Lesions present on sea mullet, Mugil cephalus L., during the first 3 weeks of red spot disease outbreaks are described. Necrotizing dermatitis is a severe, locally extensive, granulomatous lesion associated with invasion of dermis and underlying skeletal muscle by numerous, non-septate, fungal hyphac 12-I8yu,m in diameter. Erythematous dermatitis is a mild to severe, foeal, chronic active dermatitis without fungal involvement. Lesions intermediate between these two forms, with small to moderate numbers of fungal hyphae in dermis and skeletal muscle also occur. Findings indicate that erythematous dermatitis lesions and intermediate-type lesions subsequently resolve, while necrotizing dermatitis lesions consistently develop into dermal ulcers, with associated severe neerotizing granulomatous myositis. Generally, dermal ulcers occur significantly more often on posterior and dorsal areas of the body surface than on anterior and ventral areas. Lesions heal by combinations of epidermal and dermal repair, fibroplasia, destruction of fungi, removal of necrotie skeletal muscle and regeneration of myofibres. Atrophy of exocrine pancreas occurs in both diseased and clinically normal fish, but is generally more severe in diseased fish. The possible roles of suspected disease determinants, including falls in dissolved oxygen concentrations to sub-lethal levels prior to red spot disease outbreaks, are discussed.

The infection is widespread in Australian mullet, but rarely causes significant economic loss.

A cutaneous ulcerative disease (red spot) in sea mullet, Mugil cephalus L., from the Clarence River, New South Wales, Australia, was first reported in 1972. In this study, reports of disease outbreaks have been compared with rainfall and river flow records for the period from 1972 to 1988. Detailed disease prevalenee, rainfall, river flow and water quality data were compared for the period from 1985 to 1988. Significant correlations between weekly rainfall in the lower catchment and the prevalence of early stage lesions have been found. Progression to later stages of the disease occurred after rainfall and high river flows, which also caused rapid changes in various watcr-quahty eharaeteristies such as salinity, temperature, dissolved oxygen, pH and*turbidity. Organochlorine insecticide residues were not found to be associated with the disease in this area. The rainfall record since 1902 does not explain the absence of earlier reports of the disease. It is postulated that extensive structural developments for flood mitigation purposes and the increase in agricultural eultivation in the lower Clarence catchment during the last 20 years may be factors associated with the onset of the early stage of red spot disease. Fish in this early stage of the disease may then develop dermal ulcers under the stressful river conditions typically present during the autumn (high rainfall) season.