Fish deaths

Fish death (or fish kill) is an unexpected and short event where many fish die in a particular area.

Why fish deaths occur

Fish deaths are often thought to be from water pollution; however, many happen naturally. Worldwide, over half of reported fish deaths are due to natural causes, including:

lake turnover
algal blooms
blackwater events
infectious diseases
lifecycle events
rapid change in water temperature
decreasing water levels.

The Gold Coast experiences fish deaths every year. The likely cause of fish deaths is low dissolved oxygen levels due to such events as lake turnovers, algal blooms and blackwater events. The normal dissolved oxygen range in our waters is ideally between 6 and 8 milligrams per litre, however, these events can reduce dissolved oxygen levels to as low as zero. Very few fish species can survive conditions with dissolved oxygen below 3 milligrams per litre for an extended period of time.

Fish deaths can also be caused by human activity. We call this an 'artificial cause' and it includes things like:

pollutants from industrial effluent
excessive use of fertilisers and other agricultural chemicals.

If you see a fish death do not:

touch the affected fish or the water in the vicinity
collect any fish for samples or use as bait
consume any fish that have been caught in the area of the fish death.

What is lake turnover?

A lake turnover is the seasonal movement of water in a lake system. Shallow lakes have very little lake turnover. Larger and deeper lakes experience major changes as waters mix. During calm weather, water in deep lakes can separate into layers. The bottom layer tends to be colder than the top layer and is anoxic (lacking oxygen).

When a lake turns over, anoxic water is released to the surface and the two layers of water mix. This reduces oxygen levels and causes fish to die. A lake turnover can release hydrogen sulphide from the deep anoxic waters, which smells like rotten eggs.

What can be done to prevent fish deaths?

Council, residents, and businesses play a vital role in preventing fish deaths. We can:

keep organic material (leaves and grass clippings) off streets and out of stormwater drains
avoid over-irrigation
maintain septic systems
avoid using fertilisers and pesticides near the edge of water or before rainfall events.

How we are addressing the problem

We track and test the water quality of our natural and constructed waterways. Water quality data gives us information on the ecological health our waterways. It helps us to identify trends in water quality and develop strategies to deliver solutions to better manage our waterways.

When fish deaths occur, our Catchment Management Unit and Environmental Health teams investigate. 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.

Penalties

Under the Environmental Protection Act 1994, potentially allowing contaminants to enter stormwater drains, roadside gutters and waterways is an offence. It 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.