Department of Conservation acting operations manager Alex MacLeod said the planned poison drop was successful over the weekend.
Doc resorted to using poison bait after the once predator-free island had an influx of brown rats.
He believed the island’s native bird life might now have a shot at a bumper breeding season because they had a better chance of survival
Mr MacLeod said about 300kg of bait was dropped by air over a five-hour period.
He said it would be unknown until summer if the drop had been a success or not.
“This timeframe will give us confidence no rats remain on the island.”
From past experience, Doc did expect some species to be lost to the poison, but expected this season’s chicks would have a better chance of survival during the spring and summer breeding season.
“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 unaffected by rat predation,” he said.
“Short-term impacts like this are undesirable, but returning the island to its pest-free status has far more long-term benefits.”
The island had been established as a pest-free sanctuary after an intensive rat eradication programme in 1992 and was declared rat free in 1997, but from 2021, bird numbers had begun to fluctuate again.
The island’s close proximity to Rakiura-Stewart Island (less than 1km away), made it more vulnerable to re-invasion.
Murihiku mana whenua has also placed a rahui over the fishery surrounding the island.
The temporary ban covered a 1-kilometre offshore radius around Ulva Island which recommended no fish be consumed from within the banned area.
The rahui set a temporary prohibition on fishing until all traces of the toxin were no longer present within key indicator species such as blue cod, trumpeter, pāua, kina, mussels, and limpets.
Doc used brodifacoum-laced bait to poison the rats.
A second drop is planned in two weeks’ time.
— Toni McDonald