Acid sulphate soils form when iron pyrite oxidises to sulphuric acid on atmospheric exposure, lowering soil pH below 4. In the Richmond River estuary, northern New South Wales, flood mitigation, drainage works and floodplain excavations have augmented acid sulphate soil formation by increasing pyrite availability for oxidation. These engineering works have facilitated the transport of acidified water and have impeded recovery from tributary acidification. The increased frequency and duration of acidification are associated with elevated concentrations of inorganic monomeric aluminium and dissolved iron, and major iron hydroxide precipitation, with deleterious effects on exposed aquatic biota and habitat. Fish kills and an outbreak of epizootic ulcerative syndrome (EUS) have been recorded from acidified sites in the estuary.

Broad, non-septate, fungal hyphae are a eonstant finding in the neerotising dermatitis lesions and dermal ulcers eharaeteristie of red spot disease. During a one-year period, fungi with hyphae morphologically consistent with those seen in histologieal sections of lesions were recovered from 27 out of 28 lesions on sea mullet, Mugil cephalus L., yellowfin bream, Acanthopagrus australis (Owen), and sand whiting, Sillago ciliata Cuvier, collected from three widely separated river systems in eastern Australia. Cultural and morphological characteristics, including spore formation, of 24 of these isolates were examined. In addition, toleranees to different salinities, growth on different media and growth at different temperatures were studied for three representative isolates, one from eaeh species of fish. These studies suggest that all isolates belonged to a single speeies within the genus Aphanomyces. Their role in the pathogenesis of red spot disease, and their relationship with other Aphanomyces spp. affeeting fish, is diseussed.

Epizootic haematopoietic necrosis virus (EHNV) is an iridovirus confined to Australia and is known only from rainbow trout Oncorhynchus mykiss and redfin perch Perca fluvjatihs. Outbreaks of disease caused by EHNV in trout populations have invariably been of low severity, affecting only O+ post-hatchery phase fingerlings

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