A shortjaw kōkopu.
Photo: DOC

A rare and secretive native fish that lays its eggs on land is getting a helping hand to fight back against rats plundering its nests.

Last year, Department of Conservation ranger Fern Donovan was astonished to discover three nests made by the threatened shortjaw kōkopu in Northland’s Waipoua Forest.

It was the first time a shortjaw kōkopu spawning site had been found in Northland – in fact, it was the first time anywhere, apart from a discovery in Taranaki in 2001.

Shortjaw kōkopu nests consist of 50 or more tiny, translucent eggs laid on leaf litter or gravel along stream edges.

The shortjaw kōkopu is unusual among fish in that its eggs develop on land, such as here in the Waipoua Forest.
Photo: DOC / Fern Donovan

However, Donovan’s amazement soon turned to despair.

“I set up a trail camera to monitor the nest and visited it daily. I was gutted to find the nest had fewer and fewer eggs each day,” she said.

“When I checked the trail cam, it showed a rat visiting multiples times scoffing all the fish eggs.”

Within ten days the only eggs left were rotten, unviable and inedible, even for rats.

That was enough to spur DOC and local iwi Te Roroa to take action.

Together they set up an intensive predator control programme ahead of this year’s shortjaw spawning season, which peaks between May and June.

Te Iwi o Te Roroa science advisor Taoho Patuawa said Donovan’s discovery was proof of the impact rats had on the taonga species during a critical part of its lifecycle.

He said rodent tracking tunnels, which were used to gauge the presence of rats and mice, showed the predator control strategy was working.

In the predator control zone just three percent of tracking tunnels were visited by rats, compared to 96 percent in non-treated areas.

Trail cam image of a rat devouring eggs in a shortjaw kōkopu nest in Waipoua Forest last year.
Photo: DOC / Fern Donovan

Donovan said she found two more nests this year.

As of last week, tiny eyes were visible in some eggs and if she looked closely she could see the larvae twitching.

That gave her hope the eggs would survive.

Last year all the eggs had been eaten by the time they reached that stage, she said.

She had been unable to return to the nests this week due to heavy rain during the weekend, but she was optimistic the eggs had hatched.

The shortjaw kōkopu laid its eggs near stream edges in late autumn to early winter, after heavy rain when water levels were high.

The eggs developed on land until the next flood, between two weeks and two months later, triggered hatching.

The larvae would then be washed out to estuaries and the sea, eventually swimming back upstream as whitebait.

Donovan said she had been monitoring adult fish in Waipoua Forest for several months in 2023 when she made her discovery.

“When I figured they’d spawned and laid their eggs, I went out along the stream margins and searched and searched and searched, crawling on my hands and knees. So when I did find some spawning sites after weeks of looking, I was ecstatic, I was super-excited.”

Donovan said the shortjaw kōkopu was worth protecting.

“They’re endemic to New Zealand, which means they’re found nowhere else in the world. They are important piece of our freshwater ecosystem and a really good indicator of stream health. They’re just an important piece of biodiversity that represents the uniqueness of our flora and fauna,” she said.

DOC ranger Fern Donovan in the Waipoua River.
Photo: DOC / Sarah Wilcox

Patuawa said he was looking forward to seeing what could be achieved by DOC, Te Roroa and other organisations working together to restore the Waipoua River.

“It’s good to have such a concentrated effort in our forest. What we’re discovering will be valuable for protecting this taonga species both here and in other parts of Aotearoa, and ensuring shortjaw kōkopu can thrive,” he said.

The project is part of DOC’s Ngā Ika e Heke freshwater migratory fish project to safeguard populations of shortjaw kōkopu, īnanga, longfin eel/tuna and lamprey across New Zealand.

Shortjaw kōkopu are the rarest of the six native fish species caught as whitebait. Their threat status is threatened-nationally vulnerable.

Waipoua Forest is also home to New Zealand’s largest surviving kauri trees, including the famed Tāne Mahuta.