Freshwater pest fish

Freshwater ecosystems are very vulnerable to invasion by aquatic pests and weeds. Since European settlement many non-native fish have been accidentally or deliberately introduced into NSW waterways, and have become widespread. Some native Australian fish have also been moved outside their natural range for recreational fishing enhancement or aquaculture.

How are non-native fish introduced?

Some fish have been introduced deliberately, including Eastern gambusia which were introduced in the early 20th century in a failed attempt to control mosquitoes, and trout which have a long history of stocking in NSW for recreational fisheries enhancement.

Other fish have been accidentally introduced, for example through the escape of ornamental or aquaculture fish (such as carp and goldfish), and the contamination of shipments of fish destined for fish stocking (e.g. banded grunter).

While most of these species have been in NSW for many decades, recently an increasing number of ornamental fish species have been found in the wild. Some may have escaped or may have been released accidentally, but it is likely that others were deliberately (illegally) let go by owners who no longer wanted them and were unaware of the consequences. Once they are in the wild, introduced fish can establish pest populations and can spread through a river system. In some cases the spread of pests into new areas is assisted by people, either accidentally or deliberately.

What species are pests?

Introduced fish that are already widespread in NSW and are considered pests include:

Other fish, including ornamental species, that have established populations in NSW include

Aquarium and pond weeds can also cause major problems if they spread into the wild.

More information on freshwater weeds in NSW.

Of the above pest species, Mozambique Tilapia is the species most recently confirmed to have an established population in NSW. Mozambique Tilapia and some other fish species  are significant pests in other states and  could have significant impacts if they spread throughout NSW.

What impacts do introduced species have?

Some introduced species, most notably trout, are seen as having social and economic benefits for recreational fisheries and are actively maintained through stocking or other fisheries management actions.

However, other introduced species are considered pests as they can threaten NSW’s native fish species and environments by:

  • altering or degrading the natural environment, e.g. stirring up sediments, increasing nutrient levels and contributing to erosion;
  • feeding on or destroying native plants;
  • preying on invertebrates, native fish and their eggs;
  • competing with native species for food, habitat or spawning grounds.

Pest species can also degrade recreational fisheries by proliferating at the expense of native fish and dominating the catch.

What is the NSW Government doing about pest fish?

NSW DPI has a program of survey, research, education, signage and in some cases, eradication of pest fish where it is possible. Unfortunately, once pest fish become established in a waterway it is often extremely difficult, if not impossible, to remove them. Most eradication efforts have focused on pest fish which are restricted to a very small area, such as speckled mosquitofish (Phalloceros caudimaculatus) at Long Reef in northern Sydney and Jack Dempsey cichlids (Cichlasoma octofasciatum) at Angourie on the NSW north coast.

NSW DPI released the 'NSW Control Plan for the pest fish Cyprinus carpio' in 2010 and is involved in a significant national research effort directed at future control options for carp, led by the Invasive Animals CRC.

The practice of introducing fish (including ‘native’ fish) into areas outside their natural range has been listed as a key threatening process under the Fisheries Management Act 1994.

There are at least 14 introduced fish species established in the wild in NSW. This is quite a few considering that NSW only has around 55 native species. However, some of these introduced species are capable of greater impacts upon native fish species and ecosystems than others.

Many people are unaware of the damage done to our waterways by pest fish. Once pest fish have invaded a waterway and become established it is very difficult to remove them because they can spread through the whole river catchment. It is very important to prevent pest fish from being introduced into natural waterways in NSW, and from invading areas where they aren't already.

Help stop the spread of feral fish!

Members of the public, including fishers, divers and members of local environmental groups are often the first to discover a new introduced non-native fish in the wild or the fact that an existing pest has spread into a new area. This information can be very valuable in helping to manage pest problems.
You can protect our waterways and native fish by helping to stop the introduction and spread of feral fish into new areas.

For recreational fishers:

  • Don’t transfer fish between waterways – and don’t use live fish as bait in freshwater (it’s illegal!)
  • Obtain a permit to stock fish – and buy fingerlings from a registered hatchery to minimise the chances of contamination with undesirable species.
  • Don’t return pest fish to the water – if you catch a pest fish, kill it humanely and dispose of it appropriately away from the water.
  • Prevent unwanted hitchhikers – check, clean and dry boats, equipment and fishing gear between waterways.

For ornamental fish enthusiasts and backyard pond owners:

  • Don’t dump that fish! - Give unwanted aquarium fish to friends or a pet shop, rather than letting them go in the wild (Note: it is illegal to release live fish into NSW waterways without a permit, and heavy penalties apply).
  • Prevent accidental escapes – screen outdoor ponds to prevent overflow during heavy rains.

For fish farmers:

  • Prevent accidental escapes – comply with aquaculture permit conditions designed to prevent the escape of fish (e.g. screened water outlets).
  • Don’t experiment with non-native species – keep to the prescribed species list.

For everybody:

Further reading