Abstract
Sydney rock oysters ( Saccostrea glomerata ) were historically abundant throughout the intertidal and subtidal zones of the Pumicestone Passage. However, today subtidal oyster reefs are extinct, and loss of around 96% of the vertical zonation suitable for oyster habitation has occurred. European land use practices caused large increases in sediment flux into inshore regions by 1870. This may explain why subtidal oyster reefs declined after major flood events in the late nineteenth century, associated with infestations by spionid polychaete mudworms. Today, mortalities of S . glomerata in the region are associated with QX disease caused by Marteilia sydneyi . Increased virulence of M . sydneyi is likely due to a combination of increased abundance of intermediate hosts in habitat utilised by oysters, together with immunosuppression of the oysters. These processes are all driven by declining water quality derived from anthropogenic catchment development. Recently expanded marine park sanctuary zones fail to protect the ecosystem against continuing water quality decline. Rehabilitation of the ecosystem will require effective catchment management targeting reductions in nutrient and sediment loading, as well as restoration of oyster reefs using QX‐resistant oysters.