Mud Crab Culture and Fattening:
The following write-up is all about mud crab culture.
Introduction to Mud Crab Culture
This article is for the many aquafarmers, fisheries entrepreneurs, and aqua business owners to share knowledge and skill for the commercial hatchery production of mud crabs. Mud crab is a promising aqua business product for its fast growth, pricing, for its good market acceptance, as well as for good export potential. An aqua farmer or owner must have good knowledge in water quality control, husbandry of crabs, food and pond management, crab processing, and crab marketing. Mud crab have a smooth outer shell (carapace), have big claws and its last pair of legs are flattened for swimming.
Types of Mud Crabs:
Scylla serrata, Scylla olivacea, Scylla tranquebarica, and Scylla paramamosain are the four mud crab species. Scylla Serrata (green mud crab) and Scylla Olivacea (Brown mud crab) are the most commercially grown species in aquaculture in the coastal regions.
Green Mud Crab (Scylla serrata): This is the most common and larger species that are grown commercially. They have a shell width of 20 to 25 centimeters and grow over 2.0 to 2.5 kg in weight. They can be identified by their elbow which has a prominent sharp spine on the claws and the claws are dark-green to purple colored. These species are free-living animal with polygonal markings on all appendages
Brown Mud Crab (Scylla olivacea): This is smaller species is also known as red claw and more aggressive. They grow half the size of green mud crab. They have a shell width of 10 to 15 centimeters and grow over 1.0 to 1.5 kg in weight. They can be identified by their elbow which has a blunt spine or no spines at all on their claws and the claws are light brownish orange in color. This species has a burrowing habit and with no polygonal markings.
Understanding Lifecycle of Mud Crab Culture
Zoeae: The first stage of the mud crab after hatching from the egg is larvae, which is called as zoea. The zoeae feed on planktonic animals or brine shrimp in the water. At this phase in the hatchery, water must be maintained same as sea water conditions including pH, salinity, ammonia, oxygen, and water temperature (27° to 28° C). At this stage, the mud crab is about one millimeter long with no resemblance of a crab as it has no developed limbs but looks more like a tadpole.It remains at this stage for about 12 to 15 days while undergoing some changes. At the later stage, zoea larvae when crowded in the hatchery will lead to heavy losses due to cannibalism.
Megalopa: The next stage after zoea is megalopa, while in zoea stage the larva undergoes four moltings in a period of two weeks. In the final fifth molting stage, the larva transforms into a megalopa having functional claws. At this stage, it starts resembling a crab with a tail. In a weeks’ time, megalopa moves inside the waters and settles on the seabed.Megalopa starts feeding voraciously on juvenile shrimps, zoea larvae, and artificial diet. By next molting that takes place in a weeks’ time, it transforms from megalopa to a juvenile crab4 mm wide, which is the first crab stage.
Juvenile Crab: In this stage, it completely resembles an adult crab but in a miniature size. All the organs are well developed and can walk around on the seabed and also can swim in the water. The juvenile crab will be about 10 to 40 millimeter by now. Due to cannibalistic behavior, a density of 50 per square meter must be maintained. The crablets can be fed on formulated feeds, freshly minced mussel or fish meat. The juvenile crab starts to settle down in a sheltered area by moving to an estuary or brackish waters.
Young Adult: The juvenile crab grows over the coming days and grows both in weight and carapace width. It takes about 18 to 24 months for crabs to attain maturity. A matured green mud crab has a carapace width about 180 to 250 millimeters while a brown mud crab has a carapace width of 90 to 150 millimeters.
Mating: During warmer months or in the warm season, matured mud crabs start to mate. To attract males, female crab releases a chemical attractant called pheromone in the water. A suitable male climbs on top of the female by clasping her with his hind legs and carries her for up to three to four days till she begins to molt. The male turns the female upside down to mate after she sheds her shell. Sperms of the male are deposited inside the reproductive opening of the female like a capsule that will remain for a month till the developing eggs are ready to be fertilized. After mating the male flips the female and still holds the female for some more days till her carapace (shell) hardens.
Spawning and Hatching: The female digs a hole with her abdominal flap in the sand and releases the fertilized eggs. Till she releases the fertilized eggs, it uses her swimming legs to carry them for about two to four weeks under her abdomen that looks like a dark spongy mass. The female mud crab stands on her legs when the eggs begin to hatch. About two to five million eggs are released in water by moving her abdominal flap.
Mud Crab Culture Feed and Diet
Mud crabs come out from their burrows at night to search for food. Apart from the artificial formulated feed, they feed on small and slow moving animals such as juvenile shrimps, mollusks, worms, plant material, and juvenile crabs. While in search for food they use different senses such as eyes which give them 360° vision both in and out of the water, by antennae they detect minute changes in water movement around them, tiny hairs on their legs are sensitive to touch and taste. Mud crabs travel up to 500 meters at night looking for food. Crabs claws are so powerful that a medium size crab can crush shells that may require a force of forty kilograms to break. Their large claws are used to break shells or crush while the smaller ones are used to cut.
Frozen or fresh fish trash or 75% of brown mussel meat along with 25% trash fish can be given as feed. Feed is given depending on crabs weight. Crabs whose shell length is less than six centimeter can be fed up to 10% of their body weight per day while crabs whose shell length is greater than six centimeters can be fed up to 5% of their body weight per day. Feed must be given two doses in a day; the first dose is given at the evening time about 60% out of the daily ration and the remaining 40% at late evening.
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Mud Crab Culture Methods
Mud crabs are grown in two ways; grow out culture system and fattening system.
GROW OUT CULTURE SYSTEM
Ponds: Crabs can be grown in earthen ponds used for marine shrimp culture. Pond size of half an acre to one acre is most suitable and ponds larger than one acre can also be used to farm crabs. Similar to shrimp farming sites, sandy soils with 50 percent clay is an ideal site. The pond must have water inlet and outlet for water exchange. Place a netting barrier or some other type surrounding the pond or mud crab culture pond as mud crabs can spend a considerable period of time on land by leaving the water. At times, the crabs might walk away from the aquaculture pond site that will directly lead to farmer’s financial loss. To overcome this loss, farmers have to place netting with plastic topping around the pond up to 20 to 50 cm in height which will prevent crabs walking away from the culture site.
Mud Crab Culture on Raised Dry Platforms inside the Ponds: In this system, the ponds are designed with raised mounds or platforms within the pond. Keeping in mind that mud crabs will dwell both on land and water, these raised dry platforms will mimic the natural environment habitat. These raised platforms are also used as feeding stations for mud crabs. The design will reduce the water usage in the pond and these designs are also helpful in places where water resources are limited. The water levels in the pond can vary in height just like the tidal cycles giving mud crab a natural environment. There is no documented research whether this design is beneficial over conventional ponds.
Mud Crab Culture Pens in Mangrove Areas: Mangrove forests are best suitable natural habitats for juvenile and adult mud crabs, the pen design will be an ideal pond culture in mangrove areas to raise mud crabs. Each mangrove pen can be of 100 sq. meters with mangrove vegetation in the center which will provide shade and refuge for crabs during low tides. These designs will retain the crab stock while monitoring their feed and growth within the mangrove specific area. As the pens are constructed in the mangrove forest area, the pens fencing is constructed with complete bamboo wood or of netting with a wooden frame structure. Netting mesh size must be small enough to hold the smallest crab in the farm stock. To ensure that no crabs escape by swimming out on a maximum king tide, the height of the fence must be higher than the king tides. Mangrove pens are constructed with walls buried deep in the mud about 40 to 60 cm and support posts every three meters with horizontal bamboos or bracing structure to support the walls. It is recommended to construct ditches inside the mangrove pens which will hold water for crabs stay immersed even at low tide and about 20 to 30% of the total pen area must consist of such ditches. If not ditches, a canal can be dug around the edge of the pond about one meter wide and half a meter in depth that will hold water during low tide.The crab survival rate in this mangrove pen culture is 45 to 50% and the loss is due to escape of crabs and cannibalism, it is suggested the density of stock should be lower to lower the survival rate.
Crab Fattening in Mud Crab Culture
After molting, crab exoskeleton takes some time to grow and get hardened. These soft-shelled crabs do not fetch the market price as they little meat and lot of water content in it. Mud crab fattening is a process where these soft shelled crabs which are identified at the time of harvest are reared for a period of time till their exoskeleton gets hardened and are full of meat and ready to market.
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Fattening in Cage and Pen Mud Crab Culture: Cages of about 100 x 100 x 20 cm size divided into nine equal cell-type partitions can be used for carrying out crab fattening process. The cages can be made of cane or wooden sticks or any other non-corrosive materials with each cage having a lid at the top and no gaps in the flooring for easy crab movement. Each cell or compartment in the cage must hold only one crab i.e. nine crabs per cage. Cages can also be made without cells making it as the pens but this will lead to cannibalism which will lower the survival rate. In pens, stocking of five crabs per square meter is ideal. Cages can be suspended from a raft or can be made fixed in ponds, backwaters, or mangrove areas.
Mud Crab Culture Fattening in Ponds: Crabs weighing 150 to 200 grams are stocked in ponds either in the early morning or in the evening time. Two to three crabs per square meter are stocked, hollow blocks or empty used cans are placed in the pond as hiding areas to prevent cannibalism and fighting among themselves. Crabs are fed at a rate of five to eight percent of their body weight thrice a day. By two weeks the body weight increases and the crabs attain marketable weight.
It is found and recommended for all crab culture owners, the fattening system is profitable and has more advantages over grow out system. The fattening system is more popular as it takes less time for crabs to grow and will be ready for marketing while in grow out system the production of crabs for marketing takes considerable time. As for commercial production, harvesting in less time with good body weight will give high profits to aqua farmers.
Water Quality in Mud Crab Culture
For successful crab production and profits, water quality in the culture plays an important role. Crab culture must have proper water recycling with water an inlet and outlet. The ideal water quality for crab production must have a salinity of 15 to 25pptand with more than 5 ppm of oxygen levels in the water. Crabs grow well in water temperatures ranging between 26° to 30°C and with 7.5 to 8.5 water pH ranges.
Health Management in Mud Crab Culture
Though there is not much-documented source regarding diseased in mud crabs and is still at an early stage of development. However, compared with diseases in shrimp farming can be incorporated into mud crab rearing. Some diseases such as white spot syndrome virus (WSSV) and Taura syndrome virus (TSV) are common diseases with no mortality.
Disease management and Treatment in Mud Crab Culture: As said earlier that there’s no knowledge regarding diseases and treatment is still at an early stage of development. Much of the disease management has been developed at hatchery level operations in improving mud crab larvae survival. Controlling for bacterial and fungal diseases and infections is critical at hatchery stage; less mortality in hatchery stage will give more crab production at the later stages. Prophylactic treatments such as antibiotics for bacteria and fungicides for fungi were successful in improving survival rates of mud crab larva in hatcheries. Pond management is another area where substantial improvements can be made. For improved survival rate in ponds, using quicklime to disinfect the ponds as preparation before stocking and filling of juveniles and adult crabs has led to high survival rate. Some of the recommended strategies for minimizing disease risks in mud crab farming are:
- Pond or Cage or Pen Management in mud crab culture.
- Water Quality Management in mud crab culture.
- Feed Management in mud crab culture.
- Stock Density and Husbandry Practices in mud crab culture.
- Source of Stock and Record-keeping in mud crab culture.
Economics of Mud Crab Culture
The mud crab culture model project is for fattening in a 1000 sq.m size pond, the cost and items are not actual figures but are indicative project figures. While preparing the project model for a 1000 sq.m pond; estimation of capital cost and operational cost is Rs. 65,000 and Rs. 40,000 respectively. Financial viability of mud crab culture is based on the following assumptions:
|Size of the pond||1000 sq.m|
|Culture / Crop Period||21 days|
|Stock Density ( 1000 nos of 300 to 350 grams each)||1 crab / m2|
|Survival Rate||75 – 80%|
|Weight of Crab at the time of Harvest||500 grams|
|Expected Production per Crop||400 Kg|
|No.of Crops in the first year||4 – 5|
|No.of Crops in the second year||8 – 10|
|Farm gate price||Rs. 190 / kg|
Capital Investment Cost for a Mud Crab Culture of 1000 sq.m pond:
|Construction of pond ( digging, bund construction )||25,000|
|Water inlet and outlet Sluices ( 2 x 6000)||12,000|
|Water pumpset ( 5 HP diesel )||20,000|
|Fencing (bamboo, nylon netting, casuarina poles)||12,500|
|Shed for watchman||3,500|
Operational Cost for Mud Crab Culture of One Crop:
|Pond preparation and sanitizing with quicklime||750|
|Cost of soft crab (1000 nos of 350 gm each @ 85 / kg)||29,750|
|Crab Feed 735 kg (35 kg/day @ Rs. 12.50 / kg)||9,200|
|Labor cost (Rs. 250 / day)||5,250|
Mud Crab Culture Production and Profit Margin:
Cost of Production on First Crop in First Year = 75,500+45,450 = Rs. 1,20,950/-
Cost of Production on second Crop onwards =Rs. 45,450 /-
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Income on One Crop in the First Year = 400 x 190 = Rs. 76,000 /-
Income in the First Year= Rs. 1,22,700 /-
Income on each crop(21 days) from Third Crop onwards=Rs. 30,550 /-. Income and profit generation is still higher (more than 5 times) in mud crab culture which practice fattening of crab culture in cage methods. Although initial one-time capital investment will be high which can be recovered in the first 2 crops and from then onwards the profit margin will be very high in Mud Crab Culture.