Winter Mortality Winter mortality is believed to be caused by mikrocytos roughleyi , a protoctisian parasite. It occurs in NSW between Port Stephens and the Victorian border - the southern, cooler half of the growing area. Mortality can occur in winter - but most fatalities in oysters occur in the warmer spring months of September or October. The amount of kill can vary between years, estuaries, within estuaries, and even between adjacent leases. Stick oysters are less susceptible than tray oysters.
Farmers can also sell their oysters before the onset of winter.
This is believe to be caused by Marteilia sydneyi , also a protoctisian parasite, but the way it infects oysters is not known. On the east coast of America, MSX disease, similar to QX, hindered oyster growth. The wild oysters had a natural resistance, so the Americans instigated a breeding program. In Australia, however, oysters are transported from the South to the North in a half grown stage. They are then grown and sold before Autumn.