Invasive species

Koi carp originate from Asia and are an ornamental variant of the common or European carp. Unlike common carp, they are typically brightly orange coloured, often with irregular dark patches. While these fish resemble goldfish when small, they can grow to 70 cm in length and weigh up to 14 kg. Koi carp are the third most widely introduced species in the world. 

Gambusia (formerly known as mosquito fish) originate from the southern United States and superficially resemble the guppy. They are small (4-6 cm), give birth to live young and can breed several times a year. They are able to survive in a wide range of conditions and quickly reproduce to plague proportions. Gambusia are the most widely introduced fish species in the world. 

Brown bullhead catfish and goldfish were introduced into New Zealand in the late 1800s; they originated in eastern USA and eastern Asia, respectively. Catfish occur in a wide range of habitats but prefer slow-flowing waters. Goldfish are very tolerant of low dissolved oxygen and warm water temperatures, and therefore can make even highly degraded conditions worse. Rudd are a recent introduction (1972) and are native to Europe/western Asia. Rudd and goldfish prefer standing waters but also do well in weedy pools in streams and rivers. 

LERNZ researchers are also interested in the environmental effects of other widespread introduced fish such as perch and tench.

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Common name

Scientific name

Legal Status

Ecological effects


Kio carp

Cyprinus carpio

Noxious fish1

Unwanted organism2

Stir up sediments, uproot plants

Throughout North Island


Carassius auratus

No legal status

Stir up sediments

Throughout both islands (excluding Otago and Southland)

Brown bullhead catfish

Ameiurus nebulosus

No legal status

Predation, food web effects

Widespread around middle of North Island; two scattered populations in South



Gambusia affinis

Unwanted organism2

Attack native fish and compete for food

Throughout North Island


Scardinius erythrophthalmus

Noxious fish excluding Auckland/Waikato Fish & Game regions where it is Sports fish1

Eat aquatic plants; prevent plant regrowth

North Island from Northland-Waikato and Taranaki to Wellington; South Island at a few sites down the east coast

1, Freshwater Fisheries Regulations (1983)

2, Biosecurity Act (1993)