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KOREA-US AQUACULTURE -> Main Species->Crustaceans->Blue carb
Blue crab
♦ Ecological characterBlue crab is a common edible crab which occurs in Japan, Korea and China.This species is one of the most commercially important crab resources in the West Sea of Korea and the East China Sea. In the west coast of Korea, mating of blue crabs occurs from August to November with a peak in September and October. The size at 50% copulation of the males and females is approximately 10cm in carapace width. Spawning begins from March and ends to September, mainly from May to July. Sex ratio is about 1:1 all the year round. ♦ General introduction of crab culture in KoreaTechnology of crab larval production was developed by NFRDI in 1990 and millions of juvenile crabs were released into the sea for increase natural resources every year. Culture of blue crab has been attempted by several farmers since early 1990s but ended in failure because the productivity is too low. Some farmers had tried larval production in outdoor ponds and growout or hanging cultures in ponds but all these trials were not successful. The major problem in crab culture is the strong cannibalism among crabs and therefore survival rate is very low when the crabs are harvested. Secondly, the growth rate is slow because of climate in Korea and it is very difficult to produce adult crabs of marketable size. Crab size for sale should be at least 200-250g in body weight.
However crab culture using traditional methods cannot produce crabs of 200g in body weight, i.e. a traditional culture technology stocking young crabs which are produced in natural spawning season, July or August. The young crabs stocked in July are able to grow for two or three months only because crabs won't grow (molt) in the water temperature below 20°C. To extend the growing period of crabs, NFRDI developed a technique for early crab seed production in 2004. Using this technology, young crabs were produced and stocked in ponds in May and grown to marketable size,
- Collection of females and artificial induction of maturations Female crabs are collected from February to March. Females for larval production should be healthy and active and have no external wound. Female crabs are placed in a tank which is Under this condition for two or three weeks, females produce eggs out of body and become berried. The color of eggs is initially yellow but becomes dark grey in two or three weeks. - Larval production Hatched zoeal larvae pass through four zoeal stages and one megalopal stage, and metamorphose to young crab in three weeks. Optimal water temperature for larval culture is around 25°C. The zoeal larvae ingest foods as soon as hatched because they don't carry any yolk unlike shrimp nauplius larvae. The 1st to 3rd zoeal stages are fed with rotifers and the 3rd to megalopal stages are fed with newly hatched Artemia nauplii. Ground muscle of bivalves is provided from megalopal stage to young crab. Artificial feeds of shrimp larvae can be provided from zoeal stage. Density of larval rearing is about 50,000~100,000/m3. Crab larvae have very strong cannibalism. To control eating each other, air should be heavily provided and fishing net for hiding can be also provided to larval rearing tanks. When crabs grow to 1 cm in carapace width (same to the 2nd or 3rd crab stages), they can be harvested for stocking to ponds.- Preparing growout ponds Pond for crab culture is very similar to shrimp pond. Size of pond is about 1-3 ha and shape is square. Growout ponds consists of inlet, pumping system and water supply and drainage canals. Pond construction begins from February to March. First, farmers remove sludge of bottom and construct dikes using tractor. After construction of ponds, seawater is filled and chlorinated with calcium hypochloride with 10-20 ppm in final concentration in order to kill pathogens including viral vectors and others. After sterilization of seawater, lime is use far treatment. Water is maintained from 1.2 to 1.5m in depth. Bottom of pond should be sandy or mud sandy because crabs hide in the bottom during rest at daytime. - Stocking young crabs Young crabs collected from hatchery are packed in plastic vinyl bags saturated with pure oxygen or moved to outdoor ponds by vehicles equipped with water tank and aerator. Age of crab for stocking is crab 2 or 3 stage, stocking density is about 10/m2, and stocking time is May when water temperature raises up to 15°C. Young crabs can be directly released into pond or raised in restricted cage or area for two to three weeks, i.e. nursery culture. - Growout and harvest Period of growout is from four to five months in outdoor ponds. Crabs fed with ground bivalves including short-necked clams or oysters during young crab stage and frozen shrimps or trash fishes in juvenile stage and thereafter. Artificial pelleted feed was developed by NFRDI in 2005. Using the artificial feeds, some farmers produced adult crabs of marketable size last year. Feeding frequency is two to three times daily according to crab age. Management of rearing water is the most important in pond culture of crab. To maintain alkalinity and pH at optimal condition, lime is sometimes provided to ponds. To increase DO concentration, paddle wheel aerators are installed with 8-10 HP/ha. Concentration of dissolved oxygen should be maintained over 4.0 ppm. Harvest can start in late August and continue to December, depending on market price. Unlike shrimp, blue crab can survive as low as 5°C. Market price increases in winter season and farmers try to keep crabs in pond until December. In late August the male crabs can be selectively harvested and remaining females are continuously cultured because males tend to die after mating from late August. Although crab will not molt when water temperature falls below 20°C, they can eat and gain weight. The market price of live crabs is 25-40 US$/kg, depending on season.
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