The traditional trade in mud crab has been for hard-shelled crab products, namely meat crab and egg crab. However, in recent years the market for mud crab products has extended to soft-shell crab and processed crab meat.
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1. Fattening
This is the holding of fished crabs that have recently molted (also known as thin or "watery" crabs) for 10 - 30 days until meat weight has increased.
2. Grow-out
This is the process of stocking juvenile crabs (10 g to 100 g) and allowing them to molt and grow. Harvest is after 3-8 months or once the crabs reached 200 g to 500 g size.
Mud crab fattening is the most suitable for small-scale aquaculture because:
In addition, crabs can be traded in small numbers as mud crabs are valuable as individual animals. This means that farmers can hold small numbers of mud crab at a moderate stocking density and still make a reasonable profit. Meat crabs can be farmed in ponds, cages and pens both using the fattening or the grow-out systems of stocking and harvesting.
Seed supply
At present, hatchery-produced mud crab seeds are not widely available. Therefore all crabs used to stock the mud crab systems are caught by the systems’ operators themselves and/or are bought from local fishermen.
Feed
Mud crabs are carnivorous and scavengers. In all cases, the mud crabs are fed with locally caught fish species, which have a low market value. These low value fish species are harvested by the mud crab farmer themselves or are bought locally from other fishermen.
Pond culture
Earthen ponds are most commonly used in small scale culture of mud crab. Small ponds are built in the homestead (200 sq m or less). Farmers usually build a fence around the perimeter of the pond to prevent crabs from escaping to keep away predators. The fence can be made of a range of materials, e.g., roofing tiles (Vietnam), concrete slabs (Sri Lanka), polyethylene sheets (Philippines) or mesh (Vietnam/Thailand). A simple sluice gate is used to allow water exchange. The method of stocking, feeding and harvesting as well as the level of pond maintenance varies widely from farmer to farmer. The example below is of a very simple system observed in Surat Thani province, Thailand
Cage culture
Cage culture of mud crab makes use of estuaries, lagoons and brackishwater ponds. Floating cages, made of bamboo or polyethylene/galvanized wire net, are mainly used for mud crab fattening. The cage culture of mud crabs has been used in the Philippines and Indonesia. These cages can include those that are divided into compartments for individual crabs, thus reducing the mortalities due to aggression and cannibalism. The cages are floated using plastic floats and empty oil barrels to keep the upper side at surface level. A mesh screen is needed on top of the cages to prevent the crabs from escaping. Cages will allow individuals who do not have access to land to farm mud crab.
Pen culture
Pen culture has been used for grow-out of mud crab in the Philippines and
Sarawak, East Malaysia. This method integrates the system of crab rearing with
the coastal mangrove forests, which does not involve the clearing of mangroves
to construct ponds. Instead, the mangrove forest is penned-off and stocked with
crabs. The pens can be made of plastic netting supported with a bamboo network
(as in the Philippines) or made of wood (as in Sarawak). They rely on tidal
water exchange and supplemental feeding. Pens for culturing mud crabs may also
be constructed within ponds and shallow rivers.
The mud crab has been shown to be a good polyculture species, particularly with finfish species (milkfish and Tilapia) and seaweed (Gracilaria). An example of this is in the Thai Binh Province, North Vietnam. Mud crabs also have the potential to be farmed in coastal rice fields during the dry season.
1. Mud crab is a high-value species and its culture can significantly increase the income of rural coastal communities.
2. The technology required to culture crab is relatively uncomplicated (this refers mainly to ponds and pens) and can be easily disseminated to farmers.
3. Previously abandoned shrimp pond areas can be conv
erted for crab culture with relatively little additional conversion costs, e.g., fencing to prevent the crabs escaping from the ponds.
4. The mud crab has an international market (e.g., the restaurant trade in Singapore, Hong Kong, Bangkok and Taiwan) therefore the culture generates much needed foreign exchange.
5. Unlike exotic species, the mud crab is indigenous to the Indo-West-Pacific, therefore its market trading system is already well established within the region from fished mudcrab products. The transition to also include farmed crab will be relatively easy.
6. Transportation of mud crab is very simple. Individually tied mud crabs are
transported in damp conditions in baskets from the pond to the market place.
Moreover, crabs can be transported and stored in this way for three to
four days.
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Rice/crab farming in Tra Vinh Province, Vietnam Both crab growout and crab fattening are practiced in rice fields in Tra Vinh Province. In both cases, minor modifications are made to the rice fields, namely installing a mesh net around the perimeter or the area that is being used to farm crabs and deepening the waterways. The grow-out system is an extensive system where the crabs are stocked and harvested but there is no supplemental feeding involved. Fattening requires feeding to improve the weight of the crabs in a short period of time; low-cost fish species are used to feed the crabs. The grow-out systems proved to be the more profitable as smaller crabs, which were cheaper, were used to stock the ponds and there were no feed costs. However, survival was low in the extensive system, mainly due to cannibalism. Both systems are family run, extra labor only being employed during the harvest season.
The species of mudcrab found is S. paramamosain. It does not burrow, therefore, does not affect pond infrastructure. |
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7. Mud crab farming also involves other sectors of the community who do not have the capital or the land/water access to start crab farming themselves. These people may be involved in collecting food items or crab seed, providing labor for pond maintenance and guarding and harvesting of ponds, as well as transport of harvested crab to the marketplace.
8. The link between the mud crab and the mangrove provides an incentive to preserve the mangrove resource.
1. A fundamental pre-condition for the future sustainability and expansion of the mud crab industry is a reliable seed supply, particularly in soft-shell crab farming, which has a high turnover of juvenile crabs (~ 80g each). Ongoing research into hatchery production of mud crab is starting to produce reasonable survival rates of crab seed. However it is still not on a scale that can be deemed economically viable. Thus, the expansion of mud crab farming is constrained by the availability of seed supply from the wild.
2. Crabs can suffer high mortalities from diseases similar to those that affected shrimps.
3. Although pen and cage cultures have been shown to be suitable for mud crab production, the majority of mud crab is cultured in ponds. This means that any expansion in mud crab farming is going to be influenced by the same issues that faced the shrimp industry, including competition for coastal sites for ponds and pollution from over development.
4. The high value of mud crab makes it very attractive to richer sectors of the community and in some cases outside investors. This can marginalize the poorer households and prompt them to use less suitable locations to farm crab.
5. There may be competitition for locally caught low-value fish species that could be protein sources for the poor members of the community. This potential conflict needs to be outlined and efforts are needed to find alternative sources of crab food.
Mud crab culture is suitable to small-scale farmers as hardy and high value crabs can improve the income of the household even when small numbers are farmed in very simple culture systems. Soon hatchery-produced seed will be available to farmers, substantially reducing fishing pressure on wild crab stocks.
Keenam, C. P. and Blackshaw, A. 1999. Mud crab aquaculture and biology. Proceedings of an international scientific forum held in Darwin, Australia. April 21-24, 1997. ACIAR proceedings No. 78. 216 p.
Prepared by:
Julia Lynne Overton and R. K. U. D. Pushpakumara
Mud crab farming has traditionally been a small scale activity, originating
in China over 100 years ago and spreading across Asia over the last 40 years.
Its close affinity to coastal mangrove areas within the Indo-West Pacific limits
mud crab fishery to coastal, artisanal style fishing. It was, therefore, natural
for fishermen to improve the value of their catch by retaining both underweight
and juvenile crabs for rearing purposes. As a family level occupation, mud crab
farming proved to be quite profitable, provided there was a sustainable seed
supply from the local fishery.
Meat crabs are the only mud crab product most applicable to small-scale
aquaculture. The other products require high investment both in terms of
technology and food supply. Also, there are environmental considerations with
soft-shell and egg crab farming, as they tend to exploit parts of the natural
crab population that are most sensitive to overexploitation by requiring a high
turnover of juvenile and pre-spawned females, respectively. For these reasons,
the rest of the paper will concentrate on production of meat crab.
This
system is profitable as long as the salinity levels are kept low to
ensure a good rice crop. It also helps farmers save money by avoiding
the use of pesticides, which are detrimental to the health of the mud
crabs.
