Mitten Crab Farming in China
From “Aquaculture: Farming Aquatic Animals and Plants”
Check Out the Videos and Additional Information at the End of This Report
The second edition of Aquaculture: Farming Aquatic Animals and Plants contains and excellent chapter titled “Other Decapod Crustaceans” that details the farming of the Chinese mitten crab (Eriocheir sinensis) in China. To the best of my knowledge, most of this information has never been brought together in English in one place before.
Like freshwater prawns (Macrobrachium rosenbergii), mitten crabs spend most of their lives in fresh water and migrate to the sea to breed. They feed on aquatic plants and animals, tolerate temperatures from 1°C to 35°C, and can survive for more than a month without feeding. Unlike other farmed crab species, they are not aggressive or carnivorous. In China, live mitten crabs are sold from vending machines for $1.50 to $7.00, and when supplies are short, prices reach astronomical levels, as high as $50 for one crab!
Before I tell you about the chapter on mitten 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 references at the end of every chapter. Black and white pictures, tables and 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.
Mitten Crab Farming in China
The Chinese mitten crab (Eriocheir sinensis) gets its name from the distinguishing feature of dense patches of hair on its claws (chelipeds). It’s native to China and distributed widely along the eastern coast in streams and rivers that are connected to the sea. As a medium sized crab (50-80 millimeter carapace width as adults, about the size of a human palm), it is regarded as a culinary delicacy in China. During the 1960s and 1970s, however, the mitten crab fishery collapsed because of overfishing and habitat destruction, which stimulated increased aquaculture research in the 1970s and 1980s, with particularly emphasis on hatchery techniques. Successful mass production of crab seedstock in hatcheries was reported in both natural and artificial seawater in the 1980s.
It took time for the farming techniques to be adopted and improved, and it was only after 1990 that the Chinese mitten crab farming industry began to develop and spread beyond the crab’s natural distribution range.
Production Status: Chinese mitten crab farming now occurs in all provinces and regions of mainland China, although Jiangsu and Zhejiang provinces in the south and Liaoning Province in the north have the highest concentration of farms. Annual farming production reportedly reached 570,000 metric tons in 2005, more than a 40-fold increase over production in 1991. Based on statistics from the China Fisheries Bureau, hatchery production of Chinese mitten crab seed (megalopae) exceeded 200,000 kilograms (approximately 140,000 megalopae per kilogram) in 2001. Subsequent annual production more than doubled to reach 522,893 kilograms in 2003.
Despite growout all over China, the hatcheries are mainly located in the Yangtze River Delta in southern China, where most megalopae are produced and transported to other parts of the country for growout. After the Yangtze River Delta, the most important region for hatchery production is along the Liao River in northern China.
Biology: The Chinese mitten crab inhabits fresh water streams and lakes, favoring habitats with clean water and lush aquatic plants. They are omnivorous and nocturnal. Their diet includes aquatic plants and insects, small mollusks, fish, shrimp and earthworms. Aquatic plants often make up the bulk of their gut contents, probably because of their ready availability. The crab has broad temperature tolerance and can survive between 1°C and 35°C; however, in winter when temperatures drop below 5°C, they often hide in holes and feeding ceases. They can tolerate long periods of starvation and can survive without feeding for more than a month.
The Chinese mitten crab is catadromous, meaning they spend most of their life in fresh water, but return to the ocean to breed. In the wild, sexually mature crabs begin migrating downstream in late autumn and females attain ovarian maturity when they reach tidal estuaries. After mating and spawning, the extruded eggs are attached to the female’s pleopods. Females may overwinter in deeper waters, but in the spring they return to shallow water, where their eggs hatch.
Larval development of the crab occurs in brackish water. The newly hatched larvae are the planktonic first stage zoea. They progress through five zoeal stages (zoea 1-5) before metamorphosing into the megalopa stage, which is when they begin looking like crabs and begin swimming with abdominal appendages. Megalopae migrate upstream where they metamorphose into crabs and settle on the bottom of streams and lakes. The juvenile crabs continue to grow until they reach sexual maturity with a terminal molt. First stage crabs undergo 15 molts over a period of about two years to reach the terminal molt, which indicates sexual maturity and maximum size.
The production cycle for mitten crabs normally takes two years and involves three distinct culture phases: hatchery, nursery and growout.
Hatchery: Obtaining high quality broodstock crabs is the first step to successful hatchery operations. Ideally, female broodstock crabs should be 125-150 grams and males should be larger than 150 grams. Only healthy active crabs with intact appendages are selected and crabs collected from lakes and streams are generally preferred to those from farm ponds. Broodstock nutrition is crucial during the ovarian maturation process in late September to mid-November. The diets commonly used to feed broodstock are fresh natural food, like trash fish (Chaeturichthys stigmatias), the razor clam (Sinonovacula constricta), and sandworms (Nereis japonicus). Formulated diets supplemented with essential fatty acids have been developed for female broodstock.
After conditioning, male and female broodstock crabs are put together with a female:male sex ratio of about 2 to 2.5:1 and brackish water is introduced to initiate mating. The optimal salinity and temperature range for breeding are 14-20 parts per thousand and 8-12°C, respectively. Hard sandy or muddy substrate is provided to facilitate the attachment of newly extruded eggs to the female’s abdomen. Soon after, when brackish water is introduced, the crabs will pair and mate, with spawning usually occurring within two weeks of mating. Egg-carrying crabs can be found the day after mating, and in about one week 70-80% of females will carry eggs. This process helps generate synchronized spawning and allows hatcheries to arrange the most suitable time for larval culture. The fecundity of mitten crabs is relatively high, with 100 to 200-gram females producing between 200,000 and 900,000 eggs.
After spawning, female crabs carry their eggs on pleopods beneath their abdomen while the embryos develop. This takes 2-4 months depending on temperature, which can be manipulated in the hatchery either to speed up or slow down embryonic development.
Raising Larvae: Two major larval rearing techniques, namely indoor intensive and outdoor semi-intensive/extensive, have been developed for mitten crabs:
Indoor intensive larviculture is mainly performed in indoor concrete ponds or tanks with temperature control. A typical pond size is 5 x 5 x 2 meters; temperature is normally controlled at 18°C for zoea-I, but increased by 1°C at each successive zoeal stage. It is therefore about 24°C by the time larvae molt to megalopae. Microalgae, live rotifers, Artemia nauplii and copepods (harvested from the wild), frozen rotifers and egg yolk are used to feed the larvae. The initial stocking density is typically 200,000-500,000 larvae per cubic meter, and megalopa production is usually 150-500 grams/m3, or 20,000-70 000 larvae/m3. Survival is generally between 10 and 15%.
There is limited water exchange from zoea-I to zoea-3; however, a higher percentage of water is exchanged after larvae reach zoea-3 and, by the megalopal stage, water is exchanged twice daily. Antibiotics (tetracycline, for example) are often used. The optimal salinity range for larval rearing is 20-25 parts per thousand, although megalopa can be reared in 10-30 ppt salinity. Both natural saltwater or specially formulated artificial seawater can be used. The latter is mainly used at inland hatcheries and costs more; therefore it is less commonly used. To acclimatize megalopae to freshwater, salinity reduction typically begins three days after most larvae have become megalopae and salinity is reduced to less than 5 ppt by the time megalopae are six to seven days old. The whole larval culture cycle, including the time for megalopa acclimatization, is about 22-24 days. Six-to-seven-day-old megalopae, at 140,000 per kilogram, can be sold to nurseries for culture in freshwater.
Outdoor larviculture is done in outdoor earthen ponds without temperature control. The ponds, ideally with smooth hard clay bottoms, are normally built near estuaries (20-25 ppt) and a freshwater source. This outdoor larviculture technique has set a new hatchery trend in China in recent years and become very popular because it requires fewer facilities (buildings, heating and aeration systems). These systems are also substantially cheaper to setup and run, and they are easier for farmers to operate. Antibiotics are not used in these systems, the quality of the megalopae produced is generally better, and they are therefore favored by the crab farmers. This method has recently become the dominant technique for producing megalopae.
There are two kinds of outdoor larviculture technique: semi-intensive and extensive. The former uses small ponds of 400-700 square meters equipped with aeration, whereas the latter is practiced in bigger ponds of 1-1.5 hectares without aeration. Production of megalopae per unit volume of water is higher in the former, and the costs are also higher. Outdoor larviculture takes place mainly during mid to late spring when water temperature ranges between 10 and 23°C.
Typical semi-intensive larviculture ponds are either square or round and 1.5 meters deep, whereas the ponds for extensive larviculture are often rectangular and normally built in high intertidal zones. A 1.5-meter-deep channel is dug around the perimeter of each extensive pond (called a “loop channel”), which buffers the pond water from abrupt temperature fluctuations that could lead to mass larval mortality. Twenty to 50 days before larval stocking, ponds are disinfected by applying 1,500-2,000 kilograms lime (CaO) per hectare or 750-1000 kilograms bleaching powder (CI > 30%).
The ponds are filled with 40-50 centimeters of filtered seawater about seven days before stocking and may be fertilized to stimulate algal growth. Normally there is little water exchange during the larval culture period.
For semi-intensive larviculture, initial stocking density is 20,000 to 30,000 newly hatched zoea per cubic meter. If the ponds are well fertilized with a good algal bloom (often Isochrysis and Platymonas species at concentrations of 2 x 105 to 4 x 105 cells milliliter), larvae molt to zoea-2 in about five days without additional feeding. If algae are not growing well, zoea-I and zoea-2 larvae may be fed supplemental feeds, such as soybean milk and dry Spirulina powder. From zoea-3 onward, larvae are fed mainly with live Artemia nauplii as well as frozen adult Artemia and copepods harvested from the wild. The yield of megalopae is normally 15-30 grams per cubic meter.
For extensive larviculture, initial stocking density is generally less than 10,000 larvae per square meter. For all larval stages except zoea-I (not fed), live rotifers (Brachionus species) are the main feed; zoea-2 and zoea-3 larvae are fed about 35-40 kg of rotifers (wet weight)/ha/day to maintain a density of 2,000 to 3,000 rotifers per liter. Rotifer feeding increases to 70-80 kg/ha/day and 120-130 kg/ha/day at zoea-4 and zoea-5, respectively. At later larval stages, especially for zoea-5 and megalopae, rotifers may be supplemented with frozen copepods or adult Artemia. The rotifers used to feed the larvae are often reared in earthen ponds next to the larviculture ponds, with a 1:1 ratio of pond areas. Additional rotifers can also be purchased from farmers who specialize in producing them in earthen ponds. The rearing of rotifers is largely based on fermented organic fertilizer, and in a period of 20-35 days, 900-1000 kilograms per hectare can be produced. The yield of megalopa is more variable for extensive larviculture, ranging from 1 to 7.5 grams per square meter.
Nursery Culture: The hatchery produced megalopae that are sold to nursery farms are normally carried in wooden boxes (about 60 x 46 x 10 cm) with an opening on each side of the box to allow air exchange. The bottoms and the sides of the boxes are lined with mesh net to prevent escape. Before transport, the boxes are soaked in water for 12 hours and a layer of clean aquatic plants or a wet towel is laid on the bottom of the box to provide moisture and prevent accumulation of megalopae in one area. Each box can be loaded with about 0.5 kg megalopae and 5-10 boxes can be stacked together. If conditions allow, the temperature during the transportation is controlled between 14 and 18°C. Using this method, megalopae survival can be 100% if the transportation time is less than 12 hours or approximately 90% if it is 24 hours.
The nursery phase of Chinese mitten crab culture takes 5-8 months and can be divided into two stages. The first lasts about 20-30 days, during which hatchery produced megalopae are cultured in earthen ponds and reach the second to fifth crab stages with a weight between 0.1 and 0.2 grams, about the size soybeans and are called “bean-sized” crabs. In the second stage bean-sized crabs grow to 3-10 gram juveniles that are ready for stocking into growout ponds or enclosures. Mitten crabs can be kept at high densities, which is not possible for some other aggressive and carnivorous crab species. The second nursery stage takes several months and can either use the same ponds that were used for the first nursery stage or the bean-sized crabs can be stocked in other ponds or rice fields, which is more common. At the end of the second nursery stage, the juveniles are seventh-to-eighth-stage juvenile crabs and look like coins or buttons and are called “coin-sized” or “button-sized” crabs.
The nursery ponds are generally rectangular and range from a few hundred square meters to a quarter of a hectare. If the first and second stages of nursery culture are in separate ponds, the ponds for the first stage nursery are smaller. The second stage nursery can also be conducted in rice fields with appropriate modifications. Firstly, structures to prevent escape and access by predators (frogs and toads) must be built. This is normally done by setting up a mesh fence about a meter high on the perimeter of the ponds or rice fields. Secondly, an encircling channel needs to be dug around the perimeter inside the fence. The channel is typically a meter wide and 60-90 cm deep. This deeper channel serves as a refuge for the crabs on hot days. For both the first and second stage nursery, aquatic macrophytes, such as Vallisneria spiralis, Hydrilla verticillata and Elodea species, are usually planted before stocking and serve as both food and refugee for the crabs.
The initial stocking density for the first nursery stage is around 100 megalopae per square meter or 7.5 kilograms per hectare. When megalopae are first stocked, the water depth is normally maintained at 20-30 centimeters, but this is gradually increased by 10, 15 and 20-25 centimeter steps when crabs molt to the first, second and third crab stages, respectively. The maximum water level is around 70-80 centimeters.
During the nursery phase, crabs feed on natural diets, including plankton, plants and small benthic animals that grow naturally in the ponds or rice fields. Supplementary feeds, such as corn, wheat, pumpkin and minced trash fish, as well as formulated feeds, are also provided to enhance their growth.
A major problem during the nursery phase is precocious puberty commonly affecting 15-30% of the animals, which weigh 15-35 grams and do not get any larger and, therefore, have little or no market value. Research has shown that this phenomenon is linked to genetics, high temperatures and nutrition; however, techniques have not been developed to control its occurrence.
Growout: Depending on the farm and location, at the end of the year before winter sets in or early in the following year, coin-sized crabs are stocked in ponds, net enclosures or rice fields and grown to market size (80 and 200 grams).
Earthen ponds are usually between 0.6 and 2 hectares with a water depth of 1.5-1.8 meters. Fences similar to those with nursery ponds surround the ponds to prevent escapes and access by predators.
Some time before crab stocking, the ponds are normally disinfected with lime (2,000 to 3,000 kilograms per hectare) and 10 days after disinfection, aquatic plants, such as Vallisneria spiralis, Hydrilla verticillata, Ceratophyllum demersum, Potamogeton maackianus and Myriophyllum spicatum, may be introduced to the ponds. One month or so before crab stocking, live mud snails, Bellamya purificata, a favorite food of mitten crabs, may be stocked in the ponds at the rate of 3,500 to 4,500 kilograms per hectare and left to reproduce. Finally, fertilization using fermented organic fertilizers is often applied 10-20 days before crab stocking to increase the natural biomass inside the ponds.
The stocking density of coin-sized crabs is normally around 7,500 crabs per hectare, and they are often polycultured with various freshwater fish and shrimp, which may be stocked at different times. The important consideration for fish species selection is that their feeding habits should complement rather than compete with the crabs. Over the culture period, water quality needs to be closely monitored. Dissolved oxygen should be about five parts per million, pH at 7-8.5 and Secchi disc readings between 50 and 80 centimeters. The yields per hectare per year are variable, depending on intensity of polyculture, but they are normally close to 1,000 kilograms of crabs, plus twice that weight of various freshwater fish and shrimp.
Net enclosures are used to grow mitten crabs in almost all inland lakes in China. Polythene mesh (1-2 cm) is a common net material. The nets are fixed in place with bamboo stakes. The net walls are folded inwards and held on the bottom with rocks, causing them to sink into the mud. The upper edge of the net is typically 50-60 cm above the water level. Site selection is important for net enclosure culture. Sites should have high-quality, slow-moving water with a constant level, suitable depth (1-2 meters) and, most importantly, abundant submerged aquatic plants.
Stocking density for net enclosures range from 1,500 to 9,000 crabs per hectare. At some locations with good natural productivity, crabs don’t require supplemental feeding. At less productive locations, the mud snail Bellamya purificata is stocked at 6,000-7,500 kilograms per hectare before crab stocking and sometimes again after stocking. Other supplemental diets include chopped trash fish and cooked corn and wheat at 7-8% of crab biomass per day.
Survival is normally around 40-50%. Net enclosures usually produce higher profits that other growout methods because they have better water quality and utilize natural foods. Also, because stocking densities are lower, the crabs grow to larger and garner higher prices. Generally, the yield from net enclosure culture is about 150-450 kilograms per hectare, although higher yields of close to 1,000 kilograms per hectare have been reported.
Paddy-field culture system integrating rice production and crab growout are popular in northern China. Rice fields need modifications for paddy-filed culture similar to those for nursery culture, like fences, perimeter channels and aquatic plants. One of the main differences is that the channel needs to be deeper at 1.5 meters. In addition, a small pond (100-200 m2 with 1.5 m depth) needs to be dug for acclimating the coin-sized crabs and for temporary storage of market-sized crab during harvesting.
Stocking density in paddy-field culture system ranges from 1 to 1.5 coin-sized crabs per square meter. Because the paddy fields also grow rice, the water depth is normally kept low at 5-10 centimeters. Water is exchanged more regularly because lower levels of water in paddy fields and extra care needs to be taken to avoid large fluctuations in temperature. Because the natural benthic biomass in the rice fields is relatively low, the crabs need to be fed once or twice a day with supplemental feeds like com, wheat, trash fish, trash shrimp or mud snails. These feeds are normally cooked to avoid water quality problems in the shallow water. With proper management, rice field culture can achieve yields of 300-450 kilograms per hectare and occasionally 750 kilograms per hectare. Although crabs will feed on rice plants, they also feed on nuisance insects and weeds. Integrated culture may even improve rice production, and it is usually organic production with a higher value and substantial economic benefits.
Market and Marketing: The Chinese mitten crab is largely consumed domestically in China although it is also exported to countries or cities that have large ethnic Chinese populations like Singapore. Female crabs with well developed ovaries and large crabs normally fetch the highest prices.
Videos on Mitten Crab Farming, Plus Some Additional Information
Video: This minute-and-a-half, high-resolution video shows how the mitten crab industry uses authenticity tags to protect its brand from imitators. The tags, distributed by the crab farmer’s association, change each year and are only released by the government at the beginning of the crab harvest season. The tags contain the name of the farm and its phone number.
Video: When watching this video, make it as large as possible on you computer screen so you will be able to read the English language captions at the bottom of the screen. Even in China, not everyone knows how to eat a mitten crab. This four-and-half minute shows a young woman opening a gift box of “live” mitten crabs. Within the box, there is a food waste bag, several crabs, seasoning and a bottle of crab wine. You don’t have to eat the crabs immediately. You can refrigerate them for two or three days. If you give the crabs a little tap on top of the carapace, just above the eyes, the eyes will wiggle a little, demonstrating that they are still alive. With eyestalks still wiggling, the woman puts the crabs into the steamer. After that, she demonstrates the proper way to eat a mitten crab, saying, “We mustn’t eat the crab’s heart, intestines, stomach and gill,” and shows you how to dispose of them and then enjoy the tastier parts.
Some Additional Information on Mitten Crab Farming: The Chinese mitten crab (Eriocheir sinensis) is a medium-size burrowing crab native to the coastal estuaries of eastern Asia from Korea in the north to Fujian Province, China, in the south. It has also been introduced to Europe and North America where it is considered an invasive species. With a body about the size of a human palm, the crab’s most distinguishing feature is the dense patches of dark hair on its claws.
Somewhat like Malaysian freshwater prawns (Macrobrachium rosenbergii), mitten crabs spend most of their life in fresh water, but must return to brackish water to breed. In the late summer of their fifth year, they migrate downstream and attain sexual maturity in tidal estuaries. After mating, the females continue seaward and overwinter in deeper waters. They return to brackish water in the spring to spawn. After passing through several larval stages, the juvenile crabs gradually move upstream into fresh water, completing their life cycle.
The best specimens of mitten crabs are farmed in two freshwater lakes—Lake Tai and Lake Yangcheng—in Jiangsu Province. Crab fry are sourced from the mouth of the Yangtze River in March and transferred to the lake edge, where they are confined in netted-off areas and fed maize and seaweed as well as snails that are collected locally. The young crabs hibernate in the winter months by burrowing into the bottom. In the spring, they are transferred to large pens in the middle of the lake for growout. From the collection of the juveniles to the final harvest, it takes about two years for the industry to get the crustaceans ready for the table.
According to crab supplier Zhang Wei Wen, Lake Yangcheng has 55 hectares allocated to mitten crab farming while the much larger Lake Tai has 170 hectares. Zhang said, “Lake Tai is fed by the mountain streams that bring rich nutrients. The local government is very strict on the water quality of the lake and no one is allowed to use any form of feed that is not approved. Otherwise, the lake will be contaminated. Every farm here and in the more famous Lake Yangcheng is licensed and regulated.” For marketing purposes, the crabs are held together with reeds that impart a light herbal seasoning.
As for identifying an authentic mitten crab, Zhang stipulated that it should have a shiny back. Underneath, it should be ivory-colored and “ripe” with roe. The rear portion of the crab must look like it is almost bursting. When it is held, it should feel heavy, not light. Male crabs have pointed bellies that resemble triangles; females have oval-shaped bellies.
Due to the relative proximity of the two lakes to Shanghai, the crabs are often branded as “Shanghai Crabs”. The aim of eating hairy crabs is to savor the brilliant, reddish-orange roe with its delicate, creamy flavor that lingers.
With the growing affluence of consumers in Beijing, Guangzhou, Hong Kong and other major Chinese cities as well as increasing demand from restaurants around the world–including Southeast Asia–now there are fake mitten crabs on the market. They resemble the real thing, but without the sought after creamy-flavored roe.
Mitten crabs have exhibited a remarkable ability to survive in highly modified aquatic habitats, including polluted waters. They can also easily tolerate and uptake heavy metals, such as cadmium and mercury.
View Farms at Google Earth: For a great view of what appears to be the crab pens on the bottom of Lake Tai, go to Google Earth (free but you must download it from Google) and zero in on these coordinates: 31°, 02’, 8.20” N; 120°, 26’, 46.99” E. You get a much better view of Lake Yangcheng at: 31°, 25’, 38.29” N; 120°, 49’, 14.67” E. You can also copy the names of the lakes from here and paste them into Google Earth’s find window. Both lakes are clear and shallow. You can see right to the bottom of Lake Yangcheng. Zero in on it and you can see how the entire bottom is sectioned off for crab farming. Then zoom out and look at the intensity of aquaculture and agriculture around this lake. Amazing!
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.