Authorities currently searching for koi carp in Kaipara's Lake Taharoa were notified just weeks prior about angry trout anglers threatening the pest fish's illegal release.
"In late December 2020, information was passed on to council biosecurity staff that disgruntled trout anglers aim to release koi carp into Northland's Kai Iwi Lakes," Don McKenzie, Northland Regional Council (NRC) biosecurity manager, said.
Ten-year-old Zeke Voschezang's reported koi carp sighting in Lake Taharoa - the biggest of the Kai Iwi Lakes dune lakes - came just weeks later in early February.
"Whether the two are related, we don't know," McKenzie said.
"But we haven't closed the (December threat information) file," he said.
An immediate lakeside multi-agency education and information response was launched as a result of the report. An urgent much bigger multi-agency incursion response has swung into action in the wake of Zeke's early February reported sighting.
The December koi carp threat was in NRC chief executive Malcolm Nicolson's report to councillors at NRC's first 2021 meeting on Tuesday.
It is illegal under the Biosecurity Act 1993 to breed, transport or spread koi carp anywhere in New Zealand. Five years' jail and/or fines of up to $100,000 await anybody found to have spread the fish to the lake.
Illegal deliberate introduction is being looked at as one option for how the koi carp got into Lake Taharoa.
Rudi Hoetjes, Northland Fish and Game Council regional manager said accusations disgruntled trout anglers had threatened to release the fish into the lake were unfair.
He said it was not okay to blame trout anglers as a whole for the actions of a person who could be from any walk of life and have released koi carp into the lake.
"It's disastrous to think anybody would release a koi carp into the lake," Hoetjes said.
Koi carp threaten Lake Taharoa's recreational use and unique ecosystems.
Lake Taharoa is New Zealand's biggest and deepest dune lake, internationally known for its pristine water quality and ecosystems. It is also a major Northland tourist attraction and the country's most popular dune lake camping spot, just three hours north of Auckland. About 17,500 people camp there annually - 10,500 campers this summer alone. Thousands more day visitors also flock to the area.
The lakes have been used for decades as a trout "put and take" fishery meaning Northland Fish and Game, and its predecessors, releasing Rotorua hatchery-raised 1-year old trout into the lake annually for people to catch.
Hoetjes said Lake Taharoa had offered the best trout fishing north of Hamilton.
The last trout release was in 2019 when 2000 trout were put into Lake Taharoa, 350 into Lake Waikare – among about 200,000 released into these and adjacent Lake Kai Iwi since 1966.
The Taharoa Domain Governance Committee which manages the lake and its surrounds in 2017 banned trout releasing.
Ric Parore (Ngati Kuihi) committee chairman said the goal was to get the lakes back to their natural state.
Parore said the lakes were traditionally important to Ngati Kuihi, Te Roroa and Māori from across Tai Tokerau as a source of mahinga kai.
Meanwhile, about a dozen response team members were on Wednesday back at Lake Taharoa continuing their koi carp surveillance work. The response uses a drone,
boat to take responders around the lake, special baited koi carp nets and lakeside observations.
The koi carp Voschezang reported seeing has not been found so far, neither has evidence of the fish been gleaned in environmental DNA in water samples analysed in Whangārei.
McKenzie said the response would likely continue for at least another three months.
He said the trout release threat had not been substantiated. But it had been given credence and used as an important part of intelligence gathering as often happened in pest management.
"Anybody who sees any unusual fish in the lake should report it," McKenzie said.
The response team includes local iwi Te Roroa, Department of Conservation (DoC), Northland Fish and Game, NRC, Kaipara District Council (KDC) and the Taharoa Domain Governance Committee.
"I am perturbed someone would do something like this," Adam Daniel, Fish and Game national invasive species co-ordinator said.
Waikato-based Daniel recently trained incursion responders at Lake Taharoa.
Daniel said Northland was potentially at greater risk from koi carp because of its warmer water.
Koi carp are legally classed as an unwanted organism and noxious species in New Zealand. They have become a pest fish on every continent in the world except Antarctica.