People are being urged to help stop the spread of catfish in Rotorua lakes, as it is confirmed the number of the predatory pests has increased.

The species has spread and increased in numbers in Lake Rotoiti and now pose a real threat to other waterways, Rotorua Te Arawa Lakes Programme partners say.

Netting in Lake Rotoiti over the past three months has captured more than 11,000 of the brown bullhead catfish.

Although most were juvenile (40mm– 80mm) the numbers in the lake could potentially be devastating to all Te Arawa lakes.

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Bay of Plenty Regional Council biosecurity team leader Shane Grayling said catfish were opportunistic predatory scavengers and considered the single biggest threat to koura (freshwater crayfish).

"It could have been just one lake user's negligence which has resulted in these 11,000 brown bullhead catfish being caught in Lake Rotoiti – it's now up to all lake users to stop the pest incursion from spreading to other Te Arawa lakes.

"To stop the spread of the catfish to other waterways all lake users need to check, clean and dry, not only their boats but trailers, propellers, fishing, sporting and earthmoving equipment when they are moving from one waterway to another.

"The potential impact of catfish varies from predation on species of significance to competition for food and space with other desirable species. We all have to do our part now to prevent them from spreading elsewhere."

To date the regional council has spent a significant amount of its biosecurity budget on control, surveillance and research on the catfish population and it was now looking at options to prevent the pest from spreading further, he said.

The regional council was investigating whether introducing brown trout or long fin eels to Lake Rotoiti could combat the growing catfish population, as well as electric barriers and bubble curtains in the water to stop the spread to other lakes.

Grayling said the council was also working with the University of Waikato on using eDNA in water samples to help determine whether a species such as a catfish were present in it.

EDNA uses the DNA present in water samples to detect if a species is present. Once developed it could make detecting pest fish much quicker and easier.

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Co-chairwoman of the strategy group, which is the governance group for the Rotorua Te Arawa Lakes Programme, Rotorua mayor Steve Chadwick was "extremely concerned" about the increasing threat posed by catfish.

They were initially confined to Te Weta Bay but there has been a significant increase in numbers and distribution.

"The lakes are extremely important to our community and to our district's economy – they are a major reason why people live and visit here and we need to do whatever we can to protect them."

Rotorua Te Arawa Lakes Strategy Group chairman Sir Toby Curtis said they had engaged with the Ngati Pikiao Environment Society and Komiti Whakahaere on the issue.

"There is a significant concern about the impact of catfish on koura as a taonga species but also the general state of the lake."

The Te Arawa Lakes Trust was seeking every opportunity to support researchers and managers working on this issue to ensure the situation is under control quickly, he said.

"We need the Te Arawa whanau who all use the lakes to check their boats to make sure there were no catfish being transferred to other lakes – that would be a disaster."

If you see anything fishy, call 0800 STOP PESTS (0800 786 773) or send an email to STOP.PESTS@boprc.govt.nz