The occurrence of natural fish kills, and their causes, in the Darwin-Katherine-Jabiru region of northern Australia

Abstract

Approximately 50 fish kills have been reported between 1978 and 1990 in the Darwin-Katherine-Jabiru region of the Australian wet/dry tropics. A large number probably go unreported because of the region's sparse population and the absence of any formal reporting scheme. The majority of fish kills occur on the coastal plains, whilst a smaller number have been observed in inland regions. Fish kills occur throughout the year, but are most common during the wet season, particularly from October to January when thunderstorm activity is frequent and watercourses commence wet season flow. In about half reported events between 100 and 1000 fish have died.

The fish kills have been attributed to natural causes unrelated to human activity. In the Jabiru area, fish kills at the start of the wet season have been caused by biotoxic concentrations of aluminium in naturally acidic run-off. Other fish kills have been ascribed to low oxygen concentrations resulting from the entry of high oxygen demand storm run-off into receiving waters, and the oxygen depletion of floodplain waters. These events highlight the significant impact run-off can have on receiving water quality in the wet/dry tropics, and the reduced capacity of warm waters to assimilate organic material with a high oxygen demand. The small proportion of fish kills which occur in the dry season are not associated with run-off events.