Climate change and severe weather are responsible for a decline of nearly 200 Whio breeding pairs in the past year.
A count of blue ducks from the Department of Conservation found 694 pairs in monitored areas this year.
In 2021, there were 862.
Whios are only found in the fast-moving waters of New Zealand. It is estimated that there are less than 3000 throughout the country, which is rarer than some kiwis.
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The endemic species is threatened by severe weather events linked to climate change, such as torrential rains and droughts, which are wreaking havoc on an already fragile landscape, according to an expert.
Andy Glaser, head of the Department of Conservation’s recovery group, said the bad weather put the WHO in a vulnerable position.
The rain could create wet and cold conditions for the young ducklings – their survival rate has halved in the past two years, he said.
In 2019/2020, the chicks numbered 430, while this year there were only 219.
Glaser said bad weather forced them to head into side streams to look for food, where they were more susceptible to attack by predators because they lacked their main form of defence: a large body of water.
“We can try to control predator tastes that are a manageable threat, but when it comes to climate change, there’s not really much we can do in the short term,” Glaser said.
“What we like to think is that by creating resilient and hardy populations, we hope to withstand the impacts of climate change.”
New Zealand has experienced a large number of severe weather events over the past two months.
Data from MetService showed that June held the second spot for the most lightning strikes in a single month since detailed records began.
Overall, whio populations did quite well in the last few years before this decline, thanks to a long campaign to reduce predator numbers, he said.
New Zealand needed to create a stable environment for that and ensure the population was resilient enough to handle weather events, he said.