Scylla serrata is the common mudcrab occurring in the estuarine and mangrove areas and is commonly called as "red crab" and it prefers to live in low saline waters. Male crabs of S. serrata grow to 700 to 800 gm at the maximum 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.
(Source: Bureau of Agricultural Research, Date accessed 25 March 2014)
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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
Four methods of oyster culture are practised in the Philippines; broadcast (sabong), stake (tulos), lattice and hanging (bitin, sampayan, horizontal, and tray) methods
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 dykes for easy stocking and monitoring.
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.
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 150Sq. m. Within 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 centre of the pen provide shade for the crabs. Roughly 1000 to1500 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.
Cage Culture (suspended or fixed type)
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.
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.
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.
- 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.
Managing the Crab Farm
Once decided on the farming method and when the oyster spats have settled
- Condition ponds/pens before stocking mudcrabs. Plant Gracilaria or other macrophytes to serve as shelter for crabs. Stock crabs when luxurious growth of macrophytes is observed.
- To insure high survival of crab juveniles for grow-out culture while in transport, provide transport containers with fronds of mangroves. Remove chilepeds of crabs weighing less than 30g. Do not remove chilepeds of crabs weighing more than 30g but tie them firmly to prevent antagonistic behavior during transport. Frequently pour seawater into containers while in transport to keep crabs moist.
- Stock marketable size lean crabs for fattening culture at 2.0 crabs per sq m. Stock together male and female but remove movable part of the claw and apply Povidone-iodine (betadine) to the injured part to prevent infection. Acclimate before releasing them in ponds/pen.
- Stock crab juveniles (7-11g or 16-20g) at 1.5 per sq m for pond grow-out culture and 2.0 per sq m for pen (mangroves). Stock males separately from females. Stock monospecies, more or less monosize crabs. Acclimate to pond/pen water temperature and salinity before releasing them.
- Feed crabs with frozen or freshtrash fishor a mixed diet of 75% brown mussel meat and 25% trash fish. Feed grow-out culture crabs at 10% of the crab biomass per day when carapace length is less than 6cm and 5% when carapace length is 6cm or more. Feed fattening culture crabs at 10% of the crab biomass per day through out the culture period. Feed crabs in the grow-out or fattening culture twice per day: 60% of the daily ration at 5:00 PM and 40% at 7:00 PM.
- Select and remove marketable size and fat crabs several times over the grow-out culture period: 150g or more female and 200g or more for male pulang alimango; 350g or more for female and 400g or more for male giant crabs.
- Harvest fat crabs from fattening culture 20 days after stocking. Not all crabs fatten at the same time but expect to harvest about 50% fat crabs of your total stock. Replace harvested fat crabs with lean ones but remove the movable claw, disinfect, and acclimate them before releasing in ponds/pens. Harvest and replace every 10 days thereafter; this time you can harvest fat crabs of about 30% of your total stock. You can maintain this cycle for five months.
- Harvesting is done with different kinds of trap like the bamboo cage, lift net, scissors net, fish corrals and gill nets. Crabs are ready for the harvest and marketing when the piece or two reaches up to a kilo. They are sold alive and can stay out of the water even for a week. They should, however, be kept in damp containers and periodic moistening is important. Feed them with trash fish and other kitchen refuse.
- Handling adult crabs in captivity are tied with dried nipa strings. Both pincers are tied close to the abdominal cavity to prevent crawling. When transported, proper handling is important. Place them in baskets or tiklis to avoid getting trampled or crushed.
Pest and Disease Management
Mud crab fattening in bamboo cages is one of the technology verification studies tried out by Joey and Sylvia de la Cruz in Barangay Napapao, Ponteverdra Capiz. This project was conducted to provide a standard culture method for fattening crab:
- Site Selection
Mud crabs grow best in brackish water, such as tidal flats, estuarine areas, bays and lagoons. Sheltered bays and coves are selected to protect the bamboo cages from strong winds and waves during adverse weather conditions. The water at such sites should be 0.5-lm deep. Areas with low salinities should be preferred, as saline water inhibits the growth of mud crab. Areas with sufficient crab for fattening as well as trash fish for feed should be considered. The area should also be accessible to the growers and target markets.
- Cage Design
A modified bamboo cage (140 x 70 x 25 cm) subdivided into 18 compartments is fixed firmly by its comers to the substratum to prevent it from being washed away during inclement weather. The compartments are covered with 140 x 70 cm split bamboo. Holes are provided in the compartment covers for feeding.
One advantage of using bamboo cages is that selective harvesting can be done. If the desired weight has not been attained, the crab could easily be returned to their compartments and fattened further.
About 18 crabs can be stocked per unit. Stocking is done during the early morning or late in the afternoon. In Capiz, 185 crabs, each of average weight 175 g. were stocked. The weight increase after 15 days was I 10 g.
- Feed and Feeding
Mud crabs are fed twice a day at 5 per cent bodyweight for 10-15 days. Feeds may be trash fish, soft-shelled snails, kitchen leftovers, mussel meat, animal entrails or almost any other kind of food.
- Cage Maintenance
Periodic checks should be made during the culture period. Drifting seaweed, logs and other debris should be removed to facilitate easy circulation of water and prevent damage to the cages. After use, the crab cages should be lifted periodically and dried.
- Harvest and Handling
After the fattening period, mud crab can be harvested individually by hand. The crabs are then bound with straw or string to enable easy handling. A skilled laborer is hired to bind the pincers of the crab. Exposure of the crab to sun and wind should be avoided, as this may lead to weakening and eventual death.
Source: Bureau of Agricultural Research, Date accessed 25 March 2014