Pest fish are almost impossible to eradicate from our natural waterways once they’re established, which means that our best strategy is to prevent them from being introduced in the first place.

Pest fish are a problem for our waterways because they:

• Outcompete native fish for food and habitat
• Prey on native species or their eggs
• Spread diseases and parasites
• Damage waterways by degrading water quality (stirring up sediment or increasing algae)

The Wet Tropics region in Queensland has the highest diversity of freshwater fish species anywhere in Australia. Assessments have shown that fish populations in the rivers of the Wet Tropics are generally in good condition, although the Barron River is the most impacted by invasive fish species.

There are two types of pest fish – exotic and translocated 

Some of the pest fish species that have already become established in the Wet Tropics, include:

• Tilapia
• Platys
• Guppies
• Swordtails

Exotic Pest Fish 

Exotic pest fish are species that have been introduced from overseas. Examples include tilapia and European carp.

Tilapia is a highly invasive and aggressive breeder. It can reproduce so prolifically that it displaces whole fish communities. Originating from Africa and the Middle East, Tilapia was first reported in the Wet Tropics in the 1970s in the Barron River and there are now two species that have become naturalised in this catchment – the Mozambique Tilapia and the Spotted Tilapia.

Translocated Fish

Translocated fish are Australian species that have been introduced into new areas by human intervention, either intentionally or accidentally. Translocated fish species may be native to Australia but if they are moved into a natural waterway outside of where they occur naturally, they can dramatically upset the balance of the local ecosystem.

A good example is Lake Eacham, a crater lake on the Atherton tablelands west of Cairns, which was historically home to a small number of native fish including the Lake Eacham rainbowfish. However, the introduction of larger native predatory fish such as mouth mighty and archer fish, and other species of rainbowfish saw the endemic Lake Eacham rainbowfish driven from the lake through predation and hybridisation.

What Can We Do?

We can’t eradicate pest fish species once they’re established but we can take measures to prevent further exotic or translocated species from being introduced. Community awareness and education about the best ways to dispose of aquarium or pet fish is our best strategy.

Unwanted fish should never be released into natural waterways or flushed down the toilet. Instead, they should be returned to a pet shop, given to a new home with friends or family, or humanely euthanised.

Read ‘How to dispose of unwanted aquarium fish responsibly’