Catfish incursions in Lake Rotorua and Rotoiti are "quite possibly past the point of no return" according to Eastern Fish & Game officer Matt Osborne.
Four of the pest fish were caught near Mokoia Island during Bay of Plenty Regional Council biosecurity surveillance on Tuesday.
Until then, biosecurity work had focused on containing and eradicating the species population at Lake Rotoiti.
They were first discovered in Te Weta Bay in March 2016.
More than 53,000 catfish have been netted in Lake Rotoiti since, including more than 15,700 since August this year.
Osborne told the Rotorua Daily Post it was "almost inevitable catfish would get to Lake Rotorua via the Ōhau Channel as we saw them spreading up it".
"It was just a matter of time. There is a lot of concern obviously. The real question is, is the goal of eradication feasible?"
The regional council's biosecurity team leader, Shane Grayling, said that was still "the aspiration" but the incursions were "hugely challenging".
"We are proposing that in the new Regional Pest Management Plan we change it [the focus] to progressive containment and that's based on what we think is achievable."
The regional council's budget for the response was $300,000 this year and "we are going to spend quite a bit more than that" Grayling said.
"It's our biggest biosecurity programme by some way across the region."
Lake Rotoiti hosts in excess of 40,000 anglers a year and Lake Rotorua has up to 20,000.
There have been about 10,000 nets set by regional council contractors since 2016.
Lakes Community Board chairman Phill Thomass said the Lake Rotorua discovery was "extremely depressing, but not unexpected".
"While this makes the task to eradicate so much harder, it doesn't change the need to stop their spread to our other lakes."
He said the district needed "everyone to engage in the fight against catfish".
Community response co-ordinator William Anaru said residents had been "way more driven" since the discovery in Lake Rotorua.
"I've had heaps of contact, people asking how they can help."
Currently, 26 Lake Rotoiti residents have set nets outside their properties.
"Fifteen of those are netting every night. Mainly in Te Weta Bay. The residents' nets caught 1000 last week," Anaru said.
The catfish caught are being cut up and used as bait to catch others.
Kōura are a valued taonga species, considered a delicacy by Te Arawa iwi.
Freshwater consultant Dr Ian Kusabs said "only time will tell" the extent of the threat to native freshwater kōura (crayfish) from catfish.
"Catfish are well-known predators of small, juvenile kōura and also compete with them for food."
He said the regional council and Te Arawa Lakes Trust had been monitoring kōura in Lake Rotoiti for the past 15 years "and have recorded a significant decline in the abundance of small-sized kōura at one of our three sampling sites".
"We have also recorded a similar decline in the abundance of small-sized kōura in two monitoring sites off Mokoia Island in Lake Rotorua over the past five years."
He said he was currently investigating, with NIWA, whether cobbles around the Rotorua lake shore could provide refuges for juvenile kōura from catfish predation.
Eastern Fish & Game officer Matt Osborne said bullies were a "particularly important" part of the catfish diet.
"There are no estimates yet on what proportion of bullies they could take out."
He said Lake Rotorua was the "perfect habitat" for catfish "because of the shallow depth and the sheer number of bullies, as well kōura and snails for food, and breeding zones".
Biosecurity team leader Shane Grayling said the main effects on trout would be competition for food.
"With catfish consuming as much as they consume, there is less for others."
Before you leave a lake, waterway or river, make sure you remove all weeds from your boat trailer and gear and check for catfish.
Empty any lake water or ballast you may be carrying.
Don't leave your trailer in the water.
If you want to help in the netting programme, contact William Anaru on william@tearawa.iwi.nz or go to the Rotoiti Catfish Killas Facebook page.
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