In August 2022, conservationists were alarmed to find evidence of a stoat on Chalky Island (Te Kākahu-o-Tamatea), a small, rugged island sanctuary in Fiordland, New Zealand, that had been predator-free since 1999. The discovery was a major shock, as stoats had been deliberately kept out for decades. The Department of Conservation (DOC) immediately launched a full-scale incursion response, treating the single stoat as an urgent threat. They deployed trained conservation dogs, expert trappers, trail cameras, and even helicopters and boats to scour the island’s 511 hectares for the elusive animal. In New Zealand, even one stoat can spell disaster for native species.

Chalky Island shelters a wealth of vulnerable and endemic wildlife, including the Te Kākahu skink (found nowhere else), the critically endangered kākāpō (a flightless nocturnal parrot), and rare birds such as the mohua (yellowhead), tīeke (saddleback), and little spotted kiwi. These species evolved without mammalian predators, leaving them defenseless against a stealthy carnivore like the stoat. Just one can raid nests, kill chicks, and even take adult birds with alarming efficiency. DOC officials warned that if the intruder wasn’t caught quickly, it could wipe out chicks and lizards on the island, undoing years of conservation work.

The stoat (Mustela erminea) is a small carnivorous mammal in the weasel family, and it is not native to New Zealand. In the late 1800s, European settlers introduced stoats, along with weasels and ferrets, in a misguided attempt to control exploding rabbit populations that were damaging farms. Scientists at the time warned that these predators would devastate New Zealand’s unique birdlife, but the warnings went unheeded. The prediction came true: stoats spread rapidly across the country, and within a few years, native bird numbers were in steep decline.

New Zealand’s birds were especially vulnerable because many are ground-nesting or flightless. Before humans arrived, the islands had no land mammals apart from bats, so birds such as kiwis, kākāpō, and some ducks evolved with no defenses against mammalian predators. Stoats can outclimb, outrun, and outswim most of their prey. In some areas, their impact has been severe. In parts of the South Island, for example, about 70 percent of the endangered blue duck (whio) population is male because so many nesting females have been killed by stoats.

Stoats also cause boom-and-bust cycles in wildlife populations. During occasional “beech mast” years, when beech forests produce a bumper crop of seeds, rodent numbers surge, and stoats breed rapidly to take advantage. When the rodent population crashes, the stoats turn to native birds instead. In one recorded stoat “plague,” a sudden increase in their numbers wiped out more than half of the wild takahē in unprotected areas, reducing the species’ population by a third in a single year.

Today, stoats are a top target in New Zealand’s conservation work. Large amounts of resources go into controlling and eradicating them in key wildlife areas. The government’s ambitious Predator Free 2050 program aims to eliminate stoats, rats, and possums nationwide by mid-century. Until that goal is reached, conservation managers focus on creating predator-free sanctuaries, either on offshore islands or within fenced mainland reserves, where native animals can survive without constant predation. Chalky Island is one of these crucial refuges.

The island’s dense forests, cliffs, and hidden nooks offered plenty of cover for a crafty predator. Once the intruder’s footprints and later droppings were confirmed, the Department of Conservation (DOC) launched a full-scale search. Every few weeks, teams checked traps and followed any sign of activity. DNA testing of the droppings confirmed they were dealing with a single male stoat rather than a breeding pair or multiple animals, which was a small relief in a challenging mission.

For eight tense months, nothing was left to chance. The Department of Conservation (DOC) estimated the main search-and-trapping campaign cost about NZ$483,000, with a further NZ$210,000 spent on surveillance and biosecurity, bringing the total to nearly NZ$700,000. DOC argued the cost was justified to protect Chalky Island’s irreplaceable wildlife, noting that relocating the kākāpō would have cost millions and undone years of restoration work.

In April 2023, after months of false leads and hundreds of trap checks, rangers found the stoat dead in a trap. “This is a huge win,” said Rebecca Teele, DOC’s incident controller for the operation. Biosecurity monitoring continues, however, since stoats are known to swim between islands. Even as this one was caught, droppings were found on nearby Passage Island, hinting that another stoat may be in the area. Teele stressed that Chalky is “one of the highest-priority sites for biodiversity in Fiordland,” and conservation teams will “do everything we can to protect the vulnerable species living there.”

The idea of using remote islands as safe havens for endangered wildlife is not new. In the 1890s, Richard Henry, often called New Zealand’s first conservation ranger, championed the concept. Alarmed by the rapid decline of flightless birds such as the kākāpō and kiwi on the mainland, he sought a refuge free from introduced predators. In 1894, Henry moved to Resolution Island, a large uninhabited island in Fiordland’s Dusky Sound, and began an ambitious rescue effort. Over several years, he rowed back and forth from the mainland, personally relocating hundreds of rare birds to the island’s forests. By 1897, he had transferred around 500 birds, including both kākāpō and kiwi, believing they would be safe there.

Unfortunately, his hopes were short-lived. Unbeknownst to him, stoats had already begun reaching even these remote islands. First released on the mainland around 1884-1885, stoats proved to be strong swimmers, easily crossing rivers and fjords. By 1897, Henry saw his first stoat on Resolution Island, a moment that shattered his confidence in the island’s security.

A confirmed sighting followed in 1900, when tourists reported a “weasel” chasing a weka along the shoreline. Henry recorded in his diary that it “looked so like a joke that I only laughed,” but he knew the implications were serious. A single stoat could devastate the bird populations he had worked so hard to protect.

Henry acted quickly, setting traps and even moving some birds to smaller nearby islets in an attempt to stay ahead of the predators. Despite his efforts, the stoats spread, and within a few years the effects were clear. Species such as robins, kākāriki (parakeets), and kākāpō began to vanish, some showing signs of malnutrition as they struggled to survive. By the early 1900s, Henry wrote that the birds in Dusky Sound were “perishing and there was nowhere safe to take them.” In those days, there was no practical way to remove stoats once they had established themselves.

After 14 years on Resolution Island, Henry left in 1908, heartbroken that his sanctuary had failed. His work faded into obscurity for decades, but modern conservationists now recognise him as a visionary ahead of his time. Without the tools, resources, or national support available today, his transplanted birds were doomed to fall victim to the same predator that conservationists still battle more than a century later.

Even if an island is cleared of stoats once, there is no room for complacency. Stoats are resourceful travelers that, in Fiordland, often swim across channels and fjords in search of new territory. For years, scientists believed a 300-meter (984-foot) water gap would prevent reinvasion, but that theory proved wrong. Stoats have been recorded swimming for kilometers; in one study, a stoat swam continuously for two hours, covering about 1.8 km (1.1 miles) of open water.

There are many examples of stoats reaching “safe” islands. In 2010, one was found on Kapiti Island, which lies about 5 km (3.1 miles) off the coast. On Chalky Island, the recent intruder likely swam from the mainland or another island, showing that no natural barrier is foolproof, so New Zealand’s conservation teams must maintain strict biosecurity, including regular trap checks and detector dogs on islands, as well as trap lines along the adjacent mainland coast as an interception buffer.

In the late 1990s and early 2000s, the Department of Conservation (DOC) carried out groundbreaking eradication campaigns on several Fiordland islands, including Chalky (Te Kākahu) and nearby Anchor and Bauza Islands. Chalky became the first in the region to be cleared of stoats, paving the way for similar successes elsewhere. Extensive trap networks and pre-baiting to overcome stoats’ natural wariness led to rapid results. On Chalky and Anchor, all the stoats were caught quickly.

Without constant predation, native bird populations have flourished. Rangers note that within just a few years of being predator-free, the forests fill with birdsong and species once rare or absent. On Bauza Island, the diversity and abundance of native birds surged dramatically within only two years of stoat removal.