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Fish deaths in the Basin

A mass fish death is a sudden and unexpected mass mortality of wild or cultured fish. There are many causes of fish kills and often they are caused by natural events.

Fish death event can occur in both freshwater systems and estuarine systems and are more likely to occur in summer (January and February). Events in summer are more likely due to:

  • higher water temperatures (and consequently lower dissolved oxygen levels)
  • frequent severe and sudden storm/ flood events
  • lower water levels in freshwater river systems during these months. 

There are a range of causes of fish deaths, including water quality changes, pollution, infection, or a combination of these. In almost half of all reported fish kill events the cause is unknown.

Some causes include:

  • dissolved oxygen levels (e.g. caused by bushfires, flood events, decaying vegetation matter)
  • high or low temperatures
  • drought or dry conditions
  • algae and algal blooms
  • acidic runoff
  • pesticide/chemical pollution
  • dumping of waste fish or bycatch
  • disease/infections
  • dam releases.

Often, a large percentage of fish kills are caused by several factors acting together. For example drought conditions, blue-green algae and a change in temperature, resulting in reduced dissolved oxygen, collectively are thought to have led to the fish death event in the Lower Darling in January 2019.

Fish deaths are usually the result of sudden events or changes in the local environment. As far as possible, fish will avoid adverse environmental conditions and swim to another area to avoid harm. However, if the entire or a large proportion of the waterway is affected, there are barriers to movement, or the adverse conditions appear very rapidly, then fish are unable to relocate and a fish death event often results.

Areas of risk

A number of locations across the Basin have been identified as risk areas for fish death events. Basin governments are already taking action on the ground to mitigate these risks and respond to recent events.

Actions underway include:

  • adding water can protect key refuges for native fish by improving water quality. During drought, there is limited water available and water storages are low, meaning this option is not always available.
  • aerating water can increase the amount of oxygen in the water in targeted areas, improving water quality. Aerators can only improve oxygen levels for a small area (about the size of a tennis court).
  • moving fish to key waterholes can protect these animals in times of drought. This method is not an option when fish are too stressed and are unlikely to survive the journey.
  • new technologies are being considered to increase turbulence in the water, which improves water quality. Many technologies are in trial stage and a full evaluation of these methods is yet to occur.

A recommended action plan has also been provided to the Australian Government Minister responsible for water.

If you want to report on fish deaths please use the following contact numbers:
  • New South Wales Fishers Watch hotline: 1800 043 536
  • Queensland Department of Environment and Science: 1300 130 372
  • Victoria Environmental Protection Authority pollution hotline: 1300 372 842
  • South Australia Fishwatch Hotline: 1800 065 522
  • Australian Capital Territory Access Canberra: 13 22 81

Reduced dissolved oxygen and its effects on fish

Low levels of dissolved oxygen can cause stress (and even death) to fish and other aquatic animals which rely on oxygen in the water to breathe.

Dissolved oxygen in water comes from both the atmosphere—by exchange across the air-water interface, mostly from wind and wave action, or agitation during flows in streams and rivers—and is released by aquatic plants, via photosynthesis.

The normal range of dissolved oxygen for water is between 6-8 milligrams per litre (mg/L). However, this varies between coastal and inland rivers and estuarine and marine waters.

Most aquatic animals, including fish, extract the oxygen they need from the water through their gills. Very low levels of dissolved oxygen will cause suffocation and death of aquatic animals.

It is not necessary for the water to become completely deoxygenated for a fish kill to occur. The critical minimum level varies with different species and different physical conditions, but as a general guide, few fish species will tolerate prolonged exposure to levels below 3 mg/L. Larger fish species, such as Murray cod, tend to become stressed and/or die first due to their greater oxygen requirements.