A lone Norway rat on the previously pest-free island of Te Motutapu a Taikehu (Motutapu) in the Hauraki Gulf/Tīkapa Moana has finally been eradicated after having eaten toxic bait.

Over the last five months, the Department of Conservation with support from Ngāi Tai Ki Tāmaki and Auckland Council tried multiple methods to catch this rat, which was first spotted in late April.

The rat was filmed stocking up on toxic bait over four days in August, and since 8 August there have been no further sightings on the network of cameras. Last week a DOC rodent detection dog and handler, undertook a sweep of the island with no detections – giving DOC confidence to declare the rat dead.

DOC Island Biosecurity Advisor Claire Warren says the incident response team used both standard and new tools to learn more about the rat’s location and behaviour, and to eventually kill it. A network of around 50 cameras was the key tool that led to the predator’s demise.

“We used a combination of standard trail cameras and AI cameras that gave us important and surprising information about the rat’s behaviour, routes and favourite locations.

“The cameras revealed that the rat travelled from a suspected nest site to McKenzie bay on Rangitoto Island, around 5.8 km away, which is much further than we ever expected.

“The camera network helped us conclude that there was only one rat on island, and it was instrumental in determining where to lay toxic baits,” says Claire.

Two new thermal imaging cameras, named DOC AI CAM and developed by the Cacophony Project, were a key part of the mix.

“The AI cameras sent us alerts when the rat was seen on camera. This gave us real-time information about the rat’s location and direction of travel which really helped us target it,” explains Claire.

“It is always great to have a successful end to an incursion response, as they can be very challenging and a huge threat to taonga species on these beautiful islands,” says Billy Brown, Chair of Ngāi Tai ki Tāmaki Trust.

“Ngāi Tai ki Tāmaki and DOC continue to be vigilant around biosecurity risks on Te Motutapu a Taikehu and Peretū/ Rangitoto, as we manage a number of endangered species, that will only thrive if we give them a safe space to live. I want to congratulate the DOC and Ngāi Tai ki Tāmaki team on island, along with the myriad of people who run and manage these responses, for embracing kotahitanga, as together we can eliminate these threats faster.”

Although the incident team used a variety of trap types and lures such as peanut butter, rat bedding and rat recordings, the rat did not interact with any of these human-made devices – illustrating the real challenge in keeping valuable islands and areas pest-free.

“With every incursion we have, we learn more about how to track and kill mammalian pests. When they evade all your attempts to entice them into a trap, it forces you to think laterally and innovate,” adds Claire.

The eradication is a big relief as the island is a key recovery site for one of New Zealand’s rarest birds, the tūturuatu/shore plover, with just 250 individuals remaining nationwide. A single rat could have devastated the small island population.

A range of other sea and shore birds also began their nesting season in July - eggs and chicks are easy targets for mammalian predators.

Background information

Hunt for rogue rat steps up on Motutapu: Media release 26 June 2024 (doc.govt.nz)

New AI predator detection tools available: Media release 11 July 2024 (doc.govt.nz)

The islands of Motutapu and Rangitoto are considered pest-free following (at the time) the world’s largest pest eradication programme in 2011. They are now home to dozens of taonga species including takahe, tieke, kiwi and tuatara.

Rats are considered one of the most prolific pests in Aotearoa, feeding on the eggs and chicks of native birds, as well as competing with them for other food sources.

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