The Associate Minister of Biosecurity, Marian Hobbs, has announced a survey of selected South Island wetlands, rivers and lakes, to determine whether eradication of two unwanted fish species is possible.
The Department of Conservation has been allocated $362,0001 through Vote Biosecurity for the survey for koi carp and Gambusia (or mosquitofish).

“These fish were first found in the South Island last June in Nelson," Marian Hobbs said. "They pose a significant risk to our South Island wetlands, and lowland rivers and fish life.

"Gambusia are the notorious ‘killer guppies’, nicknamed for their aggressive behaviour – they have been known to gang up on fish five times their size, nibbling on their fins to immobilise them.

"Koi carp are just as bad. They prey upon invertebrates, spawn and juvenile fish as well as uprooting vegetation during feeding, destabilising river and pond banks and destroying habitat for native fish and waterfowl, as well as trout and salmon, and potentially lowering water quality.”

The Minister of Conservation, Sandra Lee, said Gambusia were originally introduced to New Zealand in a failed attempt to control mosquitoes. Research showed they pose a significant threat to native species such as fish, frogs and large invertebrates and were no more effective at mosquito control than some native fish and invertebrate species.

“The introduction of Gambusia into our waterways parallels the introduction of stoats into our forests," Sandra Lee said.

Gambusia are small, stout and dull grey in colour with a rounded tail. Females reach sexual maturity in only six to eight weeks and estimates of their enormous breeding potential has shown individual populations can expand from 7000 to 120,000 in only five months.

Koi carp grow to about 60 cm long and weigh 5kg, although a giant-sized fish was recently caught in Northland – it was 75 cm in length and weighed 9kg. They also breed prolifically and can tolerate a wide range of environmental conditions, which means that once introduced they can quickly take over water bodies.

Marian Hobbs said the survey was necessary before a decision could be made regarding the feasibility of eradication. The survey would target South Island river systems and waterways that have suitable environmental conditions for the survival of koi carp and Gambusia.

DOC along with Fish and Game are conducting a trial eradication of Gambusia and koi carp in the Nelson region, with the consent of landowners.

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