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  • At the start of October 2018, Wellington ecosanctuary ZEALANDIA received bad news – there was evidence of a weasel in the valley within the protective pest predator-free fence. This was the first reported mustelid incursion in over 10 years. The last incursion is suspected to have occurred when a tree fell over the pest predator-proof fence during a storm.

    Rights: Judi Lapsley Miller CC-BY-4.0

    Zealandia's fence

    The pest predator-proof fence at Zealandia was the world's first such fence. It successfully keeps pest predators out of the sanctuary but does require regular maintenance.

    It all started on 1 October 2018 when some weasel tracks were discovered in a tracking tunnel at the southern end of the sanctuary during a routine pest audit. This triggered an immediate incursion response, and a plan was set in motion to catch this most unwanted pest predator.

    Setting the traps

    The next day, 70 DOC200 traps were deployed, followed by another 40 traps later in the week. These traps were left baited and unset for a week. This was done to ensure that the pests got used to these new objects in their environment, increasing the likelihood that they would confidently enter the traps when they were finally set.

    One week later, on 9 October, the traps were set. By this time, the weasel had been seen a few times on the motion-activated cameras so ZEALANDIA knew what it was dealing with.

    Rights: ZEALANDIA

    Weasel at ZEALANDIA

    In October 2018, one of ZEALANDIA’s motion-activated cameras caught video footage of the weasel intruder.

    When the ZEALANDIA rangers and volunteers did the first round of trap checking on 12 October, everyone was delighted to discover that the weasel had been caught in one of the DOC200 traps.

    Tracking tunnels

    Tracking tunnels are tunnels with ink pads, paper and bait inside. Animals enter the tunnel to get the bait, on the way stepping though an ink pad and leaving inky tracks on the paper. These prints can be used to help identify which animals have walked through the tracking tunnel and are in the area. Examples of commonly spotted footprints at ZEALANDIA are from kiwi pukupuku (little spotted kiwi), giant wētā and many others.

    Rights: Zealandia

    Wētā and weasel footprints

    The footprints of a wētā and a weasel from a tracking tunnel at Zealandia.

    Baiting the traps

    Rabbit meat and eggs were used as the first bait. Once the weasel had been caught, these were changed to longer life baits for the next phase – confirming that there are no more pest predators. No poison was used in this operation.

    Rights: ZEALANDIA

    Dead weasel

    A Zealandia ranger holding the dead weasel that was caught in a trap.

    The pest predator-proof fence

    The entire 8.6 kilometre long fence was thoroughly checked for any sign of a possible entry point. Several small things needed attention, but none were particularly convincing entrance sites. For example, a very fine layer of moss was found on the top hat of the fence in some areas. It wasn’t enough for unwanted animals to use to get over the fence, but it could be if left untreated. This is an example of just one of the many ongoing maintenance tasks required to ensure no pest predators can gain entry into the fenced sanctuary.

    One possibility was a very small hole in the fence between the top hat and mesh that was found in routine checks weeks before the weasel got in. It had been fixed immediately, and ZEALANDIA thought it was an unlikely entrance site. Although weasels can get through very small gaps – a thumb-sized hole is all it takes – they don’t really like to climb.

    ZEALANDIA is confident that the fence is holding up well and will continue to do so – frequent checks and ongoing maintenance will see to that.

    How else could the weasel have got in to ZEALANDIA?

    ZEALANDIA thinks the weasel got in some time between July and 1 October when it was detected. With over 100,000 visitors per year, it may have come in a bag that wasn’t checked properly. It could have come through a gate while it was open. It may even have been dropped by a bird of prey. We may never know for sure, but all options have been covered and ZEALANDIA is confident in the effectiveness of its systems.

    UPDATE: In December 2019, a new theroy was put forward for how the weasel got in, it may have entered through a passageway of rotting tree roots, find out more in this Stuff news article.

    Hopefully, it will be at least another 10 years before another mustelid incursion, but as with any urban or mainland island sanctuary, the boundaries are under constant pressure and routine regular pest audit systems are crucial.

    Impact of the weasel incursion on wildlife

    Happily, ZEALANDIA has not seen any evidence of damage from the incursion. However, as the sanctuary is a 225 ha wild ecosystem, any impacts may not be immediately obvious.

    A Department of Conservation mustelid dog, Kōwhai, went through the sanctuary in late October with her handler Richard and found no sign of any more mustelids. To be completely sure, trapping and tracking will continue for a while longer.

    Rights: ZEALANDIA

    Kōwhai, the mustelid dog

    Kōwhai is specially trained to help the Department of Conservation track down mustelids.

    When weasels, ferrets and stoats were introduced to New Zealand in the 19th century to help reduce the number of rabbits, no one realised then the impact that they would have on our native wildlife.

    They’re incredible hunters, well adapted to finding, for example, birds’ nests, and they often will take eggs, chicks, sometimes even adult birds off the nest as well.

    Dr Danielle Shanahan, Manager Conservation and Research, ZEALANDIA

    What happened to the weasel?

    The weasel was handed over to Massey University’s Wildbase Hospital and underwent a necropsy. Nothing was found inside the stomach – which was expected as weasels have a very high metabolism – but the necropsy was very useful for confirming the reproductive status of the animal (an adult female) and that it was unlikely to have given birth in the sanctuary.

    The weasel was not lactating and had not given birth recently.

    The next steps

    ZEALANDIA is focused on continuing to bait, check and rebait the traps and monitor the tracking tunnels and cameras. Regular fence checks happen at least once a week and any repairs are made, but no one is relaxing. ZEALANDIA staff are hyper-alert to even the slightest hint of anything out of the ordinary to ensure the sanctuary remains pest predator free. The length of the current trapping period will depend on how events unfold.

    On 23 November 2018 ZEALANDIA announced the good news that after 2 months they were now satisfied that ZEALANDIA was once again weasel-free!

    A team effort

    The success of ZEALANDIA’s incursion response is the direct result of all the support, help and advice it gains from many conservation partners, including Greater Wellington Regional Council, Wellington City Council, Department of Conservation, Zero Invasive Predators, Orokonui Ecosanctuary and ZEALANDIA members and volunteers.

    All who support ZEALANDIA’s 500-year vision to restore a Wellington valley’s forest and freshwater ecosystems as closely as possible to their pre-human state help contribute towards New Zealand’s vision of Predator Free 2050.

    Related content

    Find out more about protecting native birds. The weasel in this article was caught using traps. Use this slide show to consider some of the pros and cons of various other methods of pest predator control.

    If you’d like to monitor predators in your area, try Making a tracking tunnel.

    Orokonui Ecosanctuary is an example of another mainland island sanctuary.

    In Creating connections with nature meet Dr Danielle Shanahan from ZEALANDIA who is investigating why nature is important for all of us, even those in urban areas.

    Useful links

    To find out more about ZEALANDIA, go to

    Find out more about mainland islands that are managed by the Department of Conservation.


    Thank you to the staff at ZEALANDIA for their assistance with this article.

      Published 8 November 2018, Updated 9 December 2019 Referencing Hub articles
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