Seasonal variation in the prevalence of ‘red spot’ disease in estuarine fish with particular reference to the sea mullet, Mugil cephalus L.

In March 1972 an epizootic of a non‐specific fish disease (red spot), was reported in estuarine fish stocks of the Burnett River in central Queensland, Australia. The disease, characterized by the presence of external lesions, infected a wide range offish species. Red spot has now reportedly spread to the river systems of New Guinea in the north and southwards to central New South Wales. The sea mullet, Mugil cephalus, was selected as a target species for the monitoring of epizootics within the Noosa River system of southern Queensland. Disease epizootics appeared to be related to the occurrence of certain environmental changes such as low or rapidly changing temperatures and rapid or prolonged depressions of salinity in the estuarine habitat. The effect of crowding, migration and spawning upon the transmission of this disease is also discussed. Simultaneously conducted microbiological investigations indicated that the bacterium, Vibrio anguillarum was the causative agent of red spot epizootics.