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Aquatic pests

Introduced plants and animals are an increasing problem in many New Zealand waterways, including the Waikato River. Aquatic pests often compete with or feed on our native plants and animals. They can also make our waterways less suitable for recreational uses, such as swimming.

Oxygen weed (Lagarosiphon) Oxygen weed (Lagarosiphon)

Problem plants

Introduced plant pests have invaded all of the Waikato River hydro lakes. The oxygen weeds Lagarosiphon and Egeria grow up to six metres tall beneath the water surface, crowding out native plants. Other problem plants in hydro lakes include curly pondweed, water buttercup and hornwort.

Hornwort (Ceratophyllum demersum) does not form roots and is easily dislodged by the wind or waves. It then drifts downstream where it must be removed from weed booms and the screens of hydro dams. Hornwort and Egeria are also a nuisance to boaties and swimmers. Swimmers may become tangled in the thick weed growth, which can also wrap around boat propellers and clog engines.

Alligator weed (Alternanthera philoxeroides) is found in the lower Waikato River and on nearby land. It is one of the world’s worst weeds, and is listed in our Regional Pest Management Plan as an eradication plant pest.

Other nuisance plants in the Waikato River system include the water poppy, red algae, water lillies, parrot’s feather and reed sweet grass. Blooms of blue-green algae can sometimes occur. Lindavia intermedia is a microscopic algae species that has the potential to create lake snow - a mucus-ike slimy substance can be a nuisance to water users.

Although introduced plants are generally pests in the Waikato region they can play an important role in rivers and lakes by providing food and habitat for fish and insects.

Troublesome fish

Introduced fish can also damage our waterways and the native plants and animals that live in them. Some of the worst fish in our region are:

  • koi carp
  • catfish
  • gambusia (mosquitofish)

Invasive fish management workshop - DOC

A selection of presentations from the invasive fish management workshop

Koi carp

Koi carp (Cyprinus carpio) were introduced to New Zealand as an ornamental fish, but are now present throughout the lower Waikato River.

Koi carp feed by sucking in the mud from stream and lake beds to filter out food. This makes the water very muddy, smothering other aquatic animals and plants. Koi carp also uproot native plants, damaging aquatic plant beds.

Koi carp are classified as being:

  • a ‘noxious fish’ under the Freshwater Fisheries Regulations (1982)
  • an ‘unwanted organism’ under the Biosecurity Act
  • a ‘containment (occupier control) animal pest’ under Waikato Regional Council’s Regional Pest Management Plan.

It is illegal to release, spread, sell or breed ‘unwanted organisms’. The Department of Conservation controls the possession and harvest of koi carp and is working to prevent their spread outside the Waikato River system. Contact the Department of Conservation for more information about koi carp and other pest fish.


Catfish (Amieurus nebulous) are classed as pests in the Regional Pest Management Plan. They are widespread throughout the Waikato River catchment, as well as in the Piako and Waitoa rivers. Catfish are also found in Lake Taupo. Although they are not believed to affect the lake’s trout fishery, research is underway to confirm this.

Catfish eat both plants and animals, including freshwater snails, koura, bullies, smelt and juvenile trout.

Gambusia (mosquito fish)

Gambusia (Gambusia affinis) have recently been identified as a problem in New Zealand and declared ‘unwanted organisms’ under the Biosecurity Act 1983. They are present throughout the Waikato River system.

Although small, they are very aggressive and will attack fish twice their size. As well as damaging adults, they eat the eggs and juveniles of native fish, such as inanga and the endangered black mudfish. They have also caused a decline in the numbers of New Zealand’s largest dragonfly, the baron dragonfly.

Gambusia can breed very rapidly, quickly taking over waterways and out-competing native fish. Although they were originally introduced to keep mosquitoes under control, they are not very effective. Gambusia actually ‘help’ mosquito larvae by attacking the native insects, such as diving-beetle larvae, that feed on them.

You can help

Stop the spread of aquatic pests:

  • Never release plants or fish (including unwanted goldfish, other exotic fish or aquarium weeds) to waterways.
  • Always check your boat before launching, especially the trailer and propeller, to ensure it is free of weed, juvenile fish and fish eggs.
  • Wash your boat and fishing gear carefully after you have taken it out of the water to remove any weed, juvenile fish or fish eggs.
  • Report sightings of plant pests to Waikato Regional Council and fish pests to the Department of Conservation.