Introduction

Department of Conservation staff follow up on historic pest fish records.

Over the last few months, ecologists Natasha Petrove and Ursula Brandes have been chasing pest fish around streams and ponds in Manawatu.

Single koi carp. Photo by Department of Conservation.  This koi carp was removed from a private pond during an eradication operation in 2009

Beginning in 2000, the Department of Conservation (DOC) carried out extensive surveys of pest fish species throughout Taranaki, Whanganui and Manawatu, and found gambusia (Gambusia affinis) and koi carp (Cyprinus carpio) in several sites. Some eradication and public awareness work was carried out, and now Miss Petrove and Miss Brandes have been employed by DOC to follow up on these records to determine how successful this work has been.

Gambusia, also known as mosquito fish, was introduced to New Zealand waterways to control mosquitoes. However, according to these ecologists, they don’t do a very good job of controlling mosquitoes – native freshwater species are much better at it. Worse still, gambusia attack native fish - nipping at their fins and eyes – and compete with them for habitat and food. They are also prolific breeders, and can quickly outnumber native fish. ‘Unfortunately, we’ve now got these very aggressive, invasive little fish in lots of waterways in the North Island and some in the upper South Island,’ said Miss Petrove. ‘This could lead to our native whitebait species being excluded from lowland waterways.’

Gambusia seem to be taking hold locally with confirmed sightings at several sites in the Manawatu, including Makowhai Stream near Sanson and Burke’s Drain just south of Palmerston North. ‘This is a shame, as they are very difficult to control once they are established in flowing water,’ commented Miss Petrove.

The other problem fish prevalent in the area is koi carp. They are similar to gold fish but grow much larger and are more destructive. Released into waterways as ornamental fish, koi carp undermine banks and deteriorate water quality. They are filter feeders; they suck up mud, filter out food and spit the unwanted muck back into the water. Although koi carp don’t attack native fish, they destroy their habitat.

Koi carp damage. Photo by Department of Conservation.
This drained pond shows how damaging filter feeders like koi carp can be

The good news is that koi carp were found in only one of the recorded sites in the Manawatu. However, the surveys were limited to visual observations of previously reported sites, so it is possible that there may still be some koi carp lurking out there or invading new sites. There have been recent reports of koi carp sightings in the ponds at Otira Park and the Mangaone Stream, but Miss Petrove and Miss Brandes did not see any during their surveys. If you see large gold fish in streams or ponds, please report it at the local DOC office.

Fish can’t travel over land by themselves, but new infestations are still being discovered. This means people are spreading them, accidentally and on purpose. Pest fish such as gambusia and koi carp can be inadvertently released into new waterways through transferring plants such as water lilies and oxygen weed. Fish eggs and even fry can be transported on the leaves. You can help stop the spread by following three simple rules:

  1. Don’t move plants and fish between waterways
  2. Don’t release plants or fish into waterways
  3. Check, clean and dry any gear between waterways.
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