Date:  22 August 2019

Gambusia are a concern to DOC because they prey on native insects and fish including whitebait.

Gambusia is one of the world’s most invasive species. A small, robust fish, it breeds and spreads very rapidly in spring and summer. Populations can expand to 17 times their original number within five months.

Gambusia is an unwanted organism under the Biosecurity Act 1993, and it is illegal to possess, release, buy, sell or breed them.

DOC is asking anyone who sees fish they believe to be Gambusia to take photos and email details to DOC ranger Renan Falleiros at rfalleiros@doc.govt.nz. They're asked to include their name, contact number and details of where and when they saw the fish.

Mr Falleiros, a pest fish specialist, is developing a feasibility plan to eradicate Gambusia in Nelson Tasman.

“Gambusia threaten native whitebait species and mudfish by nipping their fins and eating eggs and juveniles,” said Mr Falleiros. “They also reduce native insect populations.”

Male Gambusia grow to 3.5 cm and females to 6 cm. They have a green to brownish top, grey sides, and an upturned mouth.  The fish prefer slow moving water in creeks, wetlands, ponds, and estuaries where there is a mix of salt and fresh water.

DOC is testing new ways to identify where Gambusia are present so it can focus control efforts on affected areas. Known as the Environmental DNA (eDNA) method, rangers collect DNA from waterways by pumping water through very fine filters. These samples are tested to confirm the presence or absence of Gambusia DNA. While the method looks promising, more work is needed to improve its accuracy, says Mr Falleiros.

Commonly known as mosquito fish, Gambusia were introduced to the North Island in the 1930s in the belief they would control mosquito larvae. Recent studies have shown that they are not effective in controlling mosquitoes. In some sites they can even increase the number of mosquitoes by eating mosquitoes’ native predators.

Gambusia were first discovered in a Tasman pond in 2000. DOC initiated a Pest Fish Programme that eradicated Gambusia from over 23 sites. However, they are still found at many sites around Waimea Inlet, Moutere Inlet, Motueka and Riwaka.

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