Scylla serrata and Scylla tranquebarica are the common mud crabs occurring in the estuarine and mangrove areas along the East coast of India. Scylla serrata is commonly called as “red crab” and it prefers to live in low saline waters, whereas S. tranquebarica, the “green crab” lives in high saline waters. Male crabs of S. serrata grow to 700 to 800 gm at the maximum, whereas S. tranquebarica grows up to 3 kg. 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.
Biology and Life Cycle of Crab
Mud crabs emerge from their hideouts or burrows at night (nocturnal) and forage for food. They are primarily carnivorous and feed on slow moving, (or stationary), bottomdwelling animals such as bivalves, snails, other crabs, hermit crabs, and worms, as well as small amounts of plants and debris. They rarely eat fish under natural conditions since they lack the ability to catch them. They are attracted to a wide variety of baits, including fish, mussels, clams, and squids.
Life cycle of mud crab is female crab > egg > eyed stage > zoea > megalopa > juvenile crab.
Male and female are separate in mud crabs. Recent studies showed that the transition of immature crabs to physiological maturity occurs between 90-110 mm carapace width. While most of the life cycle of S. serrata is spent in inshore waters, especially estuaries, the females migrate offshore with the fertilized eggs attached to the pleopods, where they hatch in a few weeks. The larvae hatch out as zoea. The larval stages of mud crab include five zoeal stages, one megalopa stage and one crab instar (first crab instar).
Mud crabs are extremely tolerant to salinity and temperature variation, and can survive in a salinity range of 2 to 50 Psu and temperatures of 12 to 35 °C. But sudden variation in the salinity and temperature should be avoided. S. serrata is mainly active at night. Although many occupy burrows in the intertidal zone, the majority of adults live sub tidally, where they bury in the mud during the day.
This involves holding of large-sized crabs that have recently molted (also known as “water crabs”) for 10 – 30 days until they become hard or are completely “fleshed out.”
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
1. Pond Culture
Pond size of 1/2 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, wornout 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. meter.
2. 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 dikes for easy stocking and monitoring.
3. Pond Culture in Mangrove Areas
The ponds could be constructed as described above around the mangrove plants. But a maximum pond area of 100 Sq. meters is suitable for this type of culture. A canal of 1 m wide and 0.5 m deep, in which water will be available even during low tide, should be dug around the edge of the pond. The center of the pond forms a raised platform with mangrove vegetation, which the crabs would use as a refuge during low tide. Water exchange could be tidally controlled. Polythene nettings could be used to prevent the escape of the crabs. Feeding depends on the availability of organisms namely low-value fishes, mangrove snails, clams, mussels, etc.
4. Pen Culture in Mangrove Areas
The pens could be constructed using the locally available bamboo splits or arecanut logs or cane. These strips should be driven 1-1.5 m deep into the soil to keep the crabs inside and the potential predators outside. The manageable area of the pen could be 100 to 150 Sq. m. With in the pen, a ditch of about 0.3 to 0.9 m wide and 0.3 m deep should be dug. Mangrove trees in the center of the pen provide shade for the crabs. Roughly 1000 to 1500 crabs of 100 g each could be stocked per pen. The stocking should be continuous.
The crab could be fed once a day during high tide with low-cost fishes, mussels, clams, snails, etc. The crabs could be harvested after 4 – 7 months. The crabs could be selectively harvested after they reach 400g or more. Although this system is eco-friendly, survival rate of only 47 to 50 % could be expected. The loss could be mainly due to cannibalism, and escape of crabs. Lower stocking density is suggested to be a remedy for the low survival rate.
5. Cage culture (Suspended or Fixed Type):
a. Cage design
Crab fattening can be carried out in Cell-type Cane Cages of 1m (L) x 1m (W) x 20 cm (H) size, which can be partitioned into nine equal compartments. Each of these cages should be provided with a lid to prevent the escape of crabs. A gap of 5 mm is to be provided between the canes at the top and 2.5 cm at the sides of the cages to enable free movement of water through the cages. But, no gap should be provided at the bottom to enable easy movement of the crabs.
b. Stocking and feeding in cages
One crab should be placed in each compartment of the cages. In this method of fattening, higher number of crabs can be fattened in a square meter area, i.e. 9 crabs / m2. Based on the local availability, different types of feeds such as trash fish, mussel, chicken waste, clams etc. can be given to the crabs.
c. Deployment of cages
These cages can either be suspended from a raft deployed in bays or backwaters or mangrove areas. These cages could also be made as a fixed type in ponds, mangrove areas or coastal regions of the bays. The cages could be made without cells inside. But the survival would reduce in this method due to cannibalism.
d. Cage Maintenance
- Clean the cages as frequently as possible using brushes enabling free movement of water inside
- If nails are used in the cages, use only the anodized MS/ copper / SS nails for increased longevity of cages in seawater.
- Repair the damages in the cages immediately when it happens
- Deploy the cages where there is mild water current.
- If algal growth is found on the crabs, clean them using a brush.
source: National Institute of Ocean Technology, Narayanapuram, Pallikaranai, Tamil Nadu, India