The possibility of an unwanted freshwater fish that feeds like a "living vacuum cleaner" has set alarm bells ringing in Northland.
A 10-year-old boy recently saw a fish believed to be a koi carp in Lake Taharoa, north of Dargaville while out kayaking. His parents immediately reported the sighting to the Department of Conservation, which has now sparked an appeal from the Northland Regional Council (NRC) for other sightings of the pest.
Penny Smart, chair of the NRC, said a multi-agency response team was taking the sighting "very seriously". The worst-case scenario, said Smart, was that the fish - if it is in fact a koi - turns out to be a fertile female.
"Koi can cause devastating environmental damage and are very difficult to get rid of once they are established, with a mature female able to produce more than 300,000 eggs in a single spawning season," she said on Monday, adding the council was grateful to the boy and his family for reporting the sighting.
She said koi are already known to be in at least three of the region's major river systems and in many farm dams but it would be "disastrous" if the species was confirmed to be in Lake Taharoa, the largest of the three Kai Iwi Lakes.
Curtis Harris, biosecurity incursion management officer for the NRC, said if the fish does turn out to be a koi it is hoped it is a solitary male or an infertile female, so there's no risk of it breeding.
He said it's not known how the fish got into the lake, though possibilities include an accidental transfer of pest fish eggs or fry via a boat trailer or equipment, or a deliberate illegal release.
The hunt to find the mysterious fish was likely to be time-consuming and expensive, he said, with drones, environmental DNA detection, and purpose-built koi nets among the tools that will be used in the search.
"We're very keen to catch the fish to determine its gender and reproductive status and urge people to contact us immediately on 0800 002 004 if they've seen koi in Lake Taharoa or have any information related to this sighting," he said, adding photos or video of the fish would be even more helpful.
Harris described the fish as a "living vacuum cleaner", sucking up everything from plants and organic matter to fish eggs and small fish.
"They blow out what they don’t want and this constantly stirs up the bottom of a waterway, muddying the water and reducing light levels.
"This in turn negatively affects plant life, native fish and invertebrates, and creates unpleasant swimming conditions and bluntly, would be an absolute disaster in Taharoa, where submerged vegetation currently grows as deep as 24 metres in its clear water."
Harris said under the Biosecurity Act it is illegal to breed, transport or spread koi carp anywhere in New Zealand, and anyone caught doing so could face fines of up to $100,000 and/or five years' jail.