Production of Mangrove Crab

The mangrove crab, Scylla serrata, which has huge demand in both domestic and international markets, is commonly found in mudflats and mangrove areas of India and South East Asia. It has flat and broad body covered with a fan shaped carapace. Adult crab migrates to the sea for spawning and the larval stages are completed in seawater. The instar migrates back to the brackish water for further growth and development. Eventhough hatchery seed production technique is available with RGCA, crab farming still depends on wild seed collection.

Seed Production

Broodstock management

Broodstock collected from wild or farm is brought with its walking legs in tied condition, disinfected with 100 ppm formalin for 30 seconds, acclimatised to hatchery condition and quarantined. It is stocked at a density of 1 no./m² in an FRP tank (2 t capacity with 1.2 m height) containing washed coarse sea sand of 10 cm thickness spread at the bottom which is cleaned fortnightly. Filtered sea water is maintained in the tank to a level of 80 cm and aerated. It is fed with fresh wet feed @ 5-10% of the body weight. PVC pipes and tiles are kept in the tank as hideouts.

Mangrove crab under tied condition

Sexes are separate and it can be distinguished based on the shape of abdominal flap, which is slender and triangular in male while it is broad and almost semicircular in female.

Female crab & Male crab

Spawning

The mangrove crab spawns throughout the year. The mature female weighing a minimum of 500 g size is selected for spawning. Eyestalk ablation is carried out to induce gonadal development, if required. The colour of fertilised egg which is brilliant orange initially, gradually changes to greyish orange and finally to grey. The egg mass seen at the abdominal flap of the female crab is shifted to incubation tank (300-500 l) with moderate aeration. Egg hatches out into zoea larva within 10-12 days and it is collected.

Rearing of Larvae

The zoea larvae are stocked @ 100 no./l in tank with clean filtered sea water having 28-34 ppt and 28-30⁰C. The zoea has 5 stages of development and is fed with Nannochloropsis and Brachionus for initial two stages and Artimia nauplii for the later 3 stages. The zoea metamorphoses into megalopa, which again metamorphoses into tiny crab called crab instar. The megalopa is fed with fish or mussel meat or Artemia biomass. Larval development is completed within 25-30 days.

Rearing of crab instar

The crab instar of 0.3 to 0.4 cm carapace width (CW) is reared into crablet of 2.5 cm CW @ 25-30 no./m² for 30-40 days in HDPE or nylon nursery hapa with a mesh size of 1.5 mm supported with bamboo poles having 5x4 m size installed in pond with not less than l m water depth and 20-35 ppt salinity. Hapa can be installed prior to filling of water in the pond. Inorganic/organic fertilizer is applied to enhance the natural food production. Catwalk is constructed for feeding and monitoring the crab instar. Crab instar is initially fed twice daily with mascerated fish/mussel or clam meat @ 50% of the body weight and subsequently reduced to 30% after one week and to 20% thereafter two weeks (40% in the morning and 60% in the evening).

Rearing of crab instar in hapa

Cannibalism is a serious threat in obtaining good survival rate. Hideouts are provided to minimize cannibalism during the nursery rearing period. PVC pipes or tiles are generally used as hideouts. Grading and thinning are done periodically for maintaining uniform size. Survival rate from crab instar stage to crablet is usually 60%.

Harvest and transportation

While harvesting the crablets, care must be taken, not to expose them to direct sunlight. The crab-lets can be transported in ventilated bamboo basket, plastic tray, straw bag or thermocol box stacked with wet sand, mangrove leaves, shade net or seaweed to minimize attacking each other and to keep the temperature low in the container during transport.

Grow out Farming

Pond culture

The grow-out pond having a size of 0.2-1 ha and water depth of about 100 cm with proper inlet and outlet for water management is ideal. Net with polythene sheet enclosure should be set up along the inner side of the earthen pond dike to prevent the escape of crab. Pond preparation can be done as similar to that for the shrimp.

Pond fenced with net

Pen culture

Farming of the crab in pen is done in an earthen pond or open mangrove area and mudflat, which facilitates easy growth monitoring and stocking of different sized crabs in different pens simultaneously. It is usually reared in pen having dimension of 20 x 10 x 1.7 m. In order to install pen, trench of 50 cm deep and 20 cm width is excavated and concrete filled PVC pipe or bamboo poles of suitable height are thrust deep into the trenches one metre apart so as to fix them firmly. It can be strengthened more with horizontal or diagonal poles. Then it is wrapped with HDPE net of 10 mm mesh size and firmly tied. The bottom of the net is wrapped and stitched onto pole or sinker and placed into the trench from inside. Then the trench is filled with soil. The free top end of the net is tied firmly to the top of the poles. The inner side of the upper end of the net is tied atleast 50 cm above the water level with polythene sheet.

Pen culture

Box culture

In recent times, farming of mud crab individually in LDPE perforated box having the size of 30 x 20 x 15 cm with lid is widely practiced.

Water quality requirements

• pH : 7.5-8.5
• Salinity : 15-30 ppt
• Temperature : 28-32⁰C
• DO : >4ppm

Stocking

Uniform sized crablet of 2.5 cm CW is stocked @ 1 no./m² after proper acclimatisation.

Feeding

The crab is fed with chopped fish twice daily (40% in the morning and 60% in the evening) @ 10% of the body weight initially till it attains a carapace width of 6 cm and later it is reduced to 8% for the carapace width in between 6-15 cm and thereafter reduced to 6%.

Care and Monitoring

Highly fluctuating survival rate due to cannibalism is the major problem in farming, which can be controlled by placing proper hide-outs such as mangrove twigs, sand heaps, tiles, earthen/cement/PVC pipes, hollow blocks etc. in the culture system. Monitoring of water quality parameters such as temperature, salinity, DO and pH is done fortnightly. Water exchange is carried out to maintain water quality as per requirement.

Harvesting

During the culture period, cull harvesting with lift net or scoop net is done at required times to remove the shooters and to allow smaller crab to grow faster. After the culture period of 8-9 months, the pond is drained for complete harvesting by lift net or scoop net and hand picking. The expected size at harvest is more than 500 g with 40% survival. A production of 2 t/ha can be achieved annually.

Source : Department of Fisheries, Government of Kerala