The angler, who did not wish to be named, provided a photograph of the fish to Fish and Game, which confirmed the species and alerted the Department of Conservation (DoC) and Waikato Regional Council.
Follow-up surveys by Fish and Game later uncovered five adult perch in nearby Lake Horohoro, leading officials to conclude the species was already well established in Lake Karāpiro rather than a single, isolated incursion.
Authorities remained unsure how the perch entered the lake. Possible pathways such as illegal release, bait bucket dumping or transfer on watercraft had not been ruled out.
One angler, who also asked not to be named, said he stopped at a Karāpiro car park and hooked a perch almost every cast during about an hour’s fishing — suggesting the fish may have been present for some time.
A Waikato Regional Council spokesperson said there had been no successful eradications of perch in New Zealand in a lake the size of Lake Karāpiro.
“The focus now is on preventing further spread.”
Perch are regarded as a serious threat to native freshwater species.
As aggressive apex predators, adult perch feed on other fish, including bullies, inanga and smelt, as well as kōura (freshwater crayfish).
Their presence has also been linked to degraded water quality, with high numbers of juvenile perch capable of contributing to algal blooms.
The species could also affect recreational fishing. Perch prey on trout and, when populations become dense, are known to eat the juveniles of their own species.
The perch population in Lake Karāpiro is considered physically contained, as the fish cannot pass the dam at the lake’s southern end on their own.
There is still a significant risk of human-assisted spread to other waterways, the spokesperson said.
Perch eggs, juveniles or adult fish could be transported on boats, trailers, fishing gear or other recreational equipment, particularly through water sports and rowing activities that are common on the lake.
Anglers and lake users are being urged to thoroughly clean and dry all equipment before moving between waterways to prevent the spread of pest fish and aquatic weeds.
Fish and Game is leading monitoring efforts, including decisions around ongoing surveys and any potential escalation of the response.
Under the Freshwater Fisheries Legislation 1983, it is illegal to release pest fish into waterways. Penalties apply, and enforcement action could be taken if an offender is identified.
