DOC says the recent 1080 operation at Ōkārito has decimated rat and stoat numbers in the treatment area, lowering the risk to kiwi and other forest birds of predation during the breeding season.

DOC says the recent 1080 operation at Ōkārito has decimated rat and stoat numbers in the treatment area, lowering the risk to kiwi and other forest birds of predation during the breeding season.

Post-operation monitoring of the numbers of rats and stoats in the treatment area of Ōkārito Forest has shown that their numbers have plummeted since they were last assessed in August this year. Rodent and stoat numbers in the untreated area remained high.

Rodent and stoat numbers have been assessed every three months since 2001 using a method called small mammal indexing. The indexing result is a percentage, which gives an idea of how many stoats and rats are around.

Risk to kiwi in Ōkārito Forest from rats and stoats has lowered following a successful 1080 operation.
Risk to kiwi from rats and stoats in Ōkārito Forest has lowered following a successful 1080 operation

This year, due to the increased availability of food from the fruiting rimu and kahikatea, rat numbers in the treatment area began to climb quickly from 4% in November 2010 to 29% in August this year and were mirrored by a rise in stoat numbers from 13% in November 2010 to 48% in August. As one stoat can kill dozens of birds in a short space of time, this was a grave threat to the bird species in the area. After the 1080 operation, rat numbers dropped to 1% and stoats were down to 0%.

Biodiversity Programme Manager Jim Livingstone cautioned that there would still be a few stoats present but their numbers are now far lower than before the operation.

“We know from similar operations in other places to expect up to 40% chick loss but this is far better than the normal 95% losses that happen without control,” Mr Livingstone said.

Rats were found to be present on the edge of Ōkārito Forest, but only in the untreated ‘buffer zones’ outside the treatment area. Rat numbers in the nearby untreated ‘control’ area were very high at 95%, and stoat numbers were also comparatively high at 23%.

DOC’s Franz Josef Waiau Area Manager, Wayne Costello said, “This operation has been on hold for three years, as we waited for the right level of rodent and stoat numbers in the forest. The results of our monitoring after the operation have shown that we timed the operation just right. By taking out the main predators of kiwi and other bird species, we are ensuring that our native species have the best possible chance of raising chicks successfully in the wild. 1080 is undoubtedly the best tool available for us to achieve this goal”.

DOC is also closely monitoring the kea population in the treatment area. Out of a monitored group of 37 birds, 7 died of 1080 poisoning immediately after the operation. Two weeks of intensive monitoring after the operation have confirmed that all other monitored birds in the treatment area are alive and appear to be doing well. However, a monitored kea nest in the untreated ‘control’ area was plundered by a stoat last week, and the newly hatched chicks were killed.

“It’s disappointing to lose any of these wonderful birds,” said Mr Costello, “but the surviving kea within the area of the 1080 operation should now be able to successfully raise their chicks without facing the threat of predation by stoats or possums.”

More than 60% of kea nests fail in the Ōkārito area, predominantly due to predation by stoats and possums when no pest control is undertaken. A four year research programme is underway to assess the overall effect of 1080 operations on kea populations, and to find ways to minimise the risk to kea. So far the study has found that without predator control, only 36% of kea nests are successful in fledging a chick.

The aerial 1080 possum control operation involved three years of planning and was jointly run by the Animal Health Board and DOC. The operation covered 30,000 hectares which included the South Ōkārito kiwi sanctuary, North Ōkārito forest, and a large forested buffer zone around Franz Josef Township itself. The operation is intended to provide New Zealand’s rarest kiwi – rowi – protection from rats, stoats and possums as well as providing protection to local farms from the threat of bovine tuberculosis.   

Small mammal indexing

Small mammal indexing is a method of comparing the relative abundance of stoats and rodents in an area over an extended period of time.

Small plastic tunnels are set along numerous lines within the study area, and pieces of card with inkpads set into them are placed in the tunnels. The tunnel is then baited with peanut butter (for rodents) or rabbit meat (for stoats). The animal runs over the inkpad to get the bait leaving its footprints behind on the card.

The cards are then collected and assessed. The percentage of cards with tracks on them is then compared to the results from previous indexing work. This gives a good indication of whether the numbers of pests in an area is increasing or decreasing, and at what rate.

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