An out of control reinfestation of Norway rats on Stewart Island/Rakaura's Ulva Island, has now prompted a full scale aerial drop of poison on the treasured island sanctuary by the Department of Conservation (DOC).

A 1 kilometre rāhui is also being placed around the island by Te Whaka ā Te Weta Paterson Inlet Mātaitai committee as a precautionary measure to minimise risk to human health.

Operations are set to take place within the next few weeks (July-August), with an aim of eradicating all rats and returning the island to its predator-free status.

tieke/South Island Saddleback. Photo: Leon Berard/DOC

The Department of Conservation (DOC) action follows unsuccessfully trying to remove the rats by its usual control methods.

Ulva Island is sanctuary to some of New Zealand's rarest plants and vulnerable native bird species including the tieke/South Island saddleback, mohua/yellowhead, titipounamu/rifleman, and South Island kākā.

It is also used by marine species such as sea lions and penguins to rest and breed.

It's status and close proximity, just 800 metres from mainland Rakaura, has also made the island haven a popular destination for tourists.

The island became predator-free in 1997 but has had more than 20 incursions of rats since, including the brief establishment of a breeding population in 2010.

However a major forest fruiting event in 2021 and 2022 saw rats numbers spike and once again breach the island's defences. 

DOC Rakiura Operations Manager Alex Macleod says Ulva Island was protected by a dense network of traps, which typically hold the line against rats that have made the swim.

“Unfortunately, a number of rats were able to slip past the island’s line of defence and start multiplying.” 

DOC began an incursion response including deploying extra traps, hand laying bait, and using rodent detection dogs, but by May 2022 it became clear rats had become too widespread to remove by these methods.

“We went out to the community to discuss response options and decided to progress with a re-eradication plan, which was well supported."  

DOC said the poisoning operation was planned to start from the week of 17 July, but had been subject to weather and logistics.

It would include two applications of the second-generation anticoagulant toxin, Brodifacoum, at least 10 days apart and each application will take a day.

An Ulva Island Weka. Photo: Sabine Bernert/DOC

"Brodifacoum is very effective for eradicating rats as they can consume it over time without feeling ill and therefore don’t become bait shy so all individuals can be targeted," MacLeod said.

"We expect short-term impacts on some birds like weka and Stewart Island robin but are confident that populations of these species will recover quickly following breeding that is unaffected by rat predation."

MacLeod said DOC would be taking steps to minimise any bait falling into waterways as well as clearing any that fell on the island's beaches.

He said studies following the 2011 Ulva Island eradication have found low level brodifacoum residue in some fish and invertebrate species for short periods after the operation, but the risk to human health is considered very low. 

Public access to the island will be restricted for the days of aerial baiting. 

Macleod said DOC has also been doing work to upgrade the island’s biosecurity system to improve interception of pest invaders. 

South Island Robin. Photo: James Mortimer/DOC

“This includes using new tools like trail cameras, more traps, more frequent trap checks and a rearranged grid increasing detection and control devices in the coastal areas where the pests typically first arrive. 

 “Until we can achieve a predator-free Rakiura, Ulva Island will be vulnerable to reinvasion by rats and careful ongoing management is required.” 

The new biosecurity system will be activated immediately after the operation and intense monitoring over several months will be undertaken to determine whether rats have been successfully eradicated.