Introduction

Wairarapa landowner Mark Round alerted authorities when last year he observed “strange fish” in his pond.

Wairarapa landowner Mark Round alerted authorities when last year he observed “strange fish” in his pond.

Staff from Forest and Bird and Greater Wellington Regional Council identified it as the aggressive pest fish gambusia (Gambusia affinis) Mr Round contacted the Department of Conservation (DOC), who recently eradicated the species from his pond using the fish toxin (piscicide) cube root powder.

Gambusia are legally designated an unwanted organism under the Biosecurity Act 1993.

Sometimes known as mosquito fish, they were introduced to New Zealand in the 1930s to control mosquitos by eating their larvae. But it turned out that native fish are better at this role.

Female gambusia. Photo: Alton Perrie.
Female gambusia

Gambusia are an aggressive predatory species that will band together to attack much larger fish, swarming out of covering vegetation to bite pieces from their fins, tails, gills and even eyes. Under such concerted attacks these fish will be disabled and eventually killed.

“For relatively sedentary fish like some of the larger native galaxids it is like being mugged by a gang of piranha and small fish, like whitebait, wouldn’t stand a chance,” remarks DOC pest fish ranger and project coordinator Tom Williams.

A single gambusia female matures at six weeks old and can produce several broods a year and around 50 live offspring per brood. They breed rapidly and repeatedly enabling populations to build up to large numbers very quickly.  Consequently only one pregnant female is needed to start a new population and can take over a waterway once they are introduced.

Their unwanted organism status is well merited according to anyone who has had to deal with them

Gambusia are now in many upper North Island waterways, where they attack native fish and compete with them for food. This was the first confirmed record of gambusia in Wairarapa.

Gambusia can tolerate a wide range of habitats and could colonise almost any river system or wetland in the Wairarapa.

Other populations in the Wellington region have been eradicated by DOC.

The population was well established before Mr Round had purchased the property and he was keen to eliminate it as soon as possible. It is the responsibility of landowners to eradicate pest fish from their properties, but in many cases DOC has worked closely with landowners to eradicate pest fish species if it is a high priority or high risk. In this case, DOC led on the eradication response and as part of this, had to obtain resource consent from Greater Wellington Regional Council to apply cube root powder to the pond. 

Relieved to have the fish removed from his pond, Mr Round is urging others to contact DOC if they think they may be inadvertently harbouring the pest, or see it in local waterways.

 “Gambusia looks relatively harmless but when you take into account their numbers and the way they feed, you realise how large a problem they can become.” 

DOC Wairarapa Area manager Chris Lester said tell tale signs of gambusia presence were lots of ripples on the surface of the water. The fish will often school on or near the shallows. 

“If you are suspicious, a quick dip with a kitchen sieve is likely to produce a few little fish. They will also push into the vegetation on the edges of the ponds.”

About gambusia:

Gambusia are small fish with a greenish silvery sheen. They live in the shallow margins of slow flowing ponds, wetlands and streams, particularly around aquatic plants. They can tolerate poor water quality, high salinity levels and a wide range of water temperatures.  Populations exist throughout Northland, Auckland, Waikato, Bay of Plenty and Hawkes Bay, with isolated populations found around Whanganui.

About cube root powder:

Cube root powder is an approved piscicide which has been used successfully both in New Zealand and overseas to remove unwanted fish from wetlands, ponds and lakes. It is a natural toxin found in the roots of several tropical plants and has been used by the indigenous people of Central and South America to kill fish that are then collected and eaten.

About pest fish:

It is better to prevent the introduction of pest fish than try to control them once they have established in a waterway and they can be very difficult to eradicate. Please help by:

  • Alerting your nearest DOC, Regional Council or Fish & Game office if you know anyone spreading aquatic pests or if you suspect you may have them in your pond.
  • Contacting your nearest DOC office for advice on any approvals required before liberating any fish or pants into your pond.
  • Sourcing fish and plants from reputable outlets rather than from the wild or from friends.
  • When moving from one waterway to another, always wash down all boating and fishing equipment carefully after use to prevent pest fish and aquatic weeds from hitching a ride.

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