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 …

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.  

Habitat and diet: Mud crabs inhabit sheltered estuaries; the tidal reaches of some rivers, mud flats and mangrove forests, although females carrying eggs are present in deeper waters up to 50 kilometers offshore.  These crabs favor a soft, muddy bottom, often below low tide level. These crabs prefer salinity range of 10 to 34 ppt. and temperature range of 23⁰C to 30⁰C. Although many live in the intertidal zone, the majority lives subtidally, where they bury in the mud during the day and emerge at night and forage for food at night. Juvenile mud crabs eat planktonic animals, benthic molluscs and crustaceans of various types. Adults feed at night on a variety of slow moving bottom dwelling bivalve and gastropod molluscs including mussel, small crabs and polychaete worms.  Mud crabs are also attracted to dead fish and meat in traps.  They rarely eat fish under natural conditions since they lack the ability to catch them. The mud crab’s large claws are used for crushing and cutting their prey.  If they lose a claw, they may grow another one in successive moults. They are attracted to a wide variety of baits, including fish.

Life cycle: Mud crabs live for up to 3 years. 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.  Both the species are continuous breeders with peak breeding seasons, which vary from place to place. Each animal spawns once in two months. Mud crabs reach sexual maturity at between 18 and 24 months and mate in the warmer months. The mature females (known as ‘jennies’) release a pheromone, which attracts the males (known as ‘bucks’). The successful male picks up the female and carries her around for several days until she moults. Mating occurs only when the female mud crab is in the soft-bodied condition following moulting.  The male deposits a spermatophore, or packet of sperm inside the female’s reproductive opening where it is stored till the developing ova are ready to be fertilized.

Following mating, the female mud crabs migrate offshore to spawn. They lay 2 to 5 million eggs in each spawning. Female crabs incubate the eggs for 2 to 4 weeks under their abdominal flap.  On hatching, zoea larva emerges which are sensitive to high temperatures and low salinities, and hence require marine conditions. There are four zoeal stages, which lead to the megalopa stage.  At this stage they resemble a small elongated crab (3mm) with a well developed abdomen projecting backwards like a tail. Appendages on the abdomen also develop by this time, which ultimately help the megalopa to swim back to the estuaries. Larval life lasts about a month. Once back to the estuary, the megalopae metamorphose into juvenile crabs and settle down in sheltered mangrove areas.

To be continued …

The world’s oceans have been fished nearly to the limits of their sustainable yields. With the current state of fisheries, additional production of seafood will have to come mainly from aquaculture. In recent times, mariculture has got a tremendous boost globally owing to technological developments in the field of cage culture and related areas in Norway, other Scandinavian countries, Chile, Japan and Australia. The Norwegian technology has helped countries like Chile to make tremendous foray in the field within a short period of ten years.  

Read more about mariculture, a specialized branch of  aquaculture involving the cultivation of marine organisms, in this multi-part series.  

Candidate species: The aquaculture of high value finfish species, such as groupers, is of increasing importance throughout the Asia-Pacific region, including Australia. The development of large and affluent markets for live reef fish, particularly in Hong Kong and southern China, has increased pressure on wild stock resources. In many areas the demand for live reef fish, and the profitability of this trade, has encouraged overfishing and the use of destructive fishing practices. One such practice is the use of sodium cyanide to ‘stun’ reef fish for capture by divers. Because of the high toxicity of sodium cyanide, many more fishes are killed than are captured live and coral reef areas are devastated. The aquaculture of high value reef fish species can potentially supply product to the live reef fish markets, as well as other regional and domestic markets. The development of aquaculture technology for these species will not only support an economically beneficial aquaculture sector, but will also contribute to reducing pressure on wild stocks. Currently, the major bottlenecks to increased aquaculture production of groupers are the generally poor, and highly variable, survival in larvi culture, and the limited sources of trash fish for grow-out. The important candidate species in our context may be Asian sea bass, Groupers, Snappers and Eels.

Sustainability of commercial mariculture: The major problems pertaining sustainability of mariculture are as follows.

• Nutrient and waste loading of the aquatic environment
• Depletion of marine resource by way of consumption of fish meal and fish oil

Considering the above constraints, R&D efforts are on to meet these challenges and the present developments in this regard are as under.

• Formulation of HND fish feed with FCR of 0.83 and protein sparing effect has made it possible to cut down on the consumption of feed to 44% of what it was in 1972 and brought down nitrogen loading from 180 kg per  ton of salmon produced in 1972 to the present level of 30 kg.
• Fish meal content of fish feed has been reduced from to 35 percent today from 70 percent in 1972. Further  reduction is being attempted using alternate feed stuff such as soya, rape seed oil and corn gluten. Chinese researchers are working on a yeast-based protein supplement that could reduce use of fish meal by 50 percent. There is a possibility that Spirulina could provide the vital input in this context.

What needs to be done: As regard mariculture of fin fish in inshore and offshore areas is concerned, there are number of  gaps in development which needs to be addressed.

1. Formulation of regulatory framework

If these activities are to be increased on a commercial scale, suitable policy framework needs to be developed for leasing of sea bed and marine areas for marine cage fish farming on a sustainable basis without encountering conflict with other resource usages. The need for carrying out aquaculture in environmentally sustainable, socially acceptable and in harmony with principles of common resource use, has led to the formulation of integrated coastal zone management plan by the coastal states as a follow up of 1992 United Nations Conference on Environment and Development, in Rio de Janeiro, Brazil. Such coastal zone planning, based on assessment of holding capacity, nutrient loading from all coastal activities and common resource use needs to be worked out with clear enabling conditions for setting up such ventures.

As regard the formulation of regulatory framework involving studies related to carrying capacity assessment and site selection, suitable agencies needs to be identified with experience and facilities for under taking oceanographic studies. Institutes like National Institute of Ocean Technology (NIOT), National Institute of Oceanography (NIO) and IITs in the coastal states could be entrusted with such responsibilities on the line of country wide assessment done in Norway. 

i. Identification of potential mariculture sites based on seabed conditions, shoreline and hydrographic characteristics, etc.

ii. Assessment of carrying capacity of each site based on coastal zone planning on the pattern of Norwegian ‘lenka system’ or such other models.

iii. Monitoring of physico-chemical characteristics of water and nature of benthos of the marine sites in relation to predetermined standards should form an on-going activity while mariculture operations proceed on expected lines.

2. Assessment of potentials or holding capacity

i. Inshore cage farming: In this connection identification of marine sites, assessment of carrying capacity of the sites, framing of leasing policy of sea beds, etc. have not been carried out. However, based on general knowledge of the Indian coastline it is believed that the Andaman & Nicobar Group of Islands in the Bay of Bengal and Lakshadweep Group of Islands in the Arabian Sea would offer suitable conditions for undertaking such projects. However, a pilot study in this regard needs to be carried out if necessary by utilising services of overseas technology transfer agents, a multilateral funding agency, fisheries research institutes within the country and Indian corporates.

ii. Offshore cage farming: The potentials in this regard needs to be assessed if necessary by engaging overseas consultants. Technology in respect of offshore cage culture involving submersible cages is neither available nor any attempt has been made to develop the same within the country. The Farm Ocean cage system or SARGO^TM technology could be useful in this context. A pilot study in this context is also necessary.

3. Technology related to marine cage fish farming

The technologies for inshore cage fish farming and that for offshore areas using submersible type cages (farm ocean cages / solid wall cages) are at present not available within the country. Since the technologies in this connection are very exacting and capital intensive, only big corporate houses may be able to master the resources to undertake these activities. It may not be possible to develop in-house technology within a reasonable time frame considering the high degree of automation and the high cost / sophisticated nature of R&D involved. A pilot study with external assistance under bilateral funding could be considered for transfer of technology. The possibilities of supporting adaptive research involving our research agencies of repute and appropriate technology transfer agent could be explored. An appropriate overseas technology transfer agent in this regard needs to be identified for the purpose. Such technology transfer could involve the following exercise to start with.

 i. Identification of appropriate technology for cage fish farming in marine inshore and offshore areas and foreign technology transfer agent for the same.

ii. Identification of suitable species for cage culture in marine conditions and a suitable technology transfer agent for the same. May be sea bass will meet the requirement of the sector.

iii. Identification of suitable research institutes within the country in the field of mariculture, which could conduct an adaptive trail with collaboration with the foreign technology transfer agent that would enable verification of the end results as well as adoption of the technology and assessment of the production parameters. Probably ICAR has the requisite mandate in this regard and could associate for the marine cage fish farming trials.

 iv. Identification of a suitable research institute within the country, which could take care of the technical know-how relating engineering aspect of site selection, cage design, fabrication and installation. This could ultimately facilitate indigenization of the cage system and its manufacture by an Indian manufacturer. Since NIOT, Chennai, IIT Kharagpur and IIT Chennai are engaged in research and development related to aquaculture engineering and has several departments with requisites expertise in the related fields, these institutes may be best poised to undertake the job.

v. Before any step is taken in the above direction, it may be pertinent to conduct a meet with the industry to find out its response to the technology transfer program.  A national meet with the policy makers and research agencies to discuss the various aspects of the program and the need to develop suitable policy frameworks in this context is a basic requirement.