1080 drops, revised to deal with an explosion of rodents during last year's "mega mast", have yielded contrasting results in Kahurangi National Park.
The Department of Conservation (DOC) said the revision had proved a success overall, nationwide.
The Department of Conservation (DOC) said the revision had proved a success overall, nationwide.
Just over 20 per cent of rats were killed by the poison drops in one area of Kahurangi National Park, at the South Island's north-west tip, DOC figures indicated.
But no rats were detected after 1080 operations in two other areas of the park.
DOC reduced its planned 300,000-hectare drop in Kahurangi by more than 100,000 hectares, increasing bait application in prioritised areas of the park, after rat kill from a 1080 operation in the park's Cobb area in June was lower than expected.
Around 88 per cent of rats were killed in the area, under the 95 per cent target DOC aims for to protect native species.
The biggest beech mast for more than 40 years in South Island forests provided an exceptional amount of beech seed food for rats, and the rodents' home ranges had shrunk, because they didn't have to travel far for food, the department said.
DOC prioritised western areas of the park - around Oparara and the Heaphy lowland - for the predator control, and the Wangapeka area to protect at-risk populations of native species like tuke/rock wren, long-tailed bats, Powelliphanta snails, and the whio/blue duck.
The revised method in Kahurangi provided a more "even spread" that had "proved successful in most operations around the country", DOC Motueka operations manager Chris Golding said.
Rainy, windy weather last spring and early summer caused delays to the operation in Kahurangi, with few periods of suitable weather for carrying out helicopter bait application, he said.
1080 operations in three of the park's four designated areas were completed by November 22, including the 65,159 hectare block treated in Cobb in June.
In the Oparara-Grange area (39,324 ha), results were "excellent", with rats recorded in nearly 91 per cent of tracking tunnels before the predator control operation, in November, and in none of the tunnels afterwards.
But in the Heaphy-Gunner (37,299 ha) results were mixed.
In the Heaphy-Gunner backcountry area, rats were tracking at 71 per cent prior to the 1080 operation, and 0 per cent after.
But in the Heaphy-Gunner front-country area, results were "poor" with rats tracking at 71 per cent prior, and 61 per cent of tracking tunnels after - indicating only 23 per cent of rats in this area had been killed.
The difference may have been influenced by the staggered timing of the operations in the Heaphy-Gunner operation, Golding said
The front-country operation occurred in October, nearly a month after the backcountry, due to restrictions on applying 1080 in the school holidays in the front-country.
A 1080 operation in the remaining Wangapeka area (39,735 ha) would be undertaken in the coming weeks.
Rat abundance in Kahurangi National Park due to last year's forest mast was as high, or higher than, previous mast years in 2014 and 2016, Golding said.
The rat kill rate of last year's predator control operations was similar to operations in those years, although a detailed comparison had yet to be made.
"At all sites, the knock down of rats will have resulted in stoats being killed through eating rodent carcasses.
"At sites with high rat kills we are confident a complete knock down of stoats will have been achieved. The summer stoat monitoring is underway in Kahurangi but results won't be in for several weeks."
But no rats were detected after 1080 operations in two other areas of the park.
DOC reduced its planned 300,000-hectare drop in Kahurangi by more than 100,000 hectares, increasing bait application in prioritised areas of the park, after rat kill from a 1080 operation in the park's Cobb area in June was lower than expected.
Around 88 per cent of rats were killed in the area, under the 95 per cent target DOC aims for to protect native species.
The biggest beech mast for more than 40 years in South Island forests provided an exceptional amount of beech seed food for rats, and the rodents' home ranges had shrunk, because they didn't have to travel far for food, the department said.
DOC prioritised western areas of the park - around Oparara and the Heaphy lowland - for the predator control, and the Wangapeka area to protect at-risk populations of native species like tuke/rock wren, long-tailed bats, Powelliphanta snails, and the whio/blue duck.
The revised method in Kahurangi provided a more "even spread" that had "proved successful in most operations around the country", DOC Motueka operations manager Chris Golding said.
Rainy, windy weather last spring and early summer caused delays to the operation in Kahurangi, with few periods of suitable weather for carrying out helicopter bait application, he said.
1080 operations in three of the park's four designated areas were completed by November 22, including the 65,159 hectare block treated in Cobb in June.
In the Oparara-Grange area (39,324 ha), results were "excellent", with rats recorded in nearly 91 per cent of tracking tunnels before the predator control operation, in November, and in none of the tunnels afterwards.
But in the Heaphy-Gunner (37,299 ha) results were mixed.
In the Heaphy-Gunner backcountry area, rats were tracking at 71 per cent prior to the 1080 operation, and 0 per cent after.
But in the Heaphy-Gunner front-country area, results were "poor" with rats tracking at 71 per cent prior, and 61 per cent of tracking tunnels after - indicating only 23 per cent of rats in this area had been killed.
The difference may have been influenced by the staggered timing of the operations in the Heaphy-Gunner operation, Golding said
The front-country operation occurred in October, nearly a month after the backcountry, due to restrictions on applying 1080 in the school holidays in the front-country.
A 1080 operation in the remaining Wangapeka area (39,735 ha) would be undertaken in the coming weeks.
Rat abundance in Kahurangi National Park due to last year's forest mast was as high, or higher than, previous mast years in 2014 and 2016, Golding said.
The rat kill rate of last year's predator control operations was similar to operations in those years, although a detailed comparison had yet to be made.
"At all sites, the knock down of rats will have resulted in stoats being killed through eating rodent carcasses.
"At sites with high rat kills we are confident a complete knock down of stoats will have been achieved. The summer stoat monitoring is underway in Kahurangi but results won't be in for several weeks."
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