Gambusia or mosquitofish (Gambusia affinis)

Gambusia were first introduced into Aotearoa New Zealand in the 1930s, where they were released into a pond in the Auckland Botanical Gardens. These fish originate from the Gulf of Mexico and were brought over in attempt to control mosquito populations.

This theory was based on few observations that gambusia eat mosquito larvae, however we now know that this is only a small part of their diet.

This species is designated as an Unwanted Organism under the Biosecurity Act 1993.

Description and life history

Gambusia live for about one year and they spend all that time in freshwater. They mature at six weeks old and can breed many times throughout their life. Additionally, this species does not lay eggs but gives birth to live young, enabling a population to grow at a very fast rate.

Males and females of this species are both silvery olive in colour but grow to different sizes. Females grow up to twice the size of males, reaching lengths of approximately 6 cm, while males only grow up to 3.5 cm. Another feature to look out for is that the mouth appears upturned.

Gambusia typically eat small insects, larvae and small amounts of plant material and algae. They are also known to eat native fish eggs.

Where are they found?

Distribution

After the original introduction in Auckland, further transfers were also documented from the 1930s to Northland, Taranaki and Wellington. The distribution of these fish is now far more widespread, aided by natural spread, accidental spread, and illegal introductions.

Populations now exist throughout the North Island, with hotspots in Northland, Auckland, and Waikato. In the South Island, a few populations exist around the Nelson/Tasman area.

Habitat preference

Gambusia live in shallow margins of slow-flowing ponds, wetlands and streams, particularly around aquatic plants. They can tolerate poor water quality, low oxygen levels, and high salinity. They prefer warm waters but tolerate a wide range of water temperatures.

Why are they bad?

There are several ways in which these fish pose a threat to our freshwater ecosystems:

• Gambusia are more aggressive than our native fish and frequently nip at their eyes and fins.
• Gambusia also compete with native fish for food and have been known to eat native fish eggs.
• Due to their fast breeding, gambusia can quickly outnumber native fish in a stream and take over a waterway.
• Once in large numbers, gambusia eat larvae of native insects, such as dragonflies, affecting the populations of these species.

For mosquito control

Native fish such as whitebait, bullies and eels, and aquatic invertebrates all feed on mosquito larvae.

If you have got mosquito problems:

• empty containers around your home that contain water (saucers, jars, tyres, paddling pools etc) and clear your gutters
• if you have a pond, make it as unfriendly to mosquitoes as possible by creating steep sides, and have flowing water, and planting plants around the edge to shade the water. This will improve your pond's habitat for other species that feed on mosquitoes.