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Ecological Crab and Clam Farming in China

By Li Kangmin                          2015-12-28


To develop mariculture, the first thing we should do is to put mariculture under the guidance of the Tao. The people stand on the holistic view to develop fisheries (capture and culture). It emphasizes that we should farm fish, shrimp, mollusks (including crab) and algae between heaven and earth. All measures should be taken to fit time, space and the characteristics of aquatic species. That is to put mariculture between heaven and earth to imitate nature [1]. We have to study the bio and eco characteristics of sea crabs. Two kinds of crabs can be raised: one is mangrove crab Scylla serrata and the other swimming crab Portunustrituberculatus.

Mangrove crab: Distributed in Indo- West Pacific tropical and subtropical waters, including China's southeast coast, Japan, Vietnam, Thailand, the Philippines, Indonesia, Hawaii, Australia, NewZealand and southeastern Africa and the Red Sea. Living habit: Burrowing during the day, prowling at night, crabs are nocturnal freely, especially more active in the high tide of the night, Feeding habit: Farmed crab, no strict selection of bait, small trash fish, shrimp, small shellfish (blue clams, river clams, snails and other snail), soybean meal, peanut cakecan also be for food, crabs have habit to kill each other, often prey soft-shell crab after molt. Ecological habit: Mangrove crab adapt to salinity 6.5 ‰ ~ 33 ‰, the optimum salinity 12.8 ‰ ~ 26.2 ‰. But it is difficult to adapt to dramatic changes in salinity, which will cause two diseases: red salt and white salt spots on the shell, even death.

Swimming crab: Distributed in Japan, Korea, Malaysia, the Red Sea and China coastal provinces. Generally from north to south, from March to May and September to October are the production seasons, monthly production more in April and May in Bohai Bay and Liaodong Peninsula China. Ecological habit: salinity16-35‰, the optimum salinity between 26-32‰, the water temperatureat 4℃-34℃, the optimum temperature at 22℃-28℃, pH value between 7-9.

Sanmen County is located in the middle of Golden coastal line, which is about 227 km long, with a total area of 481.7 square km, so called “Golden and Silver Beaches”. In recent three years mariculture developed rapidly in a large scale of crab, shrimp, razor clam, blood cockle and clam. The first three products are well-known in country. However, Extensive management caused the environmental pollution and eutrophication of near sea, which is getting worse. In order to build solid ecological barrier for Blue Ocean, Sanmen adopted three measures: Pond responsibility, rotational farming and intensive eco mariculture to deal with pollution crux in mariculture to promote the transition from high polluting extensive management to eco-friendly intensive management. According to the statistics the new technologies of mariculture have been popularized by 40%, the density of per cubic meter farming raised by 30% while the discharge of waste water reduced by 60%. The detection of 18 effluent monitoring sites indicated to reach the discharging standards [2].   

Responsibility system in mariculture pond

One of three measures is to appoint an official of County Fishery Bureau to play the chief role who is responsible to manage Grade 1 culture pond; a technician to be responsible for Grade 2 pond and a general worker to be responsible for Grade 3 pond. They have to go around all ponds to look at and control water quality and give the guidance or provide service for all those culture ponds. Here is a case: when algae bloom often happens in summer and causes anoxia in waterbody due to lack of dissolved oxygen, the science and tech personnel from the Fishery Bureau proposed to culture rabbit fish Siganus fuscessens which feed on algae, leftover of feed, organic residues to control the biomass of algae effectively. Rabbit fish grow fast and can be harvested earlier. The fish itself is delicious and will bring some income as well [3].

Rotation of musk melonplantation and shrimp culture

Fuyang mariculture cooperative farm has 15 ha of pond for mariculture, where the farmers planted musk melon the first by using greenhouses within ponds in early spring. Farmers do not need to apply fertilizers. After they harvest musk melon, sea water will be introduced into the 6.7 ha pond getting ready to culture white shrimp Penaeus vannamei. This is the rotational crops between musk melon plantation and shrimp culture [4].

Polyculture of mangrove crab, razorclam and blood cockle

       Polyculture is one point of integrated fish farming features, which has been practiced in thousand years in China [5]. Integrated fish farming is often defined to be pond fish culture in freshwater [6] [7]. In fact integrated fish farming is one kind of integrated farming system in China [8]. It is not only in pond fish culture but also extended to lake fish enhancement, such as pen culture of fish or freshwater crab culture In Lakes where fish and crab are fed by tender floating and submerged aquatic plants, even under the coverage of aquatic plants in pen enclosure and thus to reduce eutrophication of the lake and even the water quality of effluents from the outlet of a pen better than the inlet. In recent decades integrated fish farming has been extended to mariculture [8] [9].

Sanmen bay has mountains in its three sides and actually is a semi-closed gulf. This area is just located in a place saline water and freshwater meet with a moderate salinity and variety of seafood, among which the most famous product is Sanmen mud crab, also called serrated black crab for its shell having serrate edge and with black color of their shell. Sanmen mud crab run amuck the world with two giant chelae. Sanmen mud crab is well known in seafood world. The shell of crab is rather thinner, its ripen gonad yellow, its meat tender, and its taste delicious. Look at these two strong chelae bigger than any other crab and they can clamp small white shrimp, cockle and razor clam into pieces. See as follows:

Crab integrated with razor clam and blood cockle

Interest in mangrove crab farming has been high due to the high demand/price for them, high flesh content and rapid growth rates in captivity. In addition, they have a high tolerance to both nitrate and ammonia (twice that of the similar sized Blue swimming crab Portunus pelagicus). Mangrove crabs are fond of eating both razor clam and blood cockle. Razor clamis a common name of Siliqua patula for long, narrow, saltwater clams (which resemble a closed straight razor in shape) and blood cockle Anadara granosa (also known as Malaysian cockle Tegillarca granosa) is a species of ark clam known as the blood cockle or blood clam due to the red haemoglobin liquid inside. It has a high economic value as food, and so it is kept in mariculture. It lives mainly in the intertidal zone at one to two meters water depth, burrowed down into sand or mud. Adult size is about 5 to 6 cm long and 4to 5 cm wide. It is served steamed, boiled, roasted, or traditionally raw for human consumption.




Razor clam farming

Sanmen County is the largest mariculture county in Zhejiang, with a total area of 13,500ha. The county has a total area of (4,667ha) razor clam farming, with annual output of 45,000 t more, annual output value about 540 million¥. In 2011, Sanmen got protected geographical origin certificate TM of razor clam, farmers in Sanmen County have been committed over the years to non-harm razor clam farming techniques, adopting razor clam farming in mud flats and pond farming. Razor clam farming industry has become the major opportunity to get rich for farmers in coastal area.

Harvesting razor clamseeds for farming in Dec.

Blood cocklefarming

Blood cockle Anadara granosa is a species of ark clam known as blood clam due to the red haemoglobin liquid inside. It is found throughout the Indo-Pacific region from the eastern coast of South Africa northwards and eastwards to Southeast Asia, Australia, Polynesia, and up to northern Japan. It lives mainly in the intertidal zone at one to two meters water depth, burrowed down into sand or mud. Adult size is about 5 to 6 cm long and 4 to 5 cm wide. The blood cockle is the main clam variety raised in the mud flats of Anhai Bay off Shuitou, Fujian. It has a high economic value as food, and so it is kept in mariculture. Just on the coast of Zhejiang Province alone, blood cockle culture occupies around 1,000 ha, These clams are also raised in the river estuaries of the neighboring Fujian Province as well [12].


Why do they all those marine organisms together in the same pond? The unit production from mud crab monoculture can get 6000¥while the polyculture of three species may reach to more than 10,000¥nearly double monoculture. In Sanmen County the polyculture model occupied 90% about 4,667 ha with better economic benefit. How do they keep them co-exist in the same pond? Farmers use two kinds of net to keep three organisms in co-existence in different locations and different layers of the pond. They put a purse net to raise blood cockle in the middle and to raise razor clam in the periphery of the purse net in the deepest layer under a net which protects razor clam from crabs. Small shrimp swim quicker than mud crab which cannot catch shrimp but exercise their muscle when they pursue shrimp. They practice blood clam culture in the center with a round screen in the center of the mud crab pond. They harvest small wild fish shrimp and clams as part of feeds for mud crabs by placing a bottom cylinder dragon net.

In the early morning about 0600 they feed crab with small white fish, and shrimp red or white. They are feeding 200 kilo of feed for 2 ha of crab pond. If one kilo costs 2 Yuan, the cost is 400¥. You do not need to increase the feed amount for small shrimp, razor clam and blood cockle; they do not eat crab feed. Mud crabs are the top consumers.The leftover of crab’s feed is the food for small shrimp. Razor clam and blood cockledwell in mud feed on algae and detritus. Crabs weighed 200g-300g usually are new raised this year, the old crab weighed 500g raised since last year. One mud crab weighs 0.5 kilo after one year fattening. The output value of mud crabs is 80 Yuan for 0.5 kilo and 50 Yuan for grown-up this year. 

Hele crab in Wanning Hainan

Mr. Zhuo Qihui was born and bred in Wuyang village Wancheng Town Wanning City. He went to Beijing in 2007 to learn in a university and in 2009 he returned home in winter vacation hoping to bring some Hele crabs (local name of mangrove crab) for his classmates, but he found that Hele crabs was getting less and less in the bay. It is on the brink of extinction. He felt it was a terrible thing. He decided to return to his hometown giving up million earning from export, merely to save the crab. In 2012 he discussed with his alumna and they unanimously agreed to do something and then they decided to rent 12 ponds about 40 mu (2.6 ha) by 80,000¥ for trial to farm Hele crabs. They could learn the theoretical knowledge from the books but lack of experience. They hired people to collect juvenile crabs and stock them into their ponds. They cruise ponds every day and night feeding the crabs. His farm site is near a small sea, which is nearly a closed bay, along Wanning County. Hele crab grow here as one of four famous dishes in Hainan. When typhoon is coming he would hurry back to his site to cruise the ponds, which are fenced round with a plastic sheet wall in order to avoid escaping when the wind blows. If the weather changes crabs often escape. Once a crab escaped, farmers will lose 100¥each.

Hele crab is one kind of serrated mud crab Scylla serrata. As one of four famous dishes in Hainan it sells by high price in the market. However its farming depends on collecting post larva from nature. Just a few people tried to do this and often failed. Somebody stocked juvenile crabs and the yield was only dozens kilo of crabs, having limited profit. In July 2012 Zhuo and group signed a contract to rent 40 mu of ponds to stock juvenile crabs into their ponds annually and it costs about 40,000 to 50, 000¥.

Prof. GU Zhifeng from Hainan Ocean College came to the site and gave them some guidance first to reduce the stocking density and do not blindly put so called water quality improver and so on. A mariculture pond is an eco system in nature. Mud crabs are fed by feeds and their leftover in turn can be fed by fish, shrimp or clams and the remaining nutrients can be absorbed by aquatic plants so that the water quality can be kept clean, no need to apply drugs or conditioning chemicals. Zhuo accepted the advice and reduced the stocking density from 5000 crabs/mu to 2000 crabs, even to 1500 crabs per unit area. In the mean time Zhuo stocked some fingerlings of various fish species and some juvenile shrimp, and even raise some ducks along the side of the pond to imitate eco mariculture model. He said duck would give us a warning: if the pond is anoxia, fish will gasp on the surface and the duck looks at and eat it. It shows that the pond must be lack of DO.

The harvest started early 2013. Zhuo and his partners pumped the water out in the evening. They put on electric torch, wow, the spotlight showed all mud crabs thickly dotted, layer upon layer, on the bottom of the pond. However, they lack experience of harvesting, many crabs died or damaged or sun-dried to death next day. The correct method should be the quick harvesting in the evening to avoid them fight against each other. If crabs are numerous, your harvesting speed is not enough the water level should be raised for crabs submerged under the water or putting some grass into the pond to avoid cannibalization.  

Finally they summarized when the stocking density of mud crabs was reduced to 1500/mu, the survival rate was the highest, achieving 700 crabs on average per year. Zhuo has found that Hele crabs are fighting each other more fiercely than other because of high temperature in Hainan. It causes the low survival rate of mud crab culture in the past. Thus, the yield of crabs has now increased to 100 kilo per mu. A pond with 7 mu can produce 750 kg of crabs. This mariculture base has ponds with the area of 200 mu so they produced 20 tons of crabs, but still not sufficient to supply the market.

When the bottleneck of low unit yield of Hele crabs is solved, it may stimulate farmers to culture Hele crabs. More producers, more seeds of crabs needed, it will promote the development of hatchery to breed more post larva of crabs and juvenile crabs, and then the whole industrial chain effectively links together to a circular economy. Furthermore, Zhuo rent another 200 mu for new trial to raise the unit yield of Hele crabs. In the near future he would like to plant mangrove to build habitats in mangrove for Hele crabs so as to protect coastal environment and to enhance Hele crab colony [13].

In Blue Economy proposed in China mariculture has developed rapidly in recent decades among which mud flats culture the most.  During 1986-1990 periods, annual increase reached 13.6% on average and years after it increased 21.8%.The production from mud flats accounted for 39% in 1986 up to 45% in 2000 of the total production of mariculture. Mariculture gained tremendous economic benefits but great impact environment in near sea as well. In the National Hi-tech R & D Plan (863 Program) project 2003AA627030 was to study on mangrove mariculture integration eco coupling system, three mangrove species: Sonneratia caseolaris, Kandeliacandel and Aegiceras corniculatum were chosen in 9 test ponds and 1 control by planting 45%, 30% and 15%. The result shows that S. caseolaris and A. corniculatum grew well, while K. candel grew poor with low survival rate. Mangroves are capable to purify water, Aegiceras species is the best. Farmed animal growth is related with the purifying effect of mangrove. Therefore, if Zhuo Qihui wants to integrate mud crab culture with mangrove planting, he should prefer planting Aegiceras corniculatum [14]. I also propose that he adopt rice crab culture system in mud flats. The sea rice 86 is the new discovered rice species grown in saline alkali mud flats and it has been a new rice cultivar in saline alkali soil after 28 years breeding. [15].

Intensive Crab Cage Culture and Facility Culture

As is well known the dominant production style of mud crab farming in Sanmen is pond culture. However, it is due to the limit of the pond in mud flats mud crabs often cannibalize each other during molt or hungry periods and this affects the effectiveness of mariculture. The mud crab farmer Yu Haichao is the first person who is trying to raise crabs in special crab cages and he has achieved annual profit quadruple that of the traditional production style [16].

It is to use a set of apparatus assembled by 4 modular rooms in 6 floors which can raise 24 crabs in one set of cage. It is easy to farm mud crabs in such a crab cage. The stereo structure can fully utilize water body and it also solves feeding and disease control problems, and especially it avoids cannibalizing each other among crabs.

Mr. Yu farmed 10,000 crabs in 2014 and he has made 20,000 sets of crab cages, preparing to farm 80,000 crabs in 2015. Crab cage farming has many benefits but also a lot of risks as well, especially Sanmen is located on the verge of typhoon, and the harm caused by red tide there as well. In addition, the costs and labor inputs have increased a little than traditional farming methods. In analysis the benefits are considerably better.

How to raise sea crabs in small setups 

Hua Bo from a mariculture farm in Qingdao Shandong Province says sea crab mariculture facility is a super intensive factory farming system. It has numerous merits:it can prevent natural enemy of sea crabs raised in ponds, avoid their cannibalization each other, resistance to natural disasters, such as typhoon,heavy storm or tsunami. A deep blue plastic setup (box) has a space of 32 cm x 20 cm to raise just one crab. Ten stacks of 10 boxes line together and cover only dozen square meters and it increases the utilization rate of land so that mariculture can potentially develop in large scale increasing production without polluting the environment near sea.

         The light is dim in a small box environment, but beneficial for crabs to take feeds, growing fast by feeding individually, to grow out to unanimous size in a controlled temperature with no season limit. The water can be recycled to guarantee the quality, salinity, DO level and pH value to clean NH4 and HS2 in effluent and then to sterilize the effluent by sand filtration and ultraviolet ray.

         Prof. Xu Quanhan Ocean Research Institute of CAS says any crab molts 13 times in its growing period. When they are molting they become soft, prone to be attacked and unable to escape if not hidden well in advance. That’s why their survival rate is so low, in general 20% in pond culture. If you raise 10,000 sea crabs in natural pond you may harvest 2,000 crabs, a great success. Now Hua Bo thought out a new method to culture crabs in setups in doors and thus it could reduce the loss and save the cost.

         This research is based on the study of biological and ecological features of sea crabs. Sea crab is a nocturnal animal and it is a typical independent animal in all zoea, megalopa, juveniles, adult and mature periods while fish like to live in colony. Each crab is being put in one box. They do not feel lonely because It is a lazy animal as long as it can find a cave to hide in daylight feeding at night. Why the boxes adopt deep blue color? Because the light can penetrate white plastic box, causing them stop feeding and activity. After the box color changed to deep blue, the environment in box becomes dim all the time and crabs adapt to be active feeding, even in daylight. Crabs often are active at night in ponds, fighting for feeds. Now all they can be fed well and thus they can shorten their growth period. Crabs need clean water, with flowing water in 24 hrs a day with DO 5 ppm above.

They use independent hit-streaming water supply, with optimal salinity, with a certain pressure to quickly aerate the water; bubbles are very delicate, hitting bottom from the surface. After being put in, the water is slowly flowing in 24 hours a day and will discharge out at acertain depth by the drainage tube with waste and food scraps, separate effluent into the purification process through high speed sand tank and sterilized by ultraviolet ray and at last water can be recycled. The water temperature can be controlled in 20℃ with apparatus to raise and reduce the temperature.

Crabs take in water from the rear, through the gill filaments; spit CO2 from both sides to supplement oxygen, do not dry out and die, crabs can survive when leaving the water for the moisture still left in gills. 

The boxes can endure for 10 years. The cost for raising 100 crabs in 100 boxes plus feeds and waterand electricity is 1800¥and harvest 90 crabs or more 400g/each 36 kg, 160¥/kg = 5760¥ to deduct the cost, you may earn more than 3000¥[17]. 

Human Consumption

Nutritional analysis shows Scylla is rich in protein and trace elements, good tonic to the body. Swimming crab is one of China's major export selling products, mainly exported to Japan, Hong Kong, Macao, etc.  It is worth noting, swimming crab will greatly reduce the survival rate after leaving high oxygen and high salinity environment. There is a big point different from river freshwater crab. Although the survival rate of swimming crab is low, but if died in a short time and with no odor they are still available for human consumption, its rich nutrition and fresh taste as much as fresh.

Generally cooked with their shells on, when they molt their shells, they can be served as one of many types of soft shell crab. Some consider them to be among the tastiest of crab species, and demand for them is large in South and Southeast Asian countries, where they are often bought alive in the markets.

Clams are served steamed, boiled, roasted, or traditionally raw. There is a safety issue. Unlike other varieties of clams that are safe to eat, the blood clam lives in low-oxygen environments, and must thus ingest more viruses and bacteria, including hepatitis A, hepatitis E, typhoid, and dysentery,


1       Li Kangmin 2015.The Tao of Fisheries Leads Us toSustainable Development The 9th World Congress on Blue Economy 2015 Surabaya April 13-15, 2015

2     New method to farm mud crab,2014-03-24 Yang Shanjin Taizhou Daily

3     Fu Jun, 2014. Three strokes to developeco mariculture in Sanmen 浙江三门:发展生态渔业有三招2014年09月21日 来源:中国渔业报 付俊http://www.ntv.cn/a/20140921/52761.shtml

4       RuǎnJīnshān; Li Xiùzhū; Lín Kèbīng; Luō Dōnglián; Zhōu Chén; Cài Qīnghǎi , 2005. Analysisof the causes of death of farmed shellfish on the mudflats in the southern partof Anhai Bay, (Fujian Aquaculture), 2005-04

5       Li Kangmin and Li Peizhen 1994. Integration of Agriculture, Livestock and FishFarming in the Wuxi Region of China In proceedings of theManagement of Integrated Freshwater Agro-Piscicultural Ecosystems in TropicalAreas, edited by J.J.Symoens & J.C. Micha, published by Royal Academy of Overseas Science, CTA& FAO 1995. Co-authored by P.Z. Li.

6     Li Kangmin1998. Fish Polyculture. In A New Hope for sustainable Development in Africaedited by Keto E. Mshigeni (Editor-in-Chief) published by the University ofNamibia 1998 and also in Sustainable Development in the South Pacific publishedby the University of Namibia 1998.

7     Li Wenhua Ed, 2001. Agro-EcologicalFarming Systems in China, Man and the Biosphere series Vol26.

8     Li Kangmin, 2008. Extending IntegratedAquaculture to Mariculture in China---New trends of integrated fish farming in Encyclopedia of Life Support Systems UNESCO/EOLSShttp://www.eolss.net 2008

9     Li Kangmin2013. New Ideas & Approaches to Sustainable Seafood Products ACOFB 2013 inBali Indonesia

10   CCTVProgram Daily NongJing 2015-12-26青蟹的养殖,三门青蟹混养卖价高效益翻翻《每日农经》

11   Razor clamfree Wikipedia

12   Zhang Chen ed. Blood cockle farming Xinhua News 2014-12-29 新华社 责任编辑:张晨

13   Zhuo Qihui: Giving up million earning merely to raise Hele crab转自: CCTV7《致富经》20151117)视频

14   Peng YouGui, Yin Min, CHEN Guizhu, She Zhongming 2004 Preliminary Study on MangroveMariculture Integration Ecological Coupling System 《中山大学学报(自然科学版)》 2004年06期

15   Li Kangmin, 2014. Feeding China with Sea-Rice 86 ISIS Journal in England

16   New method to farm mud crab,2014-03-24 Yang Shanjin Taizhou Daily

17   Farming Sea crab in boxes 

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