Fish kill facts
Fish kill or fish die−off is an unexpected and short event where a large number of fish die in a particular area.
What causes fish kills
Although fish kills are thought to be from water pollution, many happen naturally.
Worldwide, over half of reported fish kills are because of natural causes, including:
- infectious diseases
- life−cycle events
- decreasing water levels
- varying water quality
- rapid change in water temperature
- and changes in salinity levels caused by heavy rain and lake turnover.
The most common cause of fish kills is from the loss of dissolved oxygen in the water. This is as a result of decomposed organic matter, acid sulfate soil run-off and disposal of waste.
The city experiences up to two fish kills per year with low dissolved oxygen levels as the likely cause. The normal dissolved oxygen range in our waters is between six to eight milligrams per litre. Very few fish species can put up with long-term exposure below three milligrams per litre.
A lake turnover is the seasonal movement of water in a lake system. Shallow lakes have very little lake turnover. Larger, deeper lakes experience major changes as waters mix. During calm weather, water in deep lakes can stratify into separate layers. The bottom layer tends to be colder than the top layer and is anoxic (no oxygen).
When a lake turns over anoxic water is released. This causes reduced oxygen levels and temperatures which causes fish to die. A lake turnover can release hydrogen sulphide from the deep anoxic waters which smells like "rotten−egg" gas.
What can we do?
Fish kills caused by artificial occurrences can be prevented. These include pollutants from industrial effluent, agricultural chemicals, sewage and spillage.
Residents can play a vital role in helping to prevent fish kills.
Here are some things we can all do:
- keep organic material (leaves and grass clippings) off streets and out of stormwater drains
- avoid over irrigation
- maintain septic systems
- don't use fertilisers and pesticides near the edge of water, or before rainfall events.
Under the Environmental Protection Act 1994, potentially allowing contaminants to enter stormwater drains, roadside gutters and waterways is an offence and may attract an on-the-spot fine to the value of 15 penalty units (for an individual) or 75 penalty units (for a corporation), or prosecution for multiple offences. See our Penalty units page for unit values.
We track and test water quality of a number of natural and constructed waterways throughout the Gold Coast. Water quality data gives us information on the ecological health of the City’s waterways and identify trends in water quality.
Our Catchment Management Unit and Environmental Health team investigate fish kills. They determine the likely cause and recommend a course of action. Officers then coordinate a clean−up and consult with the relevant government agencies as required.
It is very important to protect our Gold Coast waterways. Report any evidence of pollution as soon as possible.
Time can be critical in preventing further environmental damage. If you become aware of pollutants in your local waterway please contact us as soon as possible.
If you see a fish kill:
- report it to the City
- do not touch the affected fish or the water in the vicinity
- do not collect any fish for samples or use as bait
- do not consume any fish that have been caught in the area of the fish kill.
Want to report a City problem, complaint, or issue? Go to Report a problem.
Have a general enquiry? Email firstname.lastname@example.org
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