The sun is beating down as a group of rafters make their way down the Tongariro River. They're on a welcome calm stretch of water, after several hair-raising sections of whitewater rapids.

Then a murmur ripples through the group. Standing on rocks at the edge of the river is a breeding pair of whio, also known as blue duck.

They're rare, there's fewer than 3000 throughout New Zealand. But now they're calmly watching the rafters drift past on the water.

It's a sight that's becoming more common, thanks to the community stepping up for a monumental pest-trapping effort.

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Nine years ago two breeding pairs of whio were on the Tongariro River.

This year there's 17 breeding pairs. Eleven pairs nested this season, and fledged 40 ducklings between them.

It all started when Garth Oakden wanted to start a conservation project through his company, Tongariro River Rafting.

Tongariro River Rafting co-owner Garth Oakden started the operation to bring a conservation element to his business. Photo / Frances Cook.
Tongariro River Rafting co-owner Garth Oakden started the operation to bring a conservation element to his business. Photo / Frances Cook.

He approached ecologist Nick Singers for ideas.

"I didn't really want to plant trees, because there's plenty of people planting trees," Oakden said.

"[Singers] suggested trapping rats and stoats, and all I could think was 'how on earth are we going to do that'?"

Parts of the Tongariro River are only accessible by raft, and other parts take a long time to walk to.

The answer turned out to be the perfect marriage of technology, community, and business.

The group decided to use a mix of traditional one-use traps, and the self-resetting A24 trap, developed by Wellington company Goodnature and which could kill 24 pests before needing to be reloaded.

Workers from Tongariro River Rafting cleared and reloaded some of the traps, hunters were responsible for others, and inmates at Rangipo Prison took on the rest.

With that, the Blue Duck Charitable Trust was born.

Rafting guides will help out clearing the traps, while tourists get to see more of the rare blue ducks. Photo / Frances Cook.
Rafting guides will help out clearing the traps, while tourists get to see more of the rare blue ducks. Photo / Frances Cook.

"It involves almost everyone socio-economically through the [Turangi] community, which is pretty cool," Oakden said.

"The way that we've done it means it can be constantly worked on, in a sustainable effort.

"If we stopped, probably within six months we would be back to where we started."

The boost to his tourism business was icing on the cake. Rafters loved seeing the ducks.

"It's like saying 'there's only 3000 of this rare car left'.

"And then you take a car nut out to see it for themselves."

Goodnature technical expert Nick Graham demonstrating one of the automatic A24 traps, while another is next to him on the tree. Photo / Frances Cook.
Goodnature technical expert Nick Graham demonstrating one of the automatic A24 traps, while another is next to him on the tree. Photo / Frances Cook.

Goodnature technical expert Nick Graham said the self-setting A24 was key to trapping stoats, which were the biggest problem for whio.

"You can set a single-set trap, and it would be set off that night by a rat, rather than the stoats you want.

"Now that trap is deactivated until you come back.

"But with this technology, you can kill multiple rats, and the trap is still available to kill the stoats."

The trapping effort is stepped up through spring and autumn when whio are most vulnerable.

Nesting ducks and ducklings find it hard to defend against stoats in spring.

In autumn, when the birds are temporarily flightless because of their annual moult, they're at risk again.

The isolation of the Tongariro River means that pest operations can be more difficult. Photo / Frances Cook.
The isolation of the Tongariro River means that pest operations can be more difficult. Photo / Frances Cook.

Singers said the rise in duck numbers couldn't have happened without the effort from the entire community, including Rangipo Prison.

Inmates check 160 traps along 8km of river.

"As an inmate's getting close to release, they're trying to engage them back into the community," Singers said.

"So getting out to do conservation work in the last few months [is] one of the last stages for them."

The inmate work alone has saved $15,000 in labour costs.

"We couldn't pay that, and we're a little community so we couldn't get enough volunteers.

"The prisoners love it, it's the perk job for them."

This breeding season has been the most successful, thanks to the 40 fledged ducklings.

The previous record was the 2015-16 season, when 29 ducklings fledged.

March is Whio awareness month.