Keeping Mud Crab Broodstock Healthy
Keeping mud crab broodstock healthy, well fed and stress-free is essential for successful larval production. The facility should have tanks for initial treatment of incoming broodstock, tanks for holding broodstock and spawning tanks.
All broodstock husbandry steps should have capacity for temperature control if ambient temperature varies diurnally more than 2 °C or goes outside the optimal range.
The highest level of temperature control is required for the incubation and hatching steps. Immersion heaters can be used but electrical cables need to be protected from damage by the crabs.
The broodstock are typically held in a maturation tank for a period of several weeks or more. Under adequate conditions, the broodstock can be held communally at up to 1.5/m2 and typically large (>10 m3), shallow (80–100 cm deep) tanks are used.
It has been found that keeping mud crab broodstock in low light conditions appears to minimize stress levels, which in turn leads to better reproductive performance.
Therefore, broodstock need to be housed in facilities constructed so that light levels can be kept low, or existing facilities need to use shade cloth or other similar interventions to enable a low light regime to be established. The inclusion of shelters in the broodstock tank provides refuge for the crabs that may further minimize stress and fighting among the stock.
Adult mud crabs are excellent escape artists. Water, air supply or heating elements provide crabs with an opportunity to pull themselves out of tanks. Often, broodstock areas will have some sort of fencing around them to ensure that valuable broodstock cannot walk out of the facility if they escape from the tanks. As mud crabs are able to escape from some plastic mesh cages by breaking the strands with their claws, the fencing should be of a suitable material.
Female mud crabs require access to a sandy bottom to spawn their eggs successfully. While mud crabs can be kept in tanks that have sand bottoms, either on the tank floor itself or an aerated raised floor (Figure 4.3), such tanks are relatively difficult to maintain. All that is required is a sand tray (Figure 4.4), so that a female that is about to spawn can access the sand when she needs to.
The female will excavate a shallow depression in the sediment and, by extending her abdomen over it, create a chamber that allows extruded eggs to attach successfully to the setae of her pleopods. When performed successfully, the mother crab can attach several million or more tiny eggs under her abdomen with very little loss. Failure to provide sand to broodstock tanks will result in poor, often aborted, spawnings and low hatching rates should the hatching be successful.
Maintenance of hygiene is very important to successful broodstock maturation, and uneaten feed and waste material need to be regularly removed. This is typically achieved by the use of nets and siphons.
Mud crab broodstock consume a large quantity of fresh diets, so a high level of water renewal per day is required in order to maintain water quality conditions.
Where water use is restricted, a recirculating system incorporating at least particulate removal and biofiltration can be used.
A foam fractionator and UV chamber will also assist water quality maintenance.
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