mud crab

2/17/2015 Mudcrab

http://www.bar.gov.ph/agfishtechhome/fisheries/213brackishwater/1320mudcrab?tmpl=component&print=1&page= 1/7

Mudcrab

IntroductionScylla serrata is the common mudcrab occurring in the estuarine and mangrove areas and is commonlycalled as red crab and it prefers to live in low saline waters. Male crabs of S. serrata grow to 700 to 800gm at the maximum The export size of the crab is 500 g and above for males and 250 g and above forfemales.Crab fattening is widely practiced in Thailand, Taiwan, Malaysia, Singapore and Indonesia. Gravid femalemud crabs with full orangered egg masses are in great demand in seafood restaurants of South East Asiancountries. Due to its high price, people started to hold immature female crabs in some kind ofenclosures and fed them until the gonads developed and filled the mantle cavity. This is how crabfattening spread, initially, throughout South East Asian countries. Subsequently, the practice ofholding postmoult water crab of market size, in some enclosures, for short period of time and feedingthem until they completely flesh out for getting quick returns also became popular. Cages, pens andsmall ponds with net are being used for holding crabs for a short period of 34 weeks.The mud crab resource is a natural bounty for our country, which has a potential to change the socioeconomic status of the coastal communities. The coastal poor fishermen and educated unemployedyouths should realize this fact and take up crab culture or fattening in ecofriendly way to raise theireconomic status.

CultureThis is the process of stocking juvenile crabs (10 g to 250 g) and allowing them to moult and grow.Harvest is done after 38 months or once the crab reaches 400 g to 500 g size. Mud crab fattening is themost suitable method for smallscale 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 growout systems (1crab/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 ratefor fattening (>90%) compared to growout systems (40%).

Different Methods of Crab CultureFour methods of oyster culture are practised in the Philippines; broadcast (sabong), stake (tulos), latticeand hanging (bitin, sampayan, horizontal, and tray) methods.

Pond Culture. Pond size of to 1 acre is most suitable for crab culture. However, large sizeponds of more than one acre can also be used for this purpose. Sandy soils with a mixture of50% clay are ideal for culture of these crabs. A water inlet system and an outlet system todrain out water during water exchange should be constructed as in the case of shrimps. Thepond should be constructed in such a way that it should hold 3 to 4 feet of water towardsthe inlet and 4 to 5 feet towards the outlet. A flow through mechanism of water exchangeshould be there in order to remove any left over organic food material and also toefficiently remove excretory material. A fencing of nylon net used for fishing can be placedon the dike to prevent the escape of the crabs during nighttime. In addition, about 1000numbers of stone ware, pipes of 6 inch diameter and 1 feet length, worn out tyres, etc.,should be kept at the bottom of the pond through out the dike. The nylon screen fencing

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should be supported with split bamboos of 1.5meter height around the pond periphery forpreventing the escape of the crabs from climbing over the bunds. The maximum stockingdensity should be 1crab per sq. metre.

Pen Culture in Ponds. Several units of pens of 4 X 4 X 2.5 m could be made inside the pondsusing bamboo strips which are driven 11.5 m deep into the soil to prevent the escape ofthe crabs by burrowing. The pens could be made nearer to the dykes for easy stocking andmonitoring.

Pond Culture in Mangrove Areas. The ponds could be constructed as described abovearound the mangrove plants. But a maximum pond area of 100 Sq. meters is suitable for thistype of culture. A canal of 1 m wide and 0.5 m deep, in which water will be available evenduring low tide, should be dug around the edge of the pond. The center of the pond forms araised platform with mangrove vegetation, which the crabs would use as a refuge during lowtide. Water exchange could be tidally controlled. Polythene nettings could be used toprevent the escape of the crabs. Feeding depends on the availability of organisms namelylowvalue fishes, mangrove snails, clams, mussels, etc.

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Pen Culture in Mangrove Areas.The pens could be constructed using the locally availablebamboo splits or arecanut logs or cane. These strips should be driven 11.5 m deep into thesoil to keep the crabs inside and the potential predators outside. The manageable area ofthe pen could be 100 to 150Sq. m. With in the pen, a ditch of about 0.3 to 0.9 m wide and0.3 m deep should be dug. Mangrove trees in the centre of the pen provide shade for thecrabs. Roughly 1000 to1500 crabs of 100 g each could be stocked per pen. The stockingshould be continuous. The crab could be fed once a day during high tide with lowcostfishes, mussels, clams, snails, etc. The crabs could be harvested after 4 7 months. Thecrabs could be selectively harvested after they reach 400g or more. Although this system isecofriendly, survival rate of only 47 to 50 % could be expected. The loss could be mainlydue to cannibalism, and escape of crabs. Lower stocking density is suggested to be a remedyfor the low survival rate.

Cage Culture (suspended or fixed type)Cage design. Crab fattening can be carried out in Celltype Cane Cages of 1m (L) X 1m (W) X20 cm(H) size, which can be partitioned into nine equal compartments. Each of these cagesshould be provided with a lid to prevent the escape of crabs. A gap of 5 mm is to beprovided between the canes at the top and 2.5 cm at the sides of the cages to enable freemovement of water through the cages. But, no gap should be provided at the bottom toenable easy movement of the crabs.

Stocking and feeding in cages. One crab should be placed in each compartment of thecages. In this method of fattening, higher number of crabs can be fattened in a squaremeter area, i.e. 9 crabs / m2. Based on the local availability, different types of feeds such astrash fish, mussel, chicken waste, clams etc. can be given to the crabs.

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Deployment of cages. These cages can either be suspended from a raft deployed in bays orbackwaters 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 cellsinside. But the survival would reduce in this method due to cannibalism.

Cage Maintenance

1. Clean the cages as frequently as possible using brushes enabling free movement ofwater inside.

2. If nails are used in the cages, use only the anodized MS/ copper / SS nails forincreased longevity of cages in seawater.

3. Repair the damages in the cages immediately when it happens.4. Deploy the cages where there is mild water current.5. If algal growth is found on the crabs, clean them using a brush.

Managing the Crab FarmOnce decided on the farming method and when the oyster spats have settled

1. Condition ponds/pens before stocking mudcrabs. Plant Gracilaria or other macrophytes to serve asshelter for crabs. Stock crabs when luxurious growth of macrophytes is observed.

2. To insure high survival of crab juveniles for growout culture while in transport, provide transportcontainers with fronds of mangroves. Remove chilepeds of crabs weighing less than 30g. Do notremove chilepeds of crabs weighing more than 30g but tie them firmly to prevent antagonisticbehavior during transport. Frequently pour seawater into containers while in transport to keepcrabs moist.

3. Stock marketable size lean crabs for fattening culture at 2.0 crabs per sq m. Stock together maleand female but remove movable part of the claw and apply Povidoneiodine (betadine) to theinjured part to prevent infection. Acclimate before releasing them in ponds/pen.

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4. Stock crab juveniles (711g or 1620g) at 1.5 per sq m for pond growout culture and 2.0 per sq mfor pen (mangroves). Stock males separately from females. Stock monospecies, more or lessmonosize crabs. Acclimate to pond/pen water temperature and salinity before releasing them.

5. Feed crabs with frozen or freshtrash fishor a mixed diet of 75% brown mussel meat and 25% trashfish. Feed growout culture crabs at 10% of the crab biomass per day when carapace length is lessthan 6cm and 5% when carapace length is 6cm or more. Feed fattening culture crabs at 10% of thecrab biomass per day through out the culture period. Feed crabs in the growout or fatteningculture twice per day: 60% of the daily ration at 5:00 PM and 40% at 7:00 PM.

6. Select and remove marketable size and fat crabs several times over the growout culture period:150g or more female and 200g or more for male pulang alimango; 350g or more for female and 400gor more for male giant crabs.

7. Harvest fat crabs from fattening culture 20 days after stocking. Not all crabs fatten at the sametime but expect to harvest about 50% fat crabs of your total stock. Replace harvested fat crabswith lean ones but remove the movable claw, disinfect, and acclimate them before releasing inponds/pens. Harvest and replace every 10 days thereafter; this time you can harvest fat crabs ofabout 30% of your total stock. You can maintain this cycle for five months.

8. Harvesting is done with different kinds of trap like the bamboo cage, lift net, scissors net, fishcorrals and gill nets. Crabs are ready for the harvest and marketing when the piece or two reachesup 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 trashfish and other kitchen refuse.

9. Handling adult crabs in captivity are tied with dried nipa strings. Both pincers are tied close to theabdominal cavity to prevent crawling. When transported, proper handling is important. Place themin baskets or tiklis to avoid getting trampled or crushed.

Mudcrab Fattenning Practicesa. Mud crab fattening in fish ponds

In New Washington, Aklan. mud crab fattening activities were initiated by fish pond owners using aseries of crude trial and error methods. Small undeveloped ponds measuring 500 m2 were utilizedfor fattening. Bamboo or plastic polyethelene netting was used as fencing material.The ponds were prepared in similar fashion to milkfish and prawn ponds. After fertilization, crabweighing 150200 g were stocked during the early mornings or late afternoons. The stocking ratewas 23 crab/m To prevent cannibalism and fighting amongst themselves, the tips of their pincerswere cut off. Sometimes hollow blocks or old cans were placed at the pond bottom to serve ashiding areas for the crab.The crab were fed three times a day at a rate of 58 per cent of bodyweight. Water was changed asoften as possible to prevent fouling. The crab were fattened for 1015 days and a growthincrement of 110g/crab was achieved. After 15 days, the crab were harvested using crab liftnets.

b. Mud crab fattening in square pensPanquil Bay in Mindanao is another mud crab producing region where mud crab fattening is widelypracticed. About 20 t of exportable mud crab are shipped every month from this area to Cebu orManila.Mud crab fattening is widely practised here because of financial assistance under the LEADBuklodYaman Project of the Department of Agriculture. Assistance has been granted to four or fivefishermens associations in this area. Each association has a membership of 25 fishermen.The method of fattening in Panquil Bay differs from elsewhere. Instead of earthen ponds, square

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pens are used. These 2 x 2 x 1.5m pens, made with bamboo poles, are erected in the muddy,intertidal areas near the fishermens houses. In order to facilitate entry, exit and feeding,especially during high tides. there is a catwalk set up near the pens.Crab weighing 150300 g are fattened over 158day periods. Chopped trash fish is given at 10 percent bodyweight as feed twice daily. Crab lift nets are used to harvest the mud crab.In Basilan Province mud crab for fattening are penned underneath the homes of the Muslimfisherfolk. These houses are often constructed on stilts and the space underneath is fenced fromtop to bottom with chicken wire and discarded netting. There is an opening in the floor of thehouse through which trash fish. kitchen refuse and fruit peelings are dropped as feed. When thecrab have attained the desired weight they are harvested.

c. Mud crab fattening in bamboo cagesMud crab fattening in bamboo cages is one of the technology verification studies tried out by Joeyand Sylvia de la Cruz in Barangay Napapao, Ponteverdra Capiz. This project was conducted toprovide a standard culture method for fattening crab:

1. Site SelectionMud crab grow best in brackishwater, such as tidal flats, estuarine areas, bays and lagoons.Sheltered bays and coves are selected to protect the bamboo cages from strong winds andwaves during adverse weather conditions. The water at such sites should be 0.5lm deep.Areas with low salinities should be preferred, as saline water inhibits the growth of mudcrab. Areas with sufficient crab for fattening as well as trash fish for feed should beconsidered. The area should also be accessible to the growers and target markets.

2. Cage DesignA modified bamboo cage (140 x 70 x 25 cm) subdivided into 18 compartments is fixed firmlyby its comers to the substratum to prevent it from being washed away during inclementweather. The compartments are covered with 140 x 70 cm split bamboo. Holes are providedin the compartment covers for feeding.One advantage of using bamboo cages is that selective harvesting can be done. If thedesired weight has not been attained, the crab could easily be returned to theircompartments and fattenedfurther.

3. StockingAbout 18 crab can be stocked per unit. Stocking is done during the early morning or late inthe afternoon. In Capiz, 185 crab, each of average weight 175 g. were stocked. The weightincrease after 15 days was I 10 g.

4. Feed and FeedingMud crab are fed twice a day at 5 per cent bodyweight for 1015 days. Feeds may be trashfish, softshelled snails, kitchen leftovers, mussel meat, animal entrails or almost any otherkind of food.

5. Cage MaintenancePeriodic checks should be made during the culture period. Drifting seaweed, logs and otherdebris should be removed to facilitate easy circulation of water and prevent damage to thecages. After use, the crab cages should be lifted periodically and dried.

6. Harvest and HandlingAfter the fattening period, mud crab can be harvested individually by hand. The crab arethen bound with straw or string to enable easy handling. A skilled laborer is hired to bindthe pincers of the crab. Exposure of the crab to sun and wind should be avoided, as this maylead to weakening and eventual death.

Reference:Entrepreneurs Guide to Mud crab Fattening. National Institute of Ocean Technology (NIOT). India.For more information contact:1. SEAFDEC Aquaculture Department. Tigbauan, Iloilo, Philippines.

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Email: [email protected]

2. BFARFIDSD. 2/F PCA Annex Bldg., Diliman, Quezon City. Email: [email protected] Web: http://www.bfar.gov.ph

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