Notes on rearing of mud crab
(Scylla serrata)
David Mann, Animal Science
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This is a regulated species. See Recreational fishing rules in Queensland - Tidal waters for the current regulations.
Seed production
Hatchery
Mature female mud crabs kept in large tanks or in ponds under suitable conditions will extrude (spawn) eggs. An egg batch contains about two to five million eggs and a female may extrude more than one batch in her life. The eggs take 12 to 18 days to hatch depending on temperature. The fully planktonic larvae, called zoea, grow through five stages over two to three weeks at 27-28° C before changing into a megalopa.
Larvae should be maintained in very clean seawater ensuring that high quality conditions are maintained throughout the entire larval culture period. They are typically fed rotifers (Brachionus sp.) at the beginning of the larval period and brine shrimp (Artemia) thereafter.
The megalopa is the first stage to have claws, becoming more crab like, but still actively swims, voraciously feeding on artemia, other larvae and fresh or artificial diets. The megalopa stage lasts about 7 to 10 days before metamorphosing into the first crab stage, which is approximately 4 mm wide and weighs 0.025g. Rearing of larvae is readily accomplished under laboratory conditions and commercial scale production is now occurring to a limited extent. Developments in four key hatchery aspects have lead to improved success:
- hygiene during the egg and larval phase is critical to success through to post-larvae in the hatchery;
- maintenance of water quality parameters such as temperature, salinity, pH, oxygen and ammonia at optimal levels is vital;
- high levels of quality live food as well as nutritious particulate diets have to be supplied to the larvae; and
- reduce crowding of larvae near the end of the cycle as it can lead to heavy losses due to cannibalism.
Nursery
Hatchery produced megalopa or first stage crabs are typically reared in nursery systems to a more advanced crablet stage, 10 to 40mm across, prior to seeding into a growout system. During the 3 to 6 week nursery period the crabs are benthic and can be successfully reared in shallow nets, tanks or ponds at densities greater than 50 per m2 . While the cannibalistic nature of the crabs can lower production rates, the inclusion of additional habitat into the system, such as mesh or shells, greatly improves survival. The crabs are fed a diet of artificial formulated feeds but will readily consume minced fresh diets such as mussel or fish meat. Water quality is less critical at the crablet stages as compared with the hatchery phase.
Growout
Growout of juvenile mud crabs is a common practise overseas but in Australia it has not yet progressed beyond pilot scale operations. In aquaculture post-nursery crabs reach marketable size and maturity in 6 to 7 months compared with 18 to 24 months under natural conditions.
During growout in ponds crab growth and survival are linked strongly to stocking rate with high stock losses due to cannibalism occurring when crabs are grown to market size at greater than 3 per m2. A stocking rate of between 0.5 - 1.5 per m2 gives the highest productivity and economic benefit in simple earthen ponds. It has now been shown that mud crabs can be grown in intensive systems where they are held separately, resulting in far less loss of stock. Commercial production in these cellular systems is yet to be established.
Juvenile and adult crabs are far more tolerant of temperature and salinity change than the larvae. Nevertheless temperature should be kept below 32° C to avoid water quality problems and above 20° C as below this temperature growth is very slow. Salinity does not appear to be important to survival provided it stays above 10 ppt and does not exceed 45 ppt. The ideal salinity range for growth is approximately 15 to 25 ppt.
Diet
In the natural environment mud crabs eat primarily shellfish and crustaceans. This is difficult to achieve in aquaculture conditions so alternatives are used. A pure trash fish or fish waste diet results in slow growth and poor condition of the crabs. Similarly a diet of abattoir waste material would be inadequate for growth. Feeding with raw animal material is also prone to problems with water quality deterioration due to fouling. Dried pelleted rations are the best option and can be formulated according to the specific dietary requirements of the crabs. Formulation of the best composition for a mud crab growout diet is currently under investigation however very good results have been achieved using pellets designed for marine prawn aquaculture.
Shelter
Crabs grow by moulting; they shed their exoskeleton, expand the new, soft shell by inflating with water and then harden the new larger shell. During moulting the crab is vulnerable to attack by hard shell crabs and an important limitation on the density at which crabs can be grown is the rate of moult related cannibalism. Provision of shelters for vulnerable crabs allows the crab to escape attack during the short period before it hardens. The inclusion of shelters or habitats into the pond improves survival and productivity however it also complicates pond management. The amount of shelters in the pond is controlled by practical limitations on the maintenance of pond quality, labour and harvesting. The maximum economic density at which crabs can be grown also depends on the crab size at harvest. The smaller the size the greater number produced. Segregation of crabs with regard to size and sex may also be beneficial in reducing cannibalism.
Commerical production
The mud crab remains a species with good potential for aquaculture due to its fast growth and good market acceptability and price. However a mud crab aquaculture operation requires significant capital input for both the seed production and growout phase. The larval stage requires controlled conditions in a properly equipped hatchery on a site with a source of high quality seawater. Pond growout requires a substantial area of earthen ponds with access to large volumes of brackish water. Advances in high density recirculating production systems may remove some of the constraints on area required and site availability. A growout operation needs expertise and knowledge of husbandry of crustaceans, water quality control, pond management, nutrition, processing and marketing. The state of knowledge of mud crab aquaculture has made significant advances with all phases conducted routinely at research facilities and also on commercial aquaculture farms.
Potential exists to make use of substantial export markets and the development of new products, both for the Australian and export markets. One of these new products is 'soft-shell' crab. Domestic and international markets exist for crabs that are harvested when they have just completed moulting. Technology is currently being developed that will automate the difficult harvest process.
An important consideration for the viability of aquaculture of mud crabs in Australia is the cost of production. This is highly dependant on the production model being used and needs to be assessed on a case-by-case basis. Overseas experience shows that it can be a very profitable enterprise.
In Queensland, a permit is required to hold female crabs which are protected under the Fisheries Act 1994* . Persons intending to develop an aquaculture enterprise in Queensland are required to apply for an aquaculture permit.
Additional fisheries information
Check FISHWEB at: www.dpi.qld.gov.au/fishweb/ or contact: Department of Primary Industries Business Information Centre: Telephone: 13 25 23 (cost of local call anywhere in Queensland).
* For further information on current regulations, please see Recreational fishing rules in Queensland - Tidal waters .
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
File No: F0019
Last
updated 10 January 2005
