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Oyster predators and pests

Both spat and mature oysters fall prey to a wide range of predators and pests. In many cases, modifying farming techniques and implementing some form of control measures can significantly reduce losses.

Mudworm

The mudworm (Polydora sp.) is a segmented worm that infects oysters grown on, or near, the ground.

Mudworms do not directly harm oysters, but they compete with the oyster for food. Infected oysters not only endure reduced food but they spend a large amount of energy secreting shell to isolate the invader, which gives them less energy to grow. The major problem with infected oysters is that their market value drops because of unsightly brown blisters.

To prevent infestation with mudworm, oysters are placed above the mid-tide level. Although this reduces feeding time for the oysters, it also reduces the chance of mudworm larvae entering them.

Oyster drills

Drills attack oysters by boring a hole in the oyster's shell using their hard radular (or teeth). Once the drill reaches the soft tissue of the oyster it secrets digestive enzymes into the oyster, killing it. The drill then digests the oyster meat, sucking up the digested fluids.

Drills can be controlled for a prolonged time by dunking oysters in fresh water, or for a shorter period by dunking oysters in hot water. Alternatively, drills can be separated from oysters by drying the oysters out and placing them on 3cm mesh where the drills will fall through.

Barnacles and mussels

Barnacles and mussels inhibit oyster spatfall by colonising the spat collecting areas first, where they compete with the oysters for food and space. These pests can be avoided by placing the spat catchers out at the right time and height, avoiding the main barnacle and mussel spatfalls.

Mussels are restricted to the subtidal zone and low in the intertidal range but barnacles usually occur over the full intertidal range, and so are more of a problem. Once these pests are established, the only option is physical removal of the barnacles and mussels by hand picking or the use of a culling iron. However, care must be taken not to damage the young oysters during this procedure.

QX disease

QX oyster disease is caused by the microscopic parasite Marteilia sydneyi.

The spore-like parasite invades the oyster's digestive system and multiplies in the digestive gland or 'liver'. These spores are so numerous that the oyster is unable to absorb food and can starve to death within 40 days of infection. Oyster fatality in an infected bank can be as high as 95%. Although QX is usually fatal to oysters, there is absolutely no evidence that it is harmful to consumers.

A sign of QX infection is a mass mortality of oysters. A thorough and correct diagnosis can only be obtained by microscopic examination of the digestive gland for the detection of QX spores. Samples of oysters suspected of QX can be sent away for examination. Once infected, oysters cannot be cured of the disease.

QX is known to occur in patches from southern Queensland to the Georges River in southern New South Wales. The parasite commonly occurs in the western areas of Moreton Bay, including Pumicestone Passage and the Southport Broadwater.

It is still possible to farm oysters in QX-susceptible areas within certain operating constraints. The risk of infection is highest in the summer months (December to March). Many operators in Moreton Bay import large spat or bottle-size oysters from disease-free areas (mainly central New South Wales) and put these out on trays about April. The oysters are then harvested as bottle-size or plate-size by Christmas when at optimum condition and market demand is high. This leaves the summer months free for equipment maintenance.

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