Winter Mortality
Cause
Protozoan parasite, Bonamia roughleyi
Species affected
Sydney rock oysters in their third winter (just before they reach market size) are most susceptible. In severe outbreaks, small spat (baby oysters) may also be affected.
Oysters grown on trays are no more susceptible to winter mortality than oysters grown on sticks.
Known locations
The area between Port Stephens and the Victorian border is particularly susceptible to winter mortality
Impacts
Outbreaks of the disease are patchy the known range and, while mortality may occur in winter, most of the oysters do not die until the warmer spring weather of September or October. Winter mortality may kill up to 80% of oysters in a local area and the severity of the kill can vary markedly between not only years, but also between estuaries, between adjacent leases and even within leases. Dry autumns (high salinities), early winters and low temperatures increase the likelihood of a severe kill.
Reducing the impacts
Farmers can, to a large extent, reduce the impact of this disease by moving oysters further upstream to areas of lower salinity before the end of autumn (May), and by increasing the growing height of the oysters to 150 mm above normal growing height. Alternatively, farmers may sell their oysters for consumption before the onset of the disease.
Although triploid Sydney rock oysters are not currently available commercially, they suffer less than half the mortality of diploids exposed to winter mortality. Triploid oysters (which have three sets of chromosomes instead of the usual two sets), reach plate size (50 g) six months earlier than wild caught oysters (diploids) of the same age, and hold their meat condition better in winter and spring. Although triploids may suffer discolouration of meat in summer and autumn, for farmers who want to grow a winter crop that is partially resistant to winter mortality, triploid oysters may be a future option.
NSW DPI has developed breeding lines that are resistant to two major diseases of Sydney rock oysters. Winter mortality disease-resistant breeding lines are available for commercial production. So far (April 2007), mortality from this disease has been cut by half through selective breeding. Further progress is being made to reduce kill from winter mortality much further. Tests need to be carried out to determine if the reduction in losses from winter mortality by triploidy and selective breeding can be combined in the same oyster to enhance survival.
Mudworm
Cause
The polychaete worm Polydora websteri.
Species affected
Mudworm infests most commercial molluscs, including Sydney rock oyster (Saccostrea glomerata), Pacific oysters (Crassostrea gigas), blue mussels (Mytilus galloprovincialis), flat oysters (Ostrea angasi), scallops (Pecten fumatus) and abalone (Haliotis rubra).
Known locations
Across NSW the siltation of shell beds and dredge beds and the appearance of mudworm early in the 20th century forced the industry into intertidal culture, which protected oysters from mudworm. Dredge bed culture, which is an effective way of oyster farming, has virtually disappeared from NSW because of siltation and mudworm.
Impacts
The adult mudworm is up to 25 mm long, 1 mm wide and red in colour. It lives on the inside of the oyster shell, where it gives rise to the formation of a mud blister but maintains a tube across the lip of the shell to the outside.
Healthy, rapidly growing oysters are able to cover the worm and its mud patch with shell and recover from the attack, whereas weak oysters usually succumb and die. Losses of oysters due to mudworm are often very high, and the remaining infested oysters become unsaleable because of their poor condition and unsightly, foul-smelling mud blisters, which rupture easily when the oysters are opened.
Reducing the impacts
Infested oysters can be left out of water in the shade for up to ten days to kill mudworm and/or flatworm (see below); some oyster farmers wash mud from oysters on their leases using boomsprays on punts, to lower the risk of infestation.
Some farmers have successfully used the following method to kill mudworm: Oysters are left out to dry for 4–5 days, followed by a 2–3 hour bath in an iodine-based disinfectant solution at 0.1 g of active iodine per litre of seawater, prior to returning oysters to the lease. It is claimed that this treatment is more successful in treating bad outbreaks of mudworm than drying alone.
Flatworm
Cause
The flatworm, Imogine mcgrathi, also known as wafers or 'leeches'
Species affected
Sydney rock, Pacific, flat and other oyster species as well as other commercial bivalves such as mussels.
Known locations
Identified as a threat to oyster production in NSW as early as the 1890's.
Recently, I. mcgrathi were found at an average density of 376 per metre of mussel culture rope in Twofold Bay and were thought to be responsible for considerable mussel losses. Since the recent increased use of fine mesh trays, cylinders, and baskets for oyster nursery culture of hatchery produced stock, mortality from flatworm has become a serious problem for Sydney rock, Pacific and flat oyster production in NSW. It seems that the fine mesh protects the flatworm from desiccation, allowing them to feed on oyster spat. Regular inspection of spat in nursery culture and increasing the mesh size assist in protecting against this predator.
Anecdotally, it would appear that flatworm numbers are greatest in periods of prolonged drought when estuary salinities are relatively high for extended periods. The reason for this is unclear, as adult I. mcgrathi can tolerate reduced salinities for some time, although reduced salinity may affect other aspects of flatworm survival (e.g., reproductive behaviour).
Impacts
In laboratory studies, flatworms have been found to eat oysters and mussels by entering the shells and everting their stomachs to engulf their prey. Generally, flatworms have been found to consume oysters at a rate of approximately 1 oyster per flatworm per month. When flatworms occur in high numbers, they can have pronounced effects.
Regardless, exposure to freshwater or saturated brine solutions is an effective means of controlling flatworms, with baths of 15 minutes sufficient to kill adult flatworms.
Reducing the impacts
Infested oysters and other shellfish can be left out of water in the shade for up to ten days to kill mudworm and/or flatworm.