Crab Farming, Mekong Delta
Mangrove aquaculture that uses mangrove functions in nutrient cycling has been popular throughout Southeast Asia. This aquaculture type commonly involves the farming of a single species, that was shrimp, fish, or oyster. Mangrove aquaculture commenced in the Mekong Delta of Việt Nam in the 1980s was mainly operated using a single species such as shrimp, brine shrimp, mud crab or finfishes. Shrimp farming was the most important and developed industry in the region because the shrimp farming system-generated enormous incomes and was promoted by the Government of Việt Nam.
Crab aquaculture probably first developed in China about over 100 years ago, to counter a shortfall in supply from the wild. In Việt Nam, mud crab farming has been widespread for only about the past 30 years. Crab farming has been increasing in the Mekong Delta for the last two decades, and it generates a good income for farmers in Cà Mau, Trà Vinh, Kiên Giang, Sóc Trăng and Bến Tre provinces. Cà Mau has many advantages that aid its aquaculture plans including the sea on three sides, a coastline of 254km and 87 river mouths. It also has 100,000ha of mangrove forests, which are a good habitat for raising shrimp, fish and crab.
The biggest wholesale crab market in the Delta is located in the Năm Căn District of Cà Mau Province. Every day, on average, around 50 local traders trade a total of US$200-250 thousand to buy 15-20 metric tons of crab. They sell most of the crabs to processing plants in Sài Gòn that export them around the world. About 70 per cent of the live crabs with roe are exported by air; the rest are sold to consumers in Sài Gòn and the Mekong Delta. Farmers harvest two to three crops a year. China has the biggest market, and if Chinese traders stop buying, prices will drop dramatically. Việt Nam exports crab to 40-45 markets, mainly, the European Union, the USA, China, Japan, Australia and Canada.
Mud crabs are highly valued for their size, high meat yield, texture and the delicate flavour, which is highly sought after in seafood restaurants, both locally and internationally. Furthermore, they are easy to keep alive for several days, as long as they are kept in cool and moist conditions, and thus can be transported without refrigeration or sophisticated facilities. As a result, mud crabs, and especially gravid females (also known as “egg crabs”), command high prices in markets throughout the region. The capture of wild crabs is an important source of income for small-scale fishers throughout the region. They are easily caught using traps or nets and form the basis of small but important inshore fisheries for many coastal communities.
There are four species of mud crab within genus Scylla are known as mud crabs, Indo-pacific swamp crabs, or mangrove crabs. The four species are; Scylla Serrata, Scylla tranquebarica, Scylla olivacea and Scylla paramamosain. In the wild, they inhabit tropical to warm-temperate zone mangrove swamps and nearby tidal muddy habitats. Scylla serrata is the largest and most widespread species of genus Scylla. Scylla serrata has been reported in most tropical and subtropical coastal regions of Indo-West-Pacific such as Bangladesh, India, Sri Lanka, Indonesia, Philippines and around the oceanic islands of Indo-Pacific, except around the South China Sea.
Naturally, the culture of the mud crab Scylla serrata is being commonly used in semi-intensive conditions in small family-scale ponds. Juventile mud crabs are collected mainly from the wild. In addition to harvesting wild stock, mud crabs are now being farmed in many coastal provinces in Việt Nam. In Việt Nam, Scylla paramamosain is the most prevalent species used in mud crab aquaculture farms.
The exploitation of the wild mud crabs has increased dramatically over the past 40 years. Demand for mud crabs has exceeded supply from the wild-catch fisheries, and this over-exploitation of the mud crab has affected production in the country. As a result, there has been a speed decline in production in the wild for the last 20 years. It’s difficulties with obtaining wild-caught juveniles or wild-caught crab “seed” for farming operations and it would not be sustainable due to over-exploitation of wild crab stocks have led to major investment into research into reliable hatchery techniques over the last 20 years.
To counter the threat to the wild population and ensure the sustainability of mud crab farming, there is an urgent need to produce juveniles in hatcheries. Over the past 10 years, more than 250 commercial mud crab hatcheries (mainly for Scylla paramamosain) have been established in Việt Nam. These hatcheries provide crablets (early stage juvenile crabs), accounting for one-third of the demand.
Mud crab farmers benefit in several ways from stocking their pens with hatchery-reared mud crablets rather than wild-caught mud crablets. Mud crabs are aggressive animals, but cannibalism is reduced when ponds are stocked with hatchery-reared crablets that are uniform in size. Typically, hatchery-reared crablets also are disease-free, and generally healthier because they were raised in hygienic conditions and had suitable feed and shelter. Consequently, it has been shown in field observations and preliminary trials that it is possible to use higher stocking rates of hatchery-reared seed than for wild-caught seed, and to achieve faster growth rates and higher survival rates, so long as the seed is stocked into disease-free ponds.
To develop fruitful and healthy crablets, broodstock crabs need to be specially selected, carefully handled and attentive breeding techniques planned ahead. 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. The controversy “eyestalk ablation” may also be used to increase total egg production (and increase the percentage of females in a given population which will participate in reproduction) and spawning during the breeding season. 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 spawning. Because female mud crabs with advanced ovarian development have normally already mated and so, no males are needed. Healthy, female broodstock are fed clams, marine worm, fish, squid and other crustaceans daily until they spawn. About 80% of the water is changed daily. Water temperature and salinity are maintained at 26-29 Celsius. 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. 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 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.
Egg hatches to zoea 9-14 days after spawning. Zoea passes through five stages (zoea 1 to 5), after which it becomes megalopa. The megalopa moults once and assumes a crab-like appearance. The small crab moults several times until full maturity.
Zoea larvae eat rotifers (microscopic aquatic animals) but Nannochlorum (microalgal species animal) is commonly used in hatchery because it is easy to cultivate. The production of natural food has to be synchronized with the hatchery operation so that food will be available as soon as the eggs hatch. The water is replaced at 30% daily starting day 3 and up to 80% as larvae grow longer.
Megalopa is also fed Artemia (aquatic crustaceans are also known as brine shrimp). Small crab is fed minced trash fish, mussels, and small shrimps. At the crab stage, about 30% of the water is replaced daily during the first five days and every two days thereafter. Stocking density of megalopa in a tank is two individuals per litre. Black nets and cut PVC pipes are distributed as shelters when megaloga becomes crab instar.
In net-cage culture, megalopa is stocked at 50-70 individuals per meter. About 50% of the water is replaced weekly. After harvest, crab juveniles can be transported even without water, ideally early in the morning or late afternoon.
Mud crabs are hardy animals that are easier to farm than for example shrimp as they can withstand salinity fluctuations and low oxygen levels, and tolerant of some of the diseases that can devastate cultured shrimp. It has a high yield, especially harvest for Tết (Lunar New year) as demand is high for the biggest and most important holiday of the year. With stable selling prices, breeding mud crabs provide farmers with sustainable higher incomes compared to black tiger shrimp and white-legged shrimp. For these reasons, many farmers had switched from breeding three shrimp crops a year to one shrimp crop and two mud crabs a year, or some switched completely to breeding three mud crab crops a year. Many farmers also stagger the breeding of mud crabs so they can have harvest year-round to avoid an oversupply during the main harvest seasons.
How to pick meaty live mud crab
To choose a good and meaty crab, you need to understand the life cycle of the crustacean. So often, some crabs turn out to be disappointing even when they are so active and solid-looking when you pick them out of the tank.
Very soon the crab takes on a more familiar form and continues to grow. The exoskeleton protects the crab-like a suit of armour. This hardshell cannot expand as the crab grows, so periodically the crab must shed its shell and develop a new and bigger shell in a process called moulting. Moulting begins with secretion of hormones from the female crab, after which both female and male crabs fast. Surviving off of its fat reserves, a crab will absorb as much calcium from its shell as possible (which aids in the development of the crab’s new shell). When the crab is ready to evacuate its old shell, a fracture opens along the underside of the back and the crab literally backs out of its shell. The crab’s new shell (which was developing while the crab was fasting) is soft and flexible. The crab expands its new shell by filling its body cavity with water but with little flesh. Once the crab’s shell starts to harden (which can take several days), the crab will resume feeding. Soon the crab will consume enough food to restore its fat reserves and replace the excess water in its shell with muscle. Then the cycle starts all over again. Hence, the key to choosing a good, meaty crab is to be able to pick one that is nearing the end of its current cycle rather than the beginning. Consequently, and recently moulted crabs will often contain mostly liquid or a jelly mass with little edible flesh.
Obviously, choose from the feisty and aggressive ones, or at least, those that are moving.
The trick then is to be able to tell the difference between an old shell and a new shell. Here are five signs to look for:
1.Colour
The new shell will have a nice shiny surface with bright colours and a distinct mottling pattern on its claw. Sometimes the shell will appear translucent. Whereas the older shell will be duller and more weathered with pockmarks and scratches. A worn shell suggests that the crab is ready to moult (shed its existing shell) and is well fed.
2.Crab’s claw
The large molar of the claw is a telling sign of whether the crab has been eating or not. The newly molted crab would have a molar that is well rounded while the old shell crab has a flattened molar. The smaller teeth in the new shell crab would be chiselled and sharp while those on the old shell crab would have been ground down and rounded. Mud crabs use their right claw to crush shellfish and their left to slash into flesh, so the most telling sign that the crab has been busy eating is to look at the molar of the larger right claw – for blunt or flattened teeth.
3.Underside
The underbelly of the new shell crab would look slightly blue and be quite flat whereas the older shell crab would look more yellowish and the segments would have a tell-tale bulge.
4.Abdominal plate
One of the most reliable signs that the crab is full of meat is to press on the abdominal plate. The empty crab would have a lot of giving while the meaty crab would have very little or hardly any give at all. This sign is the most reliable because a crab might have all the signs of a worn-out shell, but if it is kept for too long without any food, it would also start using up its energy stores and the flesh will start to atrophy. Once the flesh starts to shrink, the way to tell is by pressing on the abdominal flap as well as the segments.
5.Suss out the smell
Fresh crustaceans should be odourless. Steer clear of those that reek of ammonia.
Lastly, you should never buy crabs that are stored in water. When crabs first arrive from overseas, they are first refreshed by putting them into brackish water for 6 hours to get their gills wet, then they are stored out of the water in order to limit their activity so that they don’t use up their energy stores. Subsequently, the crabs are put into water for only 2 hours a day. Crabs can survive out of the water as long as their gills remain wet. If crabs are kept in a water tank, they tend to be very active and so they will use up their energy stores quickly and their flesh will start to shrink. So always buy crabs from a good supplier who stores its crab out of the water where they stay resting or asleep!
