The Department of Conservation and Northland Regional Council have jointly declared war on koi carp and rudd, and are calling for public to tell them where the fish are lurking.
"The immediate task in Northland is making sure we know where populations of these serious pests are. We will also be getting expert advice on our options for eradicating and controlling them, and where to focus our efforts," DoC freshwater technical adviser Amy Macdonald said.
"In Northland we've still got the opportunity to protect our waterways from pest fish and stop their spread. We don't want to end up with koi carp in every pond and river like the Waikato, so now is the time to act.
"Pest fish upset the balance in our freshwater systems, affecting ecological, cultural and recreational values. Koi carp and rudd are both bad news for water quality, so they are also a problem for our agriculture and tourism industries. We've all got a reason to work together to stop them spreading and taking over."
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Koi carp, big and brightly coloured, bred prolifically, and could have devastating impacts on waterways. Rudd, smaller but also prolific breeders, ate the tips of native aquatic plants, and could turn lakes into barren algae-filled bodies of water that nobody wanted to swim in.
Pest fish expert Helen McCaughan, from Wildland Consultants, had travelled from Christchurch to support the operation, funded from DoC's Budget 2018 allocation of $76 million over four years to invest in targeted biodiversity initiatives across land, freshwater and marine ecosystems to address the country's biodiversity crisis.
That funding included $4.5 million over four years to contain key aquatic pest species populations to prevent further spread to sites with high biodiversity values.
The aim was to contain at least four freshwater pests (koi, gambusia, rudd and hornwort), all with the potential to expand to other regions.
DoC and NRC were now seeking reports of "mysterious" fish that could be koi carp or rudd. New reports would be added to the regional pest fish dataset, informing surveillance that was now under way.
To report suspect fish email DoC (email@example.com) or phone the NRC on 0800 002-004.