Plan to eradicate aggressive pest fish across top of the south

Tuesday, 1 October 2019

Small but aggressive, tiny gambusia threaten our freshwater fish.

A Nelson-based fish specialist is working on a plan to eradicate an aggressive invasive fish species that preys on whitebait and eats its eggs.

Gambusia, also known as mosquitofish, were first found in the region when a group of schoolchildren playing on an Upper Moutere property spotted them in an irrigation pond in April 2000.

It was estimated at the time there were up to 30 fish in the pond. Almost 20 years on, their numbers have grown exponentially, with the small invasive fish found at more than 50 sites across Nelson and Tasman.

Department of Conservation ranger and pest fish specialist Renan Falleiros said the top of the south was the only place gambusia had been found in the South Island and special efforts had been made to prevent it spreading into other regions.

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Gambusia caught in the Tasman District. The small pest fish can breed rapidly, their population can expand to 17 times their original number within five months.

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'Gambusia threaten native whitebait species and mudfish by nipping their fins and eating eggs and juveniles.

'They also reduce native insect populations.'

Since they were discovered, there had been 64 operations to eradicate gambusia from waterways.

They were successfully removed from more than 23 sites, but when ex-Cyclone Gita hit, the flooding helped the fish to spread to new sites, particularly around Lower Moutere.

Recent monitoring showed gambusia were found in 54 sites around Riwaka, Motueka, Lower Moutere and Waimea Inlet, mostly near the coast.

Recent monitoring showed the fish were found in 54 sites around Riwaka, Motueka, Lower Moutere and Waimea Inlet, mostly near the coast. Falleiros said it was impossible to estimate how many there were given the fish populations could expand to 17 times their original number within five months.

He is working to determine if it is feasible to fully eradicate gambusia in the region.

Falleiros had contacted several whitebaiters since the fishing season opened on August 15 asking for sightings of the fish, but he had not received any reports which was promising.

There were four other pest fish species in the top of the south; rudd, tench, perch and koi carp but gambusia was the only species spreading to new sites which is why they were focusing on its eradication.

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

The fish look like a guppy with a greenish-olive-brown back, grey sides and silver belly. The females grow up to 60mm while the male only reach 35mm in length. They prefer slow moving water in creeks, wetlands, ponds, and estuaries where there is a mix of salt and fresh water.

This summer, DOC plan to test different methods of eradication in flowing water.

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

Falleiros said anyone who saw fish they thought was gambusia to take photos and email him at rfalleiros @doc.govt.nz with their name, contact number and details of where and when they saw the fish.