Red spot disease (RSD), a cutaneous ulcerative disease of estuarine fish, was first reported from the Burnetfc River in Queensland in 1972. By 1985, when the present application was submitted, seasonally recurrent RSD outbreaks were adversely affecting commercial fisheries in many east coast estuaries. For example, in 1983, commercial fishers on the Clarence River, which supports the most productive estuarine fishery in NSW, expressed concern that RSD was causing substantial economic losses and possible long term damage to the fishery. A preliminary study, conducted in 1985 in response to these concerns, showed that approximately 20% of the commercial sea mullet catch in that year on the Clarence was discarded due to RSD (Virgona, 1992). A previous FRDC-funded study in Queensland had suggested that RSD ulcers were initiated by the bacterial agent, Vibrio anguillarum, after susceptible fish were stressed by exposure to rapid salinity and temperature changes following rain events (Burke and Rodgers, 1981; Rodgers and Burke, 1981). The subsequent preliminary Clarence River study failed to confirm the association of V. angulllarum with early ulcers ( Callinan and Keep, unpublished data).

The present project was intended to examine the impact of RSD on the fishery and to identify causes of the disease.