Year in Review: "The population just went boom." This story about an explosion in Lake Rotoiti's catfish numbers was one of The Country's most popular reads of 2018.
"Business as usual won't suffice."
That's the message from Bay of Plenty Regional Council's biosecurity team leader Shane Grayling, leading the fight against catfish in Lake Rotoiti, the only known incursion in the region.
Nearly 35,000 were caught in the lake in the year to June, more than 10 times the number the year before.
Seven-thousand were caught in one night in Te Weta Bay.
Grayling told the Rotorua Daily Post the shallow, weedy, sheltered bay was the "perfect habitat" for catfish.
"They feed on just about anything, they are very good breeders, they predate and outcompete other species and they are considered the single biggest threat to kōura.
"We are assuming what the impacts are going to be but the only way you're really going to know is the worst-case scenario if they establish, and we don't want to leave that to chance. There is also the possibility that they impact on water quality by suspending sediment."
Grayling said the regional council had only found the fish in "a few isolated places" until earlier this year.
"Then something happened and the population just went boom."
He said abnormally warm lake temperatures last summer may have been the cause.
Many of the netted specimens are put in freezers and sent to the University of Waikato where their stomach contents are examined.
When asked what the chances are of the fish reaching other Rotorua lakes, Grayling said it was "definitely a possibility".
"We are going to be doing quite a lot of work in Lake Rotorua and the Kaituna River to see if they have spread into there because at this stage there is no barrier stopping them."
He said people posed the biggest risk of spreading the pest species.
"People need to be fairly careful if they are moving from one water body to the next and check their gear."
Grayling said catfish were "hardy creatures", that could live up to 48 hours out of the water, and were known to hide away in boat trailers.
He said $300,000 a year was budgeted for the catfish response.
A community catfish co-ordinator and a new biosecurity officer are being hired.
The co-ordinator will provide training and catfish trapping gear to anyone keen to "get their hands dirty".
Meanwhile, more than $90,000 worth of research strategies are being carried out by NIWA and the University of Waikato, covering water-quality effects, catfish behaviour, surveillance and ways of controlling the population.
Grayling said "aquatic pests are always very tricky to control" and individuals need to be responsible.
"One small mistake by them can cause a huge problem for everybody else."
Te Arawa Lakes Trust chief executive Karen Vercoe said "we know from recent monitoring that kōura are doing well in Rotorua so keeping them [catfish] out of Rotorua and our other lakes is important".
Rotorua Lakes Community Board chairman Phill Thomass said the Rotoiti community had been "extremely concerned" since the discovery of catfish, "but the explosion in their numbers, and finding them around many parts of the lake this year has shocked and galvanised everyone into action".
He said there were "no easy fixes".
"The saddest comment I've heard is: 'It's too late to do anything. The horse has bolted.' I can't accept that, and neither can our lakes."
Brown bullhead catfish
• Introduced to New Zealand in 1877
• Throughout the Waikato River system
• Typically grow 23cm to 30cm in length
• Can lay thousands of eggs