Mud crab is highly popular due to its great demand in the export market. The commercial scale mud crab culture is developing fast along the coastal areas of Andhra Pradesh, Tamil Nadu, Kerala and Karnataka.
Types of mud crabs
Mud crabs in the genus Scylla inhabit coastal areas, estuaries and backwaters.
i. Larger species :
- The larger species is locally known as `green mud crab'.
- It grows to a maximum size of 22 cm carapace width and 2 kg in weight.
- These are free living and distinguished by the polygonal markings present on all appendages.
ii. Smaller species :
- The smaller species is known as `red claw'.
- This grows to a maximum size of 12.7 cm carapace width and 1.2 kg in weight.
- It is without polygonal markings and has a burrowing habit.
Both species have good demand in the domestic as well as in the foreign market.
Mud crab farming is done by two methods.
i. Grow- out culture
- In this method, young crabs are grown for a period of 5 to 6 months till they attain desirable size.
- Mud crab grow-out systems are generally pond based, with or without mangroves.
- The pond size varies between 0.5-2 ha, with proper bunds and tidal water exchange.
- Fencing is advisable if the pond is small. In large ponds where natural conditions are prevailing, strengthening is necessary along the outlet area.
- Wild collected juvenile crabs of 10-100 g size are used for stocking.
- The duration of the culture varies between 3-6 months.
- The stocking rates are commonly between 1-3 crabs/m2, with supplementary feeding.
- Feeding is usually with trash fish (wet weight feeding rate-5% per day of the biomass), along with other locally available items.
- Regular sampling is necessary to monitor the growth and general health, and to adjust the feeding rate.
- Partial harvesting of marketable sized crabs can be started from 3rd month onwards. This 'stock-thinning' provides chances for better survival by reducing the mutual attacks and cannibalism.
Soft shelled crabs are reared for a period of a few weeks till their exoskeleton gets hardened. These 'hard' crabs are locally known as "mud" (meat) and fetch three to four times better price than the soft crabs.
a. Fattening in ponds
- Fattening can be done in small tidal ponds between 0.025-0.2 ha with a water depth of 1 to 1.5 m.
- Before stocking the soft crabs in the pond, the bottom is prepared by draining the pond water, sun-drying and adding sufficient quantity of lime.
- Care is taken in strengthening the pond bunds without any holes and crevices. Special care is taken in the sluice area as these crabs have a tendency to escape through nearby areas of sluice gate. The inlet areas should be reinforced with bamboo matting inside the bund.
- The ponds are fenced properly using bamboo poles and nets along the sides of the bund, which incline towards the pond to prevent the escape of crabs.
- Soft crabs collected from local fishermen / crab merchants are stocked in the ponds preferably in the morning hours @ 0.5-2 crabs / m2 according to the size of the crabs.
- For crabs of 550g and above, market demand is more. Hence it is ideal to stock
crabs belonging to this size group. In this case, the stocking density should not be more than 1 crab /m2.
- Depending upon the location and availability of water crabs, 6-8 cycles of "fattening" can be carried out in a pond by repetitive stocking and harvesting.
- If the culture pond is big, it is better to divide the pond into different compartments of suitable sizes for stocking crabs of uniform sizes in the same compartment. This is good for manipulating the feeding and is easy to monitor and harvest.
- When the interval between stockings is wide, crabs of similar sizes can be stocked in one compartment.
- Sex-wise stocking in compartments is advantageous to reduce the attacks from more
aggressive male crabs. It's better to provide shelters like old tyres, bamboo baskets, tiles etc. to minimize mutual attacks and cannibalism.
b. Fattening in pens and cages
- Fattening also can be carried out in pens, floating net cages or bamboo cages in shallow estuarine waterways and inside large shrimp ponds with good tidal water influx.
- HDPE, netlon or bamboo splits can be used as the netting material.
- The size of the cage shall be preferably 3 m x 2 m x 1 m.
- The cages have to be arranged in a row so that feeding and monitoring can be easily carried out.
- A stocking of 10 crab/m2 in cages and 5 crabs/m2 for pens is recommended. Since the stocking rate is higher in cages, and to minimize the mutual attacks, the tips of the chelae can be removed while stocking.
- However, these methods are not commercialized like `fattening' in ponds.
Between these two methods, Fattening is more advantageous since the culture period is short and profitable, when enough stocking material is assured. In India, grow-out culture is not popular, mainly due to non-availability of crab seeds and commercial feed.
Crabs are daily fed with trash fish, brackish water clams or boiled chicken waste @ 5-8% of their body weight. If the feed is given twice a day, major share must be given during evening hours.
The water quality parameters shall be maintained within the range shown below:
> 3 ppm
Harvesting and marketing
- Periodically crabs are to be checked for their hardening.
- Harvesting should be done in the early morning hours or evening hours.
- The harvested crabs should be washed in good brackish water to remove the dirt and mud and carefully tied without breaking its legs.
- The harvested crabs have to be kept in moist conditions. It has to be kept away from sunlight, which has a negative effect on survival.
Source: Central Marine Fisheries Research Institute, Cochin