(Gambusia affinis)

Progressive Control Pest

Gambusia are small, silvery-green fish (males up to 3.5 centimetres, females up to 6 centimetres long) that were introduced into North Island waterways from the southeast United States in the 1930s in the mistaken belief that they could control mosquito larvae. They are intermittently widespread in the upper North Island; the first South Island populations were discovered in the Tasman-Nelson region in 2000 and an active campaign has been conducted against them, along with other pest fish, in the Tasman-Nelson region by the Department of Conservation.

Reasons for the Strategy

Two gambusia swimming.

Gambusia has the ability to rapidly increase population numbers. It matures at six weeks old and gives birth to live young; one pregnant female can start a new population. Although they are small fish, they can be very aggressive and attack fish much larger than themselves. They attack native fish by nipping their fins and eyes, and preying on their eggs. Whitebait and mudfish species are especially vulnerable to Gambusia, as they inhabit similar habitats. They prefer the shallow margins of slow-flowing ponds, wetlands and streams, especially around aquatic plants. They can tolerate poor water quality and a wide range of water temperatures. They are considered to pose a major threat to aquatic organisms throughout the Tasman-Nelson region.

Gambusia is assessed at “2” on the infestation curve. It has been recorded in a small number of ponds in the Tasman- Nelson region. The low incidence of Gambusia in the Tasman-Nelson region, extensive areas of suitable habitat, and the potential for it to cause significant adverse effects, mean the benefits of progressive control far outweigh the costs.


To reduce the distribution and density of Gambusia in the Tasman-Nelson region during the term of the Strategy.

Alternative Measures

The alternative option of “do nothing” or relying on voluntary control will not achieve the objective of reducing the distribution and density of Gambusia, and will result in significant additional costs to the community with respect to lost natural values and the increased cost of control in the future.

Strategy Rule for Gambusia

The occupier shall report any suspected sightings of Gambusia and allow access to the Management Agency, the Department of Conservation or their agents, to destroy all adult and juvenile forms of Gambusia in water bodies on land that they occupy. A breach of Strategy Rule 5.4.5 is an offence under Section 154(r) of the Biosecurity Act 1993.

Biosecurity Act Requirement

No person shall knowingly sell, propagate, breed, release, or commercially display Gambusia, under Sections 52 and 53 of the Act.