This year's build-up of the rat population in New Zealand forests can be attributed to unusually high seed production, called a mast, experts have said.
DOC has completed three out of more than 20 aerial 1080 operations planned across New Zealand. But recent monitoring of the Cobb area in Kahurangi National Park showed a rat survival rate of nearly 20 per cent - above what was hoped.
“The biggest beech mast for more than 40 years is underway in South Island forests and the exceptional amount of seed means rats don’t need to travel far for food and their home ranges have shrunk,” says Mr Morton. “It also means any gaps in bait coverage is leaving pockets of rodents that are not exposed to the bait.
“Despite killing an estimated 80 per cent of rats in the Cobb area, the surviving rats will continue to breed and increase in numbers until the seed germinates or rots in spring."
DOC is reassessing predator control for the rest of Kahurangi National Park, of which the Cobb block is just one fifth, and is planning to apply 1080 bait more intensively over a smaller area of the park than originally planned to better protect high priority areas.
The agency's technical advisory team has also revised bait application for the next three scheduled predator control operations at Arthur's Pass, the Clinton and Eglinton rivers and Te Maruia to ensure more even bait is spread at an increased rate of 2 kilograms per hectare, up from 1.5 kilograms.
“These adjustments aim for a complete bait coverage to reach all rodents given the elevated levels," says Mr Morton. "Once we have the results from these operations, we will evaluate and adjust our approach as necessary.”
DOC says the more thorough 1080 bait application has a high chance of success but will increase costs and flying time and require longer periods of fine weather to complete operations.