Growing mud crabs

Growout of juvenile mud crabs is a common practice overseas, but in Australia it has not yet progressed beyond pilot-scale operations. In aquaculture farms, post-nursery crabs reach marketable size and maturity in 6–7 months compared with 18–24 months under natural conditions.

Farm design

Mud crab growout requires a substantial area of earthen ponds with access to quantities of brackish water. Farm design requirements for mud crabs are very similar to that for marine prawn production. Advances in high-density recirculating production systems may in the future remove some of the constraints on site area required and site availability, but this method of production will be capital-intensive.

Growout is currently being undertaken to a limited extent on prawn farms, which require only minor adaptation of facilities to accommodate the crabs. This is being explored as an enterprise option for remote northern communities.

Stocking rates

During growout in ponds, crab growth and survival are linked strongly to stocking rate. High stock losses due to cannibalism occur when crabs are grown to market size at greater than 3 per square metre. A stocking rate of around 0.5–1.5 per square metre may give the highest productivity and economic benefit in simple earthen ponds.

Mud crabs can be grown in highly intensive systems where they are held separately, resulting in far less loss of stock as there is no cannibalism. This is new technology and commercial production in these cellular systems is yet to be established.

Water quality and temperature

Juvenile and adult crabs are far more tolerant of temperature and salinity change than the larvae. Nevertheless, temperature should be kept below 32oC to avoid water quality problems and above 20°C as growth is greatly reduced at temperatures below this level.

Salinity does not appear to be important to survival provided it stays above 10 parts per thousand (ppt) and does not exceed 45ppt. The ideal salinity range for growth is about 15–25ppt.

Feeding

In the natural environment, mud crabs eat mainly shellfish and crustaceans. This is difficult to achieve in aquaculture conditions, so alternatives are used. Overseas it is common practice to feed trash fish or fish waste or other waste streams; however, this can be inadequate for maximum growth. Feeding with raw animal material is also prone to problems, with water quality deterioration due to fouling.

For this reason, most aquaculture operations should predominantly use formulated dried pelleted rations. Diet development work is underway to find a cost-effective formulation specifically for mud crabs. Good results have been achieved using pellets designed for marine prawn aquaculture.

Shelter

Crabs grow by moulting. They shed their exoskeleton and expand the new, soft shell by inflating it with water and then harden the new larger shell. During moulting, the crab is vulnerable to attack by hard-shelled crabs. This moult-related cannibalism is an important limitation on the density at which crabs can be grown in ponds.

Providing shelter for vulnerable crabs reduces the risk of attack and cannibalism during the short time their shells are hardening. Onion bags or shadecloth, masonry blocks, short lengths of pipe or car tyres can be used to provide shelter. Providing shelter in the pond improves survival and productivity; however, it also complicates pond management. The amount of shelter 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.

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