Freshwater ecosystems are very vulnerable to invasion by pests and weeds. Since European settlement many exotic 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.

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 invading areas where they aren’t already.

How are exotic fish introduced?

Some fish have been introduced deliberately, including gambusia which were introduced in the early 20th century in a mistaken attempt to control mosquitos, and trout which have a long history of stocking 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 (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 been released accidentally, but it is likely that others were deliberately let go by owners who no longer wanted them and were unaware of the consequences.

What species are pests?

Once they are in the wild, pest fish can spread naturally through a river system. However, in some cases the spread of pests into new areas is assisted by people, either accidentally or deliberately.

Members of the public, including fishers, divers and members of local environmental groups, are often the first to discover a new exotic 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.

• Find out about identifying and reporting pest species

What impacts do they have?

Some of these 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, others are considered pests as they can threaten NSW’s freshwater species and environments by:

• altering or degrading the natural environment eg. 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 NSW DPI doing about pest fish?

NSW DPI has a programme 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 (Cichlosoma octofascitum) at Angourie on the NSW north coast.

NSW DPI is developing a NSW carp control plan and is involved in a significant national research effort directed at future control options for carp, led by the Invasive Animals CRC.

• Assessing carp biology in NSW: The first step in a successful implementation strategy for daughterless carp – non-technical summary
• Invasive Animals CRC - Freshwater Products and Strategies (www.invasiveanimals.com)

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.

What can you do to help?

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. Obviously, some species cause more problems than others.

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

• Carp
• Redfin
• Banded grunter
• Gambusia (previously called mosquitofish)
• Weatherloach
• Goldfish

Other ornamental fish that have established feral populations in NSW include:

• Green swordtails
• Jack Dempsey cichlids
• White cloud mountain minnows
• Speckled mosquitofish or one-spot livebearer

Aquarium and pond weeds can also cause major problems if they spread into the wild. More information on freshwater weeds in NSW.

There are also some species such as tilapia which are important pests in other States but are not currently found in NSW. They could have significant impacts if they spread here.

Help stop the spread of feral fish!

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.
• Prevent unwanted hitchhikers – check, clean and dry boats and gear between waterways.

For ornamental fish enthusiasts and backyard pond owners:

• Don’t dump that fish! – Give unwanted aquarium fish to friends or a petshop, 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 exotics – keep to the prescribed species list.

For everybody:

• Keep a lookout for suspected new pest fish in local waterways.
• Report suspected pests to NSW DPI’s Aquatic Biosecurity Unit