How Long Do Mud Crabs Take To Grow

Once a female spawns and has an egg mass under her tail, she should be transferred to an incubation system where the water quality and hygiene conditions can be controlled to a high level. Typically, females are put into a hatching tank, with just one crab per hatching tank, so that larvae from each female can be monitored.

A separate system is required for incubation because the egg mass is highly susceptible to parasitic, bacterial and fungal infection. Incubation tanks can be relatively small, 100–500 litres, and, generally, several are required in a hatchery to accommodate multiple spawners held individually.

During the egg incubation period, the mother crab does not require feeding, which means that there is far less waste produced and maintenance of a high level of hygiene and water quality is achieved by simple aeration and modest flow through of seawater treated to the same level as used in the hatchery.

In some locations, fungal infection is very difficult to prevent, and antimicrobial treatment of the mother and the eggs, such as a formalin bath, may be required to control it. The egg mass should be inspected regularly to identify infections and monitor egg development.

The incubation tank can be used for hatching and collection of the newly hatched larvae, but because of the very large number of larvae that hatch at once, it is recommended that a separate hatching tank of 400–1 000 litres is used. This tank requires a very high level of water treatment, as newly hatched larvae are susceptible to a range of virulent bacteria and fungi.

LARVAL REARING

The larval rearing section of a hatchery does not require the very low light levels of the broodstock component of the hatchery, but direct strong sunlight should be avoided.

Mud crab larvae have been found to eat more at light levels of 1 000–6 000 lux, while below 1 000 lux, larvae both eat less and have increased mortality rates. As crab larvae exposed to 24-hour light exhibit decreased survival, compared with a 12-hour light/dark cycle, natural lighting should be the primary light source for larval rearing areas.

The larval rearing areas should be well ventilated, with a reasonably high ceiling to minimize humidity (Figure 4.7). There should also be adequate space between tanks and equipment to facilitate operation and provide access for equipment to be removed or serviced.

Adequate physical separation from broodstock and feed production areas is required in order to minimize aerosol drift and maintain a high standard of biosecurity. Appropriately located walls, or barriers of some sort of sheeting, can assist in separation. When tanks are not being examined, they can be covered with plastic or similar sheeting (Figure 4.8). This reduces temperature fluctuation in tanks, in addition to controlling aerosol sprays.

Larval rearing tanks of various designs have been used to culture mud crab larvae, including circular tanks with a conical base, hemispherical round tanks, parabolic tanks and rectangular tanks.

The colour of the tank in which mud crab larvae are grown has been shown to have a major impact on survival. Larvae grown in black tanks have significantly higher survival rates than any other tanks, with increasing larval survival recorded in increasingly darkly coloured tanks.

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