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Government of Western Australia - Department of Fisheries

Fish kills

Mass deaths of fish or other aquatic animals such as crabs or prawns (‘fish kills’) have been recorded in Australia (and overseas) for thousands of years and are often the result of natural events.

However, many are due to the impacts of humans on the natural environment. An increasing population, poor land-use practices and a drier climate have the potential to increase the frequency of fish kills.

As soon as a fish kill is reported, we action an Incident Management Response Plan to investigate the cause.

What is a fish kill?

A fish kill is usually characterised by a large number of fish dying over a short period of time, often within a particular area.  Their severity can vary, according to the circumstances and how quickly the kill is reported – if the cause can be rapidly identified, further deaths may be preventable.

For this reason, it is very important that accurate data is quickly gathered at the site of the fish kill by an officer who has been trained to respond to this kind of incident.

Reasons for a fish kill

Man-made fish kills can be caused by pollutants, such as industrial effluent, agricultural chemicals, sewage and spillage from transport accidents.

Natural causes include algal blooms and infectious diseases (caused by viruses, bacteria, protozoans and fungi), gas bubble disease (similar to the ‘bends’ in humans), bushfire ash, coral spawning, lifecycle events (such as spawning and migration) and oxygen depletion (caused by high organic loads in the water after rain, algal blooms, high temperatures and low water flow).

Further details are given in the table below.

​Conditions Causes​ Major signs​
​Low dissolved oxygen Algal blooms, eutrophic conditions, decomposition of organic matter, coral spawning, recent rains.​ Gulping at the surface, larger fish dying first – affects a range of species – particularly benthic, all sizes.​
Diseases​ Stress caused by sub-optimal conditions (secondary disease) and primary disease due to susceptibility to a particular pathogen.​ Bleeding on the skin surface, lesions, flashing behaviour, growths; inability to maintain buoyancy – affects fish of the same species or from various species.​
Sunburn​ Animals caught in shallow water​ White patches of peeling skin – mainly surface-dwelling species.​
Life cycle​ Spawning (fish or coral)​ Natural death after spawning, same species and same size animals, re-occurring event.​
Poisoning and chemical pollution​ Lethal and sub-lethal effects​ Affects a large range of species.​
Sudden change in physical water parameters​ A drop in salinity, dissolved oxygen, pH due to large rainfall in saline water, habitat disturbance; change in water temperatures; turbidity and dissolved solids.​ Usually affects species that are territorial and/or unable to get away from the adverse conditions.

If you see a fish kill

Report it immediately to your nearest Fisheries office or Department of Water office.

You can also report a fish kill to our 24 hour FishWatch service:
T: 1800 815 507

Keeping yourself safe

If you see a fish kill do not:

  • Touch the affected fish or the water in their vicinity.
  • Collect any fish for samples or use as bait.
  • Consume any fish that have been caught in the area of the fish kill.

Last modified: 4/03/2016 5:10 PM

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