Possum control in the Rimutaka Ranges

The Animal Health Board (AHB) and Greater Wellington Regional Council (GW) will be joining forces later this month to put nearly 30,000 hectares of the Rimutaka Range under possum control.

The pest control operation – scheduled to begin later this month – will help primarily to protect cattle and deer herds from bovine tuberculosis (TB), and improve the health of native forest.

“In order to eradicate bovine TB from a possum population, numbers need to be kept extremely low – around one or two animals over 10 hectares,” said Southern North Island Programme Manager Alan Innes.

“Monitoring within the Rimutaka Range during 2011 has indicated that the possum numbers are up to five times the level required for effective TB control. It is vital to minimise the risk of TB-infected possums sustaining the disease in their own populations and passing it to farmed cattle and deer.”

The possum control operation includes Greater Wellington Regional Council’s Wainuiomata-Orongorongo water collection area, one of three main water sources for the Hutt Valley, Porirua and Wellington. Good forest health plays an important part in maintaining relatively clean and uncontaminated source water for treatment, before supply, which in turn helps to minimise the health risk to the public.

Coordinating AHB and Greater Wellington possum control work in the Rimutaka Range provides potential gains for native plants and animals and cost savings too. The combined operational area covers approximately 28,000 hectares of public and private land. These areas have all undergone aerial possum control operations in the past. The last within the operational area was in 2007.

A helicopter using differential global positioning systems (GPS)  – to ensure accurate bait application – will sow cereal baits containing biodegradable sodium fluoroacetate (also known as 1080). This will be done in conjunction with hand-laid baiting in some areas, such as around huts and water intakes. While possums are the primary target of this operation, it will achieve a ‘triple hit’ of the worst conservation pests by also taking out rats and, through secondary poisoning, stoats. Native birds breed more successfully when there are fewer pest animals eating their eggs, chicks and the native forest they feed on.

Sodium fluoroacetate is particularly suited for this operation given the size of the area to be treated and the rugged nature of the terrain. It is a highly effective, cost-efficient and safe method of controlling non-native pests. It is highly soluble in water and biodegradable, so does not persist in water or soil. The water in the supply rivers and streams will be independently monitored and GW will not take water from the Water Catchment Area during or directly after the operation. Over 100 water tests have been carried out within the Wellington Region during previous aerial 1080 operations. None have shown any trace of 1080 contamination. In June 2011, the Parliamentary Commissioner for the Environment strongly endorsed its continued use in New Zealand.

Warning signs will be erected at all main public access points to the operational area and will remain in place for at least six months. Baits and poisoned possum carcasses are extremely toxic to dogs. The AHB and GW strongly advise all dog owners to keep their dogs safe from accidental poisoning by keeping them out of the operational area until all the warning signage has been taken down. Occasionally, poisoned carcasses may be washed downstream into areas outside the operation boundaries – usually after heavy rain. While every effort is made to recover these carcasses, dog walkers are advised to remain vigilant. Free dog muzzles will be provided on request. Please contact Debbie Viner on (06) 353 2712.

Similar dog walking opportunities are available at Akatarawa Forest, Belmont Regional Park, East Harbour Regional Park’s northern forest or other local parks. For more information on these see www.gw.govt.nz/parks or your city council website.

Additional information

Bovine tuberculosis (TB) is a highly-infectious disease that can jeopardise New Zealand’s access to premium beef, deer and dairy export markets.

In high-risk areas, possums are linked to more than 70 per cent of new TB outbreaks in herds.

The project includes the bush clad hills surrounding the Kaitoke Basin and extends to the Rimutaka Summit. South of State Highway 2, the project includes the Pakuratahi Forest and extends into the Wairarapa, including the Rimutaka Rail Trail and up to Pigeon Bush. The project extends southwards along the Rimutaka Ranges to Ocean Beach and also includes the Wainuiomata-Orongorongo Water Collection Area (WCA).

Maintaining a healthy and intact forest catchment ensures the continuation of a high-quality water source for treatment and supply. The roots of the trees bind the soil and help keep sediment out of the water, making it much easier and less expensive to treat. Possum browse damages our forests and makes them less effective at filtering our water.

Possums are targeted in the water collection area because they carry diseases such as bovine tuberculosis, and transmit organisms such as giardia and cryptosporidium, which are a contamination risk to the water supply and may lead to serious illness. International best practice advocates effective catchment management as part of a multi-barrier approach in guarding against contaminated water supplies.

The Wainuiomata-Orongorongo WCA provides about 20 per cent of the tap water for the Wellington metropolitan area.

The Medical Officer of Health requires that any trace of 1080 in water supplies must not exceed a concentration level of two parts per billion parts of water. At two parts per billion, a 60kg person would need to drink 2,300 litres every day for several weeks for any illness to occur.

For more information on how and why 1080 is used in New Zealand, visit www.1080facts.co.nz. You can download the Parliamentary Commissioner for the Environment’s 2011 report from www.pce.parliament.nz