Photo credit: Kevin Fitzsimmons (USA)
Review: Abdel Rahman El Gamal (Founder of the website)
Increased market demand as well as the price of mud crabs, coupled with dwindling wild stocks, resulted in great interest in mud crab aquaculture in many countries especially in Asia where crabs are at highest demand.
Among the four mud crab species, the largest and the most broadly distributed species is Scylla serrata while Scylla paramamosain is the most important farmed species because of its dominance in China and Vietnam. At the present, most of mud crab production comes from China while sizeable amounts of farmed crabs are also produced in other Asian countries, like Vietnam, the Philippines, India and Indonesia.
Most mud crab farms rely on wild caught 10-100 g juveniles. The collection of mud crab “seed” is often very tedious and less predictable and would negatively impact natural crab populations. Based on that, the establishment of mud crab hatcheries is highly justifiable. In fact, there are a growing number of hatcheries for mud crabs in Asian countries.
Grow-out: The grow-out of mud crab can be done in monoculture or in polyculture with a variety of species, including seaweeds, marine shrimp (black tiger prawn, Penaeus monodon) and marine fish (milkfish, Chanos chanos). Stocking densities vary significantly depending on the farming system and may range from as high as 20,000 to 30,000 carb juveniles/ha to as low as 500 crabs per hectare such as in mangrove ponds. In case of earthen ponds, its sizes are generally between 0.3 and 0.5 hectar with a water depth between 0.8 and 1.5 meters; ponds are usually fenced to prevent escapes.
The grow-out of mud crab in mangroves could be extensive in mangrove ponds or/and semi-intensive culture in enclosures. In the extensive pond culture, mud crabs are often stocked at very low densities of about one crab/20 m2. In such system, crabs feed on natural food and hence no external feed is provided. In mangrove enclosures, semi-intensive system is practiced where the stocking density is generally between one and five crabs per square meter. In this system, supplemental feed such as trash fish is provided. It needs to be noted that higher stocking densities are usually accompanied by higher levels of 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 crabs grow. The culture period for grow-out from juvenile to market size is generally five to eight months, with survivals of 30-70%.
The major constraint restricting further expansion of mud crab culture is the limited supply of crab ‘seed’ required for grow-out even at the current size of the mud crab culture industry. In other words, the sustainable farming of mud crabs as well as future expansion has to depend on hatchery-produced crab seeds.
Note: The inserted picture is for an Indonesian woman holding a specimen of mud crab grown in a shrimp-seaweed polyculture pond.