Pest fish koi carp species found in Hawke's Bay

Hawke's Bay Regional Council freshwater ecologist Daniel Fake with one of two pest koi carp found in a Tukituki valley farm pond. Photo / Supplied

It's a case of one down, one to go after two koi carp, an invasive species of fish, were discovered in an isolated farm pond in the Tukituki valley.

A tip-off from a farm manager to the Hawke's Bay Regional Council, followed by an eDNA sample has resulted in a process to remove two fish that had been in the pond for some time.

Koi carp can dramatically impact water quality through their feeding strategy of sucking up mud from banks, eating bugs and plants, and expelling the muddy water through their gills.

This can turn clear waterways and wetlands muddy, and they are prolific breeders and can be very difficult to eradicate when they get established.

They have wreaked havoc in waterways in the Waikato, Auckland and Northland where they have proliferated, and the council doesn't want that to happen in Hawke's Bay, regional council freshwater ecologist Daniel Fake said.

"In this instance we're lucky that they haven't bred in the pond, or escaped into a larger waterway, and we're very grateful that a member of the public has alerted us to their presence."

Evidence suggest that in the 1970s and early 1980s, koi carp were deliberately placed in New Zealand waterways by some misguided individuals who wanted to fish for them.

"We think this koi carp is related to that."

Council confirmed the presence of the fish using new technology called eDNA sampling which picks up traces of biological material in the water and matches their DNA to known species.

One of two detected koi carp has already been caught, and the council had placed gill nets in the pond, which are about 20m long, to catch the remaining one.

"The large mesh size in the gill nets means that native species such as eels and bullies won't be caught," Fake said.

"This is the preferred option over poisoning or draining the pond which will kill or harm all of the other species in there."

The last koi sighting in Hawke's Bay was more than 20 years ago, Department of Conservation senior community ranger Chris Wootton said.

"But there's a chance there's still some lurking in isolated ponds," and he encouraged the public to report any sightings to DoC or the regional council.

"Koi carp resemble large goldfish, but can vary in colour with patchy gold, red, black and white colourations and have two pairs of whisker like feelers at the corner of their mouth."