Big increase in whio numbers in Kahurangi National Park

Date:  21 September 2023

The current survey, started in 2020 and still underway, has found to date a 340% increase in whio/blue duck numbers in Kahurangi since a DOC whio survey in 1998-2000.

In the latest survey, 846 adult whio, with 335 breeding pairs among them, have been observed. In the 1998-2000 survey, 191 adult whio, including 58 pairs, were counted.

DOC Science Technician Jason Malham says the combined endeavours of DOC, the community, business and a captive breeding facility have greatly improved the fortunes of whio in Kahurangi National Park since the 2000 survey.

“We started two DOC whio security sites in the early 2000s to protect whio with stoat trapping and boosting breeding with techniques that have enabled more rapid growth in their numbers. The Ōpārara-Ugly and Wangapeka-Fyfe Whio Security Sites have both surpassed the goal of 50 breeding pairs.

“Support from Genesis through the Whio Forever programme, The Isaac Conservation and Wildlife Trust in raising whio ducklings, and contractor and community volunteer help in maintaining traps have been instrumental to the security sites protecting whio and increasing their numbers. The community volunteers include Tapawera Area School students and other locals.

“Aerial 1080 predator control over large areas of Kahurangi National Park has also increased protection for whio and other vulnerable native wildlife from high rat and stoat numbers in years when there has been beech seeding, known as a mast.

“More whio pairs generally have been found on security site rivers where stoat trapping is supplemented with aerial 1080 predator control. The average pair density on rivers with both forms of predator control was 0.76 pairs per kilometre compared to 0.58 pairs per kilometre with 1080 alone and 0.12 pairs per kilometre before predator control or any whio management was carried out.

“Community groups are undertaking trapping which contributes to protecting whio. These include the Friends of Flora, operating in the Flora Stream catchment, the Friends of Cobb, in the Cobb Valley, and the Mokihinui-Lyell Backcountry Trust on the Old Ghost Road.

“An Air New Zealand-supported biodiversity project in Gouland Downs has a trapping network to protect whio, takahē, roroa/great spotted kiwi and other native species.”

DOC staff have walked more than 700 kilometres of waterway in the current whio survey. Exact counts of whio in the 516,000-hectare national park aren’t possible, but DOC staff are highly encouraged by the substantial increase in the Kahurangi whio population indicated by the numbers being observed.

Although overall numbers have risen, in some places, setbacks for whio have been noticed since the current survey started that reflect challenges the ducks face in the wild.

Survey results indicate climate change may have impacted whio in some parts of the national park. Repeated flooding of rivers in successive very wet winters appears to have led to fewer breeding attempts in places, with very few ducklings or juveniles encountered in most surveys at those sites.

Repeated surveying of some rivers showed a decline in whio numbers that is thought to be due to the 2019 mega beech mast. It was the biggest beech mast for more than 40 years in South Island forests and with abundant beech seed for food, rat and mice numbers soared. Stoat and feral cat numbers also escalated due to the increased numbers of rodents they feed on. Aerial 1080 predator control was carried out over a large part of Kahurangi National Park but it wasn’t possible to cover all the park.

It’s thought that as the beech seed ran out and rodent numbers dropped off in 2021, hungry stoats and feral cats then turned to preying on even large native birds such as whio and kea. Predators moving in from land adjoining the park particularly appears to have impacted whio numbers on some rivers near the park edges.

Background Information

Whio

• The whio is a threatened species of native duck only found in New Zealand’s fast flowing waters. Featured on New Zealand’s $10 note and with an estimated population of less than 3000 birds, whio are rarer than kiwi.
• Whio are adapted to live on fast-flowing rivers. Finding whio means you will also find fresh, fast-flowing water with a good supply of plants and underwater insects.
• This makes whio important indicators of ecosystem health – they only exist where there is quality fresh water and an abundance of life.

Whio Forever

• Genesis has a strong historic association with whio through the Tongariro Power Scheme. In 2010 this association grew through the establishment of Whio Awareness Month (March).
• Today, Genesis and the Department of Conservation (DOC) continue their partnership through The Whio Forever Project, which aims to secure the future of whio in the wild and ensure New Zealanders understand and value whio in our rivers.
• The support of Genesis and the work of DOC has enabled the Whio Recovery Plan to be implemented.

Conservation issue

• The whio are predated by stoats, ferrets and cats with the largest impact during nesting time when eggs, young and females are vulnerable, and also when females are in moult and can’t fly.
• Extensive trapping can manage these predators and work in key whio habitats by DOC and Genesis on the Whio Forever Project has already seen an increase in whio numbers.
• Whio cannot be moved to predator-free islands like other species because of their reliance on fast-flowing rivers.
• Pairs occupy approximately 1km of water – so they need a lot of river to sustain a large population and they fiercely defend their territories, which makes it difficult to put them with other ducks in captivity.
• They are susceptible to flood events which, destroy nests, fragment broods and wash away their valued food source.

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