There are fears koi carp entered Lake Karāpiro during Mercury’s routine maintenance of the dam.

Six to 10 koi were spotted in the bulkhead after an underwater tunnel was opened. Electric fishing in Lake Karāpiro the following day caught one female fish.

Koi are considered a pest fish in New Zealand. Sometimes called rats of the river they’re long-lived, breed prolifically, and cause damage to waterways and native fish. The Karāpiro dam had been thought to prevent carp moving upstream in the Waikato River.

A Mercury spokesperson said the incident happened during maintenance. The same maintenance procedure has been done four times in the past 20 years.

“During a maintenance project at Karāpiro Dam on June 18, a tunnel below the waterline was opened that may have provided for passage of koi carp from below the dam to Lake Karāpiro above. An observant operator on site noticed six-10 koi carp in the bulkhead slot and realised that upstream passage was possible.”

It is illegal to possess, breed or release koi. Offenders can receive a maximum sentence of five years’ imprisonment/and or a fine of up to $100,000.

A single fish 

Mercury notified several organisations as soon as the fish were spotted. The Department of Conservation (DoC) led a team the following day and began fishing in the lake.

DoC’s Waikato District operations manager Ray Scrimgeour said fishing began the next day using a University of Waikato electric fishing boat.

Within 90 minutes a single female fish was caught. The flesh was analysed and it was concluded it had lived in the lake for some time.

No further fish were found and fishing was halted after 10 days due to cold weather.

“DoC is planning to meet with the other partners to develop a strategy for monitoring the situation. Fish visibility is highest in spring when the fish are likely to be closer to the lake surface for spawning.”

Ticking time bomb

Fish & Game’s Auckland/Waikato fisheries manager Dr Adam Daniels believes if the same procedure had been used four times in the past 20 years, it couldn’t be ruled out the caught koi entered during one of the other maintenance operations.

He worries others may have entered as well. Koi females usually produce 100,000 eggs per kilogram of body weight. For larger females this means they can carry millions of eggs.

“So if you’ve got two in there in that huge lake and they haven’t met up yet, they just have to meet up one time in the spring when the water temperature is 18 degrees over that 10-to-20 year period of their lifespan. It’s sort of like a ticking time bomb that may or may not go off.”

Koi carp are prolific breeders. A female can have hundreds of thousands of eggs. Photo: Getty Images

University of Waikato Dr Grant Tempero said koi numbers can rapidly grow to levels which cause damage. 

“We just recently published a study where we fished a population down in a lake. We got it down to 10 kilograms per hectare and within four years it was back at about 100 kilograms per hectare.”

He believes the risk to Lake Karāpiro isn’t great due to its depth. It’s the risk to the smaller tributaries which is the biggest concern. 

“They’re likely to have more of an impact, as they disperse out into those shallower drains and things like that. In that case, they will probably going to be disturbing vegetation on the bottom, stirring up some sediment.”

Koi cause damage to waterways. Feeding like vacuum cleaners they eat plants as well as animals. They suck up the good bits, and expel the inedible. This is thought to add to water turbidity. When water is too murky, plants are less likely grow, and other fish struggle to find food.

They would also be competing with native fish for food, he said. Around 76 percent of New Zealand’s indigenous fish species are threatened with extinction.

In Huntly, which is well below the Karāpiro dam, 8.6 tonnes of koi were caught in a lake in one weekend as part of a competition.

Has it happened before?

A Mercury spokesperson was asked if koi had entered on any of the other four occasions.

“It is possible, but unlikely, koi could have entered on these occasions, as they would have been seen and reported at the time.”

Newsroom specifically asked if the risk of pest fish entering the lake during the procedure had been raised previously to June 18. The spokesperson said it had not.

“If we had identified the inadvertent transfer of koi carp as a risk we would have had a mitigation plan in place.”

The spokesperson said Mercury would ensure future procedures for opening the bulkhead were modified to “limit chance of a repeat risk”.

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