There is good and bad news from the front in the war the Department of Conservation and the Northland Regional Council are waging on Northland's koi carp and rudd populations.
The good news is that a survey of covered 12 pest fish sites in Northland has confirmed that two historic koi populations have been eradicated, and no new sites were found.
The bad news though is that the battle has just begun, with significant pest fish populations in waterways across the region.
DoC freshwater technical adviser Amy Macdonald said DoC and the NRC would use the survey information to develop a pest fish surveillance and eradication programme.
"This will enable us to prioritise sites for control and eradication, and to work with communities to prevent the pests spreading to new sites," she said.
"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."
Both species were prolific breeders, and could have devastating impacts on waterways, potentially creating barren algae-filled lakes that nobody would want to swim in.
Pest fish expert Helen McCaughan, from Wildland Consultants, travelled from Christchurch to support the operation.
The survey was funded from the Government's Budget 2018, which allocated $76 million to DoC over four years to invest in targeted biodiversity initiatives across land, freshwater and marine ecosystems to address the country's biodiversity crisis.
That included $4.5m over four years to contain key aquatic pest populations and reduce the likelihood of them spreading to sites with high biodiversity values, where it would be more difficult and expensive to control them. The aim was to contain at least four serious freshwater pests - koi, gambusia, rudd and hornwort - which had the potential to expand to other parts of the country.
The fund would also be used to control invasive aquatic plants with a high risk or impact on freshwater biodiversity values, to reduce their impact on river, lake and wetland ecosystems and reduce the likelihood of dispersal to other indigenous habitats.
The aim was to increase the reduction of invasive aquatic plants to at least 10 sites each year.
New Zealand's freshwater ecosystems faced a number of threats - pollution, deforestation, loss of habitat, changing land uses, sedimentation and nutrient in-flow, and the impact of invasive aquatic animals and plants such as koi carp and rudd, Macdonald added.
■ Koi carp superficially resemble goldfish, but grow much bigger and have two pairs of whisker-like feelers (barbels) at the corners of their mouth. They are highly variable in colour, from blotchy shades of black, red, gold and orange to white. They can often be seen close to the surface in ponds and slow-flowing water during the day over summer.
Rudd are stocky, with distinctive red fins and large, shiny scales that range from silver to pale or burnished orange in colour. They normally grow to about 25cm and around about 500g. New Zealand's only herbivorous freshwater fish, they are regarded as "the possums of the waterways", feeding voraciously on native aquatic plants.