A large natural fish kill which resulted in the deaths of a total of more than 3400 fish comprising eight different species occurred in a freshwater billabong (lagoon or small permanent waterbody) on the Magela Creek in the subtropical Northern Territory of Australia in January, 1980. The fish deaths were assessed to be the result of a combination of natural acid water runoff into the billabong and resultant elevated aquatic biotoxic aluminium levels. Details of water quality changes which occurred during the event and analytical data for the fish killed are provided, together with some explanation of the sequences of occurrence of the fish kill.

In 1987, Toronto and Region (formerly Metro Toronto) was recognized as one of 43 Great Lakes Areas of Concern (AOCs) under the amended Great Lakes Water Quality Agreement (GLWQA). AOCs are locations where water quality and ecosystem health are considered to be severely degraded as a result of local sources of pollution caused by human activities that may affect the wider Great Lakes system. Under the GLWQA, the governments of Canada and the United States committed to restoring ecosystem health in the Great Lakes (IJC, 1987).Toronto and Region was identified as an AOC as a result of complex environmental challenges from several centuries of agriculture, industrialization and urban development that have dramatically reshaped the terrestrial and aquatic environments. The health of the waterfront is linked to conditions and activities in local watersheds, which provide a conduit to Lake Ontario by way of the rivers and creeksas well as storm and sanitary sewer systemsthat run through local communities.The boundary of the Toronto and Region AOC encompasses 42 km of waterfront together the north shore of Lake Ontario, along with the six watersheds from Etobicoke Creek in the west to the Rouge River in the east that drain into Lake Ontario (Figure 1). The watersheds of the AOC, which drain an area of approximately 2000 km 2 , originate from the southern slopes of the Oak Ridges Moraine (north of the City of Toronto), and are composed of more than 40% rural landuse, and contain Rouge National Urban Park, one of the world's largest national urban parks. At the same time, more than three million people live in the AOC drainage area and the City of Toronto is at the centre of one of the most densely urbanized and fastest growing areas in the Great Lakes.Historically within the AOC, a vast number of habitats have been degraded: wetlands have been drained and filled (e.g. Ashbridges Bay Marsh); forests and riverbank vegetation were removed; creeks were buried, channelized or concreted; shorelines were hardened; and dams and weirs were built that obstructed fish movement in rivers. Contaminants associated with runoff from stormwater and snow melt from local watersheds, and direct discharge of industrial and domestic wastewater, have created serious water quality impacts in local rivers, and along the waterfront. Spills, road runoff and historically unregulated chemical inputs to sewers from industries and residences contributed to a highly degraded aquatic environment.These multiple stressors polluted the local environment, impacted fish and wildlife habitats and populations (both directly and indirectly) and degraded water quality at beaches, leading to the impairment of several beneficial uses in the Toronto region, and forming the basis for the AOC designation.

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