Ulva Island in Paterson Inlet. Photo / ODT files
Ulva Island is famous for being a predator-free sanctuary for native birds, but its status is at risk after rats invaded the island.
The Department of Conservation (DoC) yesterday said a rat was detected there in February which prompted an incursion response to the island.
DoC's bid to remove all rats has failed and rats have spread to other parts of the island.
DoC Rakiura Operations Manager Ren Leppens said it was disappointing that the intensive efforts to stamp out the incursion had failed.
The priority now was to make a plan with the Rakiura community which could include ongoing trapping to maintain lower rat numbers, or eradicating rats using ground-based or aerial toxins.
"Despite ongoing trapping efforts, at least one rat a year has infiltrated the island since 2012, most likely swimming over, although they may also have arrived on visitors' boats or stowed away in gear.
"If rats are not stamped out, we're facing a loss of some species from the island and impacts on tourism businesses that operate there.
"Our team on Rakiura is highly motivated to protect this special place, as are the locals."
Rakiura Adventure's Rakiura Herzhoff said the issue was not new for Stewart Island's residents and his company had been transporting DoC staff to the island.
"It is not unusual - the problem is they seem not to be able to get all the rats."
He hoped DoC could exhaust all the options possible before it used ground-based or aerial toxins but if this was the only option, they would need to live with the consequences.
"No one wants or likes to have sprays or toxins used there, but if it is what it takes to get rid of the rats, we all will need to live with the consequences. They have done this before.
"I don't think it will impact the business as much because Ulva Island is a beautiful place which many people visit and easily see some of New Zealand's native birds."
He believed even with the use of toxins, Ulva Island would still be one of New Zealand's most protected destinations to see birds.
Southland District Councillor Bruce Ford said this was very important to be sorted in order to reach DoC's target to make Stewart Island the first predator-free island in New Zealand by 2025.
He was worried the use of the poison to eradicate the rats could have a huge impact on the number of birds there.
"This is a cost that needs to be weighed up, but if this is what they need to do, we don't know.
"I am curious with this approach because they are supposed to be the experts, rather than everybody else.
"The fact is [even with] control and eradication [efforts] in the early days they [DoC] were not able to keep [the island] clear - my guess is that they don't know what to do."
The Stewart Island Community Board chairman said there would always be rats on the Ulva island due to the proximity to the main island.
He thought this would not impact tourism in the island, he said.
"It would be lovely to get rid of them, but there are rats in the mainland still, so it is always going to have the risk."
According to a DoC spokeswoman, rats have reinvaded Ulva Island more than 20 times since 1997, with most incursions successfully thwarted.
In 2010, an incursion resulted in a breeding population on the island and an eradication operation was undertaken to restore its predator-free status.
"DoC's technical advisers are currently reviewing options to suppress rats on Ulva Island to keep the population at a low level to protect the most vulnerable native species.
"However, this approach won't provide the same level of long-term protection as a rat-free island sanctuary."