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.

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.

Some of the pest fish species that have already become established in the Wet Tropics, include tilapia, platys, guppies and swordtails.

There are two types of pest fish:

• Exotic pest fish – introduced from overseas
• Translocated pest fish – introduced Australian species that are not naturally occurring in that specific location

Tilapia: A Noxious Exotic Species

Tilapia is a particularly noxious species because 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: Native But Not Always Nice

Translocated fish are fish that have been moved from one area to another 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, then 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.