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Mud Crab Farming in Asia

From “Aquaculture: Farming Aquatic Animals and Plants”

 

The second edition of Aquaculture: Farming Aquatic Animals and Plants contains and excellent chapter titled "Other Decapod Crustaceans" that details the farming of mud crabs (Scylla spp).

Before I tell you about the section on mud crab farming, written by Chaoshu Zeng (Australia), Yongxu Cheng (China), John S. Lucas (Australia) and Paul C. Southgate (Australia), I would like to tell you a little bit about the comprehensive book on aquaculture where it appears. Edited by John S. Lucas and Paul C. Southgate and published in January 2012, Aquaculture: Farming Aquatic Animals and Plants is a softcover book with 26 chapters, 648 pages, a 13-page index and a long list of references at the end of every chapter. Black and white pictures and tables populate almost every page. The book begins with chapters on the general aspects of aquaculture like water quality, genetics, disease, nutrition, feeds, disease, harvesting, processing and marketing and ends with chapters on all the economically important aquaculture species like carp, salmon, trout, tilapia, catfish, marine fish, shrimp and mollusks. It even has chapters on seaweed and microalgae culture, turtle farming and ornamental fish farming. It concludes with predictions on the future of aquaculture.

It can be purchased at Wiley-Blackwell for $129. Here's what Wiley-Blackwell's webpage says about the book:

Since the first edition of this book was published in August 2003, the global aquaculture industry has rapidly expanded and made huge technological advances. The output from world aquaculture, a multi-billion dollar global industry, continues to rise at a very rapid rate, and it is now acknowledged that it will soon over take fisheries to become the main source of fish and shellfish globally.

The book is recommended as a text for students and as a concise reference for those working in or entering the industry. Providing core scientific and commercially useful information and written by around 30 internationally-known and respected authors, the expanded and fully updated second edition of Aquaculture provides essential reading for all students and professionals studying and working in aquaculture. Fish farmers, hatchery managers and suppliers to the industry will find useful background information and a great deal of commercially important data in the book.

 

 

Mud Crabs

 

Mud crabs, also known as mangrove crabs, occur widely in estuaries and along tropical, subtropical and warm temperate coasts from eastern Africa to the southern tip of Japan and northeast Australia.  They also occur in many Pacific island nations, such as Papua New Guinea, Fiji, Samoa and Tonga.  Large with a delicate flavor, they are highly sought after and command high market prices wherever they occur.  In the recent past there was considerable confusion about their taxonomy, but it is now widely accepted that there are four species of mud crabs: Scylla serrata, S. olivacea, S. tranquebarica and S. paramamosain, with different species occurring at the same location in many countries, including the Philippines and Indonesia.  Among the four mud crab species, the largest and the most broadly distributed is S. serrata.  It appears that S. paramamosain is the most important species in aquaculture because it is the dominant species in both China and Vietnam, where mud crab farming is most successful.

 

 

During the latter half of the twentieth century, increased fishing efforts driven by market demands and destruction of mangrove habitats led to the decline of mud crabs in many Asian countries.  There are only a few exceptions, like Australia, where the mud crab fishery is strictly regulated and the wild stocks have remained relatively healthy.  Increased market demand and price, coupled with dwindling wild stocks, have generated substantial interest in mud crab aquaculture in many countries.  Although the mud crabs have been farmed in southern China for more than 100 years and in other Southeast Asian countries for a few decades, the rapid expansion of the industry has occurred since the turn of the century.  Further expansion of the industry is highly dependent on whether wild caught crab seed can be replaced by hatchery produced juveniles.

 

Production Status: With annual aquaculture production reportedly reaching 106,000 metric tons in 2003, China farms more mud crabs than any other country.  The actual total production could be higher because mud crab farming is traditionally a small-scale, family-run business; hence production from some small farms may not be included in the official statistics.  Because mud crabs like warm temperatures, most of the production is in southern China.  Other Asian countries, like Vietnam, the Philippines, India and Indonesia, also produce sizeable amounts of farmed mud crabs, and their industries are growing rapidly.  For example, Vietnam’s farmed mud crab production tripled from l0,000 tons in 2004 to an estimated 30,000 tons in 2009.

 

Biology: Mud crabs inhabit shallow coasts and estuaries and favor soft-mud or sandy-mud bottoms.  They are also commonly associated with mangrove forests, hence their other common name, “mangrove crab”.  Mud crabs often burrow in the substrate or hide under shelter during the daytime and forage at night.  They are largely omnivorous scavengers with diets including a wide range of animal and plant materials, including mollusks, small crustaceans, polychaetes and detritus.  Mud crabs have broad tolerance to salinity and venture up estuaries and streams to forage.  S. serrata in particular possesses strong osmoregulatory ability and has been reported to survive well in brackish water at salinities as low as 3-5 parts per thousand.  It is also found in full-strength seawater at salinities of 34 parts per thousand and higher.

 

Under captive conditions, mud crabs can reach sexual maturity in one year.  The carapace width at sexual maturity is generally between 9 and 14 centimeters, depending on species.  Before copulation, male and female crabs often pair up for several days and copulation takes place soon after the female crab undergoes a molt and is still in the soft-shell condition.  During copulation, the male crab transfers spermatophores to the seminal receptacles of the female, where they remain viable for a long time and are used for fertilizing eggs during subsequent spawnings, or until the female’s next molt.  After copulation, the newly molted female crab often remains under the protection of the male for several days until her new shell has become hard.  The ovaries of female crabs start to develop after copulation and may take several months to ripen.  In nature, female S. serrata migrate offshore to spawn, which largely explains why egg-carrying females (berried females) are rarely found in estuaries and near-shore waters.  Mud crabs are highly fecund, producing one million to eight million eggs per spawning, depending on the species and size of the female.  After spawning, females attach the eggs to their pleopods and care for them until hatching, which can take ten days to more than a month, depending on water temperature.

 

Larval Development: The pattern of larval development is like that of the mitten crab.  The eggs hatch into zoea, small planktonic larvae, and progress through five zoeal stages (zoea I-V) and then metamorphose into megalopae, which swim with the plankton and resemble adult crabs.

 

Under optimal culture conditions, zoeal stages typically take three to five days each, whereas the megalopal stage lasts seven to ten days.  It is believed that megalopae migrate from offshore to coasts and estuaries and then undergo a second metamorphosis and settle on the bottom as first-stage crabs.

 

Wild-Caught Seedstock: Despite substantial efforts over recent decades to develop hatchery techniques for mud crabs, most farms rely on wild caught juveniles for stocking, although commercially viable hatchery operations have recently been reported in Vietnam.

 

Mud crab juveniles, often between ten and a hundred grams, are collected with traps, nets and bare hands.  Because mud crab juveniles are not normally densely distributed, the collection of wild mud crab “seed” is often very tedious and inefficient.  For example, it was reported that in Vietnam, using scoop nets in mangroves, on average only 100-200 juvenile mud crabs could be collected per person per day.

 

In recent years, in southern China as well as in Taiwan, collection methods for wild, late-stage megalopae have been developed to collect megalopae as they move from the ocean into the estuaries and rivers.  These methods allow the collection of large quantities of wild megalopae, often in the millions, in a short period, as they swarm for their inland migration.  Such large-scale removal, however, will undoubtedly impact natural populations and fisheries and, in the long term, it is probably an unsustainable practice.  The megalopae usually molt to first stage crabs in two to four days after capture and are then further grown in nursery ponds for one month or more before being sold to farmers.

 

The reliance on wild seed for mud crab farming is a major bottleneck for the industry.  It is clear that even at the current harvest rate in most Asian countries, quantities of wild-caught seed are not sufficient to meet demand.  Furthermore, collected juveniles are often not uniform in size and consist of mixed species.  Most importantly, there is evidence that wild juvenile mud crabs are already over-exploited in many places, which will inevitably impact natural populations and local fisheries.

 

Hatchery Production: Over the past few years, more than 250 commercial mud crab hatcheries (mainly for S. paramamosain) have been established in Vietnam.  These hatcheries provide crablets (early stage juvenile crabs), accounting for one-third of the demand.  In other Asian countries, like the Philippines and China, hatchery production of crablets is also increasing, albeit still at a small scale, and this is often done by research institutes and universities subsidized by government funding.

 

Broodstock: Although wild, female mud crabs with advanced ovarian development migrate offshore to spawn, spawning can be readily achieved under captive conditions.  The natural breeding season in subtropical and warm temperature regions occurs from late spring and into the summer.  By manipulating water temperature and eyestalk ablation, mud crabs can be induced to spawn year-round.  Eyestalk ablation may also be used to enhance ovarian maturation and spawning during the breeding season.  Although re-maturation of spent females and captive breeding of animals produced in hatcheries have been achieved experimentally, commercial hatcheries normally do not try to bring spent crabs back to gonad maturity.  It’s just more efficient and economical to start with newly caught crabs with fully developed ovaries.  Hence, after they spawn, broodstock crabs are discarded and a new batch of mature females is introduced for subsequent spawnings.  Because female mud crabs with advanced ovarian development have normally already mated, no males are needed.  Healthy, female broodstock are fed bivalves, squid and other crustaceans (shrimp) daily until they spawn.  Females attach the eggs to their pleopods, and farmers add habitats/shelters to protect the females from cannibalism.

 

Newly extruded eggs are bright yellow to orange, and they gradually turn grey and then dark grey when they are about to hatch.  After spawning, berried females can either be immediately transferred to separate tanks for hatching or the transfer may occur after eggs turn dark.  Berried females do not need to be fed during egg incubation.  The hatching time can be accurately predicted with regular monitoring of embryonic development.  Hatching normally occurs in the early morning, and high fertilization and hatching rates can often be achieved under captive conditions.

 

Larval Culture: Larval culture of mud crabs is usually performed indoors in either concrete ponds or large circular tanks.  The size of the culture ponds or tanks varies substantially, depending on the hatchery.  With slight modifications, marine shrimp hatcheries can be used for mud crab larval culture.

 

The optimal culture temperature for mud crab larvae is between 28 and 30 °C.  These warm temperatures substantially reduce the culture period and improve survival.  However, controversy exists over the optimal salinity for early zoea larvae: some researchers think salinities of about 25 parts per thousand are best, while others argue for salinities of 33 ppt (full strength seawater).  The differences may reflect the salinity preferences of different species.  Despite different views on the optimal salinity for culturing zoeal larvae, it is generally agreed that from megalopae onwards, salinity should be gradually reduced.  Additionally, the background color of larval culture tanks has been shown to affect larval survival and development, with darker colored backgrounds producing better results.

 

The “green water” method is often used for larval culture, particularly during the early zoeal stages when rotifers are used as the main food.  From late zoea-II to zoea-III, as their foraging ability increases, live Artemia nauplii are added to their feed because rotifers are an inadequate food for the later-stage larvae.  In Vietnamese hatcheries, it has also been reported that the umbrella stage of Artemia (the first few hours after hatching when the embryo hangs beneath the cyst shell) is used for feeding early larvae; therefore, eliminating the need to use rotifers for larval culture.

 

Problems in Mud Crab Hatcheries: Although successful culture of mud crab larvae on a large scale has been reported in various countries, including Japan, China, Vietnam and the Philippines, the main problem with hatcheries is their inconsistency and the low survival rates of the larvae, often attributed to the following issues:

 

• Significant variation in the quality of newly hatched larvae from different females.

• High mortality rates often occur at the zoea-I stage if antibiotics are not used (a problem

in countries like Australia, where the use of antibiotics is strictly controlled).

• Mass mortality routinely occurs around the time zoea-V metamorphose to megalopae

(often linked to “molting death syndrome”) and again during the megalopal stage.

• Cannibalism at megalopal and early juvenile stages.

• Species-specific larval culture conditions.  For example there is evidence suggesting

that larval culture of S. serrata is more difficult than S. paramamosain.

Feeds that don’t satisfy the larvae’s nutritional requirement.

 

Despite these problems, it is very encouraging to see that in Japan, where mud crabs are raised to increase wild stocks, and more recently in Vietnam, where the operations are commercially viable, consistent hatchery production has been achieved.

 

Nursery and Growout: A nursery phase, a relatively new development in crab farming, is often used to grow wild-caught megalopae and hatchery-produced juveniles to sizes that are acceptable for stocking ponds.  These systems include shallow concrete ponds, earthen ponds, hapa nets, cages and mangrove pens.  The initial stocking density for nurseries is generally less than 100 megalopae crablets per square meter.  During nursery culture, substrata and shelter, such as artificial seaweeds, are normally provided to reduce cannibalism.  The feeds used in mud crab nurseries are again highly diverse, including minced trash fish and shrimp tails, small bivalves and gastropods and commercial shrimp feeds.  In southern China, small enterprises specializing in the collection and production of small bivalves and gastropods, which are unsuitable for human consumption, but ideal for mud crab juveniles, supply nurseries.

 

Growout: Growout of megalopae and crablets to marketable sizes can be done in monoculture or polyculture enclosures.  For the latter, mud crabs have been cultured with a variety of species, including seaweeds, marine shrimp (black tiger prawn, Penaeus monodon) and marine fish (milkfish Chanos chanos).  In Taiwan, polyculture of mud crabs with tiger prawns is practiced in ponds with stocking densities of 10,000 to 16,000 crablets and 150,000 to 250,000 tiger shrimp postlarvae in a pond of about 0.5 hectare.  In Vietnam, mud crabs are stocked in mangrove farms for extensive culture at very low densities of approximately 500 crabs per hectare.

 

Fattening: Lean and newly molted “empty” crabs are often fattened for a couple of months so that they put on meat weight before they are marketed, and female adult crabs with undeveloped or spent ovaries are fattened to produce crabs with ripe ovaries.  Crabs with ripe ovaries fetch much higher prices.  Fattening is usually done in ponds, mangrove pens and bamboo cages at relatively high densities, but sometimes it’s done with individual crabs in small baskets.  With relatively short culture periods and no molting, fattening survival rates range from 70-85%.

 

Soft-Shell Production: Soft-shell crab production has been conducted in several southeast Asian countries, particularly Thailand.  Through regularly monitoring (every three to four hours), any crabs undergoing molting are discovered and harvested within a few hours of their molt, so their shells do not have time to harden.  Because soft-shell crabs can be eaten whole, they fetch a high price and are often exported to lucrative overseas markets.  The production of soft-shell crabs is done by either holding individual crabs in perforated plastic boxes suspended on the water surface or stocking them communally in big cages.  In communal holdings, their claws are often removed to prevent cannibalism.  Soft-shell crabs (60 and 130 grams) are often wild-sourced.

 

Growout Systems: The most common culture systems for mud crab growout are pond culture and mangrove culture.  In China, pond culture is exclusively used, while both pond and mangrove culture are practiced in other southeast Asian countries.  Mangrove culture has been dubbed “mangrove-friendly” aquaculture.

 

Earthen ponds are commonly used in pond culture.  With simple modifications, existing marine shrimp ponds can be utilized for mud crab growout.  The size of the ponds are generally between 0.3 and 0.5 hectare with a water depth between 0.8 and 1.5 meters.  Various structures, most often simple net fences, but sometimes also concrete or lined pond banks, are constructed to prevent escapes.

 

Stocking densities between 2.5 and 5 crabs per square meter are most common for pond culture because higher densities are likely to lead to cannibalism.  Various feeds, like trash fish, animal offal, cheap mollusks and sometimes formulated feeds for marine shrimp, are used to feed mud crabs.  Feeding rates are generally between 3 and 10% of biomass, with a decreasing feeding ratio as they mature.  The culture period for growout from juvenile to market size is generally five to eight months, and survivals of 30-70% are commonly achieved.

 

There are two types of growout in mangroves: extensive culture in mangrove ponds and semi-intensive culture in enclosures.  In the former, the mud crabs are often stocked at very low densities of around 0.05 crabs per square meter.  The crabs are normally not fed after stocking because they feed on naturally occurring food in the mangroves.  With low culture density and abundance of natural food, both survival and growth rates of mud crabs are often high, although there is a low overall yield.

 

The more commonly used system is semi-intensive enclosures, often constructed in intertidal zones using easily harvested bamboo.  The stocking density is generally between one and five crabs per square meter.  Although crabs can utilize natural diets, supplementary feeds such as trash fish are also supplied daily at a rate of approximately 5% of cultured biomass.  The survival rate for mangrove enclosure culture is generally between 50 and 90% from juveniles.

 

Markets: Mud crabs are popular throughout the Indo-Pacific region, particularly in countries where they occur naturally.  They remain alive out of water for a considerable periods (more than 1 week when kept moist) after capture and are traditionally marketed alive.  However, they are also sold as a cooked product in Australia or frozen for the newly emerging soft-shell crab market.  Although the price may vary substantially from country to country, mud crabs are generally a high-priced seafood item in local markets.  The size and sex of the crabs may affect the market price: for example, female crabs with well-developed ovaries may fetch prices several times higher than those for male crabs in ethnic Chinese markets.

 

Also see Shrimp News International’s review of the FAO Mud Crab Farming Manual and get a copy of it by clicking on the link in the Source below.

 

Sources: 1. Aquaculture (Farming Aquatic Animals and Plants, Second Edition).  Editors: John S. Lucas and Paul C. Southgate.  Chapter 22: Other Decapod Crustaceans.  Page 514.  Chaoshu Zeng, Yongxu Cheng, John S. Lucas and Paul C. Southgate.  Wiley-Blackwell.  2012.  2. Pictures. Mud Crab Aquaculture/A Practical Manual. Food and Agriculture Organization of the United Nations (FAO). A Free PDF Download. Colin Shelley (FAO Consultant, Australia) and Alessandro Lovatelli. Fisheries and Aquaculture Technical Paper No. 567. ISBN 978=92-5-106990-5. 2011. 3. Bob Rosenberry, Shrimp News International, November 29, 2012.

 

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