencing a height between 0.5 to 1.0 m over the dyke is done with materials like bamboo sticks, bamboo poles and knotless nets, asbestos sheets, fibreglass panels, etc. As the crabs are highly cannibalistic on the freshly moulted animals, 'hide outs'made out of hollow bamboo pieces, cement pipes or stones are required to be placed inside the pond to minimise mortality
5.4 .Water supply and drainage
Water exchange is through tides. Sluice gates may 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, brackish /sea water is pumped in.
5.5. Pond management
The pond bottom is prepared by draining out the water and allowing it to dry before liming is done. This process would help to get rid of obnoxious gases as also the pathogens and microbes which are detrimental to the cultured crabs. The water is let in during the high tide or sea water is pumped into to maintain a maximum height of 1.5 m.
5.6. Stocking
Soft-shelled crabs of size 8 cm carapace width and above or crabs of more than 550 gm are stocked with a stocking density of 1- 3 crabs/m2. Crabs of similar size are preferred so as to reduce cannibalism. Ponds can be divided into compartments so that crabs of the same size can be stocked together to avoid cannibalism. Male and female crabs can also be stocked separately.
Polyculture along with varieties of fishes like Chanos and Mullets are practised by farmers in India, however, it is not recommended. Monoculture of crabs based on scientific practices yield better results.
5.7. Feeding
Crabs are fed with bivalve meat/ trash fish with a daily ration at about 5-10% of body weight (10% during the first week). The duration of fattening is normally 20 days. Care should be taken to provide sufficient food for better growth and also to avoid cannibalism.
5.8. Harvesting
The expected survival rate would be 70 to 80%. The crabs are harvested after the shell hardens and before the next moulting. The harvesting can be effectively done in tide-fed ponds by letting in water through the sluice into the pond during high tide. As the water starts flushing in, the mud crabs tend to swim against the incoming water and congregate near the sluice gate. The crabs can be caught with the help of a scoop nets and also by hand picking at the lowest low tide levels. Harvesting should preferably be done in the early morning hours or evening. 6 cycles of fattening can be possible in a pond in a year.
5.9. Packing
Crabs should be washed and the first pair of chelate legs (the largest legs with pincers) of each crab should be firmly tied up to the body by jute/nylon thread to avoid fighting and consequent injury. Care should be taken to keep the chelate legs intact and the threads not touching the eyes. These crabs are packed either in bamboo baskets or in perforated thermocole boxes for export purposes. Moist conditions should be ensured while transporting crabs.
6. Production & Income
The expected production per crop for an area of 0.1 ha. is around 240 kg. The income per crop from 0.1 ha. has been assessed to be Rs.72000/-.
7. Marketing
The scope for live mud crabs export has opened up good opportunities for crab farming / fattening. Export market of live crab especially to South East Asian countries is picking up steadily. Kerala is a major supplier to the export trade and Chennai is the main centre of live mud crab exports to Singapore and Malaysian markets. Crab is poised to be the next potential sea food in the world market among the marine crustaceans next to shrimp and lobsters.
8. Financial outlay
Details of the requirement of funds for fattening in 0.1 ha area have been indicated in Annexure I. The items and cost indicated under the model are indicative and not exhaustive. While preparing project reports for financial assistance from the banks the investment cost projected in this model could be modified taking into account the actual field conditions. The projected capital cost for 0.1 ha.unit has been estimated to be Rs.68000/- and operational cost for one crop to be Rs.47500/-. The production cost for three crops in the first year is capitalised. Lease rent if any may be included in the recurring cost.
Rate of interest
Interest rate to be charged to the ultimate borrowers would be as indicated by bank/RBI/NABARD from time to time depending on quantum of loan amount and the agency providing the loan
9. Financial viability
The following assumptions have been made for working out the financial viability of the activity.
| Sl No | Particulars | Parameters |
|---|
| 1 | Pond Size | 0.1 ha |
| 2 | Culture Period | 20 days |
| 3 | Stocking density | 1 crab /sq.m (400 Nos of 550-600 gm each) |
| 4 | Survival | 90% |
| 5 | Expected Production | 240 Kg |
| 6 | Crops / year | 1st yr 3
2nd yr onwards 6
|
| 7 | Farm Gate Price | Rs. 300/kg |
| 8 | Lease Rent /annum | Rs. 12000 |
The financial analysis (Annexure III) reveals that the IRR is >50% and BCR 1.34
10. Margin money and bank loan
The farmer is expected to bring margin money out of his own resources. The margin requirement will be in conformity with the RBI norms under priority sector and the bank may insist that the entrepreneur brings in the stipulated percentage thereof as his own contribution.
11. Rate of Refinance
NABARD provides refinance assistance for mud crab fattening to Commercial banks, Cooperative banks and Regional Rural Banks. The rate of interest on refinance is fixed by NABARD and undergoes changes from time to time.
12. Rate of Interest
Repayment of bank loan is possible in 8 years in equated annual insatments with a moratorium on repayment of principal for the first year . A tentative repayment schedule is given in Annexure E -IV
13. Security
Security from the ultimate beneficiaries may be obtained as per the guidelines of RBI issued from time to time.
14. Repayment Period
Repayment of bank loan is possible in 5 years with one year moratorium.