The Perth Valley. ZIP's rat and possum eradication in the Perth Valley will be the first time anyone has attempted to clear a mainland tract of pests, then hold the line. As such, it might be a roadmap to a Predator-Free 2050.

By the time it joins the Whataroa River, not far from the West Coast town that borrowed its name, the Perth is on its best behaviour – stately and composed. Up here though, in the steep foothills of the Butler Range, it's a ceaseless, churning roil – meltwater, grey and glacial, hurled headlong down a chaos of giant schist boulders.

A tramper wouldn't even think about trying to cross this mayhem, and Al Bramley is hoping possums and rats feel the same way. There are roughly 10,000 hectares of upland rata-kamahi forest and leathery subalpine scrub between here and the similarly rambunctious Bettison and Barlow Rivers, and the reaches delimit a triangle, closed off behind by the snowfields of the Southern Alps, that Bramley, chief executive of Zero Invasive Predators (ZIP), reckons he can defend.

ZIP is a public-private R&D venture that's bringing pest control into the 21st Century. Bramley, an engineer by training, has spent the last three years figuring out how to do pest control at high resolution, using technology to add precision to a task that, historically, has simply worked on percentages.

supplied Redfield Swingbridge on the Perth River. Rats and possums might be wary of raging rivers, but will happily use a swingbridge. Pest-proof gates will deny them access.

Such that, here at the Perth, ZIP means to kill every last possum, every last rat, in this triangle, then hold the borders against any subsequent invaders. If that sounds like a tall order, consider that the government has already committed to a similar plan to rid the entire country of possums, rats and stoats, called Predator-Free 2050, and ZIP's model – Remove and Protect – is already its de facto M.O.

READ MORE:

* Predator-free New Zealand: No fences allowed at Zip facility

* Government sets target to make New Zealand 'predator-free' by 2050

* 1080: Braving the storm for our wildlife

* Why 1080 is a dirty word

Not everyone's happy about that, because it means using 1080. But Bramley says he has no other choice: the technique calls for an initial knockdown that only aerial poisoning can deliver, and there's only one toxin approved for general aerial use in New Zealand, and that's 1080.

supplied A ZIP ranger beside the Perth River. 1080 will get rats and possums down to zero inside the project area: pest control experts hope these churning waters will keep others from re-invading.

Franz Josef motelier Logan Skinner has a few concerns, manly around collateral casualties like deer and kea. "There's a known downside to 1080." But if it works, he says, "and the area can be kept predator free, it would be amazing for Glacier Country from a both environmental and business perspective."

He'd sooner ZIP didn't use 1080, but the "Protect" part of Remove and Protect is, he says, a step in the right direction: "The potential, long term, for a reduction in 1080 use is very positive."

Bramley's candid about the reality of bykill. "On our website right now, you can read about the non-target kills during our Arawhata-Jackson operation – what we killed that we didn't want to kill." In nearly 400 hectares, searchers found one dead chaffinch, a song thrush, three blackbirds, two tomtits, and three deer.

supplied Every trap and camera inside the ZIP project area will be connected to a low-frequency radio network, letting rangers know the moment they encounter a rat or possum.

Historically, 1080 operations have consisted of one drop of non-toxic baits, to get pests used to the idea of eating them, then a poisonous follow-up a few weeks later. But ZIP's research has shown that one pre-feed doesn't convince enough pests to get the sort of knockdown it needs.

The ZIP model – 1080 to Zero – calls instead for two, says Bramley, "So that every pest gets hooked into it. Then we allow for rain to wash every trace of pre-feed away before dropping toxic versions of the bait."

Then, he says, "We check if we left anything behind."

It seems fanciful to suggest you can detect a single surviving rat in a tract of dense bush, but ZIP has already done it – twice: at Taranaki Maunga (1600 hectares), and at Arawhata/Jackson (2300 ha). "People don't appreciate how intensive our

detection network is," says Bramley. "At Taranaki Maunga, we put two detection devices into every rat's home range. Then we watched, for 50 days."

Estimates of rat numbers before the Taranaki drop ran to around "a couple of thousand". After it, the system told Bramley there were 12 survivors, "and we knew where they were. We could just about give them names."

Clearly, they hadn't liked the cinnamon-based lure, so Bramley offered them another two pre-feeds of an orange lure instead. Then, he set out cages in the rats' territories, each holding a toxic 1080 bait. "We caught 12 rats."

Taranaki was just a trial, as was Arawhata-Jackson. Within weeks of ZIP leaving, possums and rats were moving back in. But Whataroa, says Bramley, is not a drill. This time, he means to keep them out for good.

Hopefully, the rivers will do most of that for him. ZIP's research has shown that possums can swim substantial rivers, but they won't if they can possibly avoid it. "The reason we've picked this site is because South Westland is the most naturally defendable place in New Zealand."

But they're happy to use a footbridge, so Bramley's designed a predator gate so that only humans can cross the rivers.

If anything does get back in, Bramley's hoping his snooping devices will reveal them. ZIP will also lay out traps across a representative sample of the treatment block: newly-developed rat traps, and leg-hold traps for possums.

Although they're already used all over the country by possum trappers, leg-hold traps are contentious, but ZIP's trials have shown that they're 20 per cent more effective than any other kind. Bramley points out that, like the rat traps, his will be connected to a low-frequency radio network that will send a message to a cellphone the moment they go off, minimising the time a possum might be caught in one.

The traps are placed high on a tree to avoid catching weka or kea, and will automatically switch off during the day.

Bramley's confident he can take out every last possum. "Rats, we're not so sure about. If too many survive the initial drop, then we might need to repeat the prefeed and toxin drops."

If a rat gets back in from the outside, it'll encounter a trap every 20 hectares. "That network is looking for any emerging first family" he explains. "We don't expect it to catch every rat, but if a pregnant female has a litter, we'd expect to catch at least one of that litter before it spawns a second generation."

The block won't be closed to hunters and trampers during the 1080 drop, but they'll notice a few changes this time. Historically, helicopter pilots have been directed not to drop 1080 around huts, or on riverbanks, but leaving those gaps means leaving pests behind, the nucleus of a re-invasion. So, on the day, ZIP staff will disconnect each hut's water supply, and make sure the roof is clear of baits before reconnecting it.

"To ensure maximum effectiveness," says Bramley, "we need to sow right up to the river edges, and possibly into the water itself. We can't afford to leave a rat or two behind."

That's anathema to anti-1080 activists, and strikes at the heart of Māori values around water, and land. The Whataroa is just one of hundreds of courses that flow within the rohe of Ngāti Māhaki ki Makaawhio.

In 2010, the runanga banned aerial 1080 from its vast lands, and placed strict exclusion conditions on a few already-consented operations.

But the tribe has endorsed the Whataroa operation, despite the fact that it'll be using twice the normal 1080 sowing rate – 4 kg/ha – and may well do that twice, depending on whether sufficient pests survive the first drop.

Makaawhio is looking to the future, says Tim Rochfort, runanga chair. "We still oppose the use of 1080 or other poisons in the management of our whenua. What's changed is that DOC and ZIP have been much more happy to talk. We've been given an opportunity to talk to them, work with them, to help with the design."

The people of Makaawhio, he says, "have a lot of dreams and ambitions around educating our young people about our stories, keeping traditions alive, so that they become part of who we are. So that our kids see themselves as part of this great heritage, which is tied to the landscape.

"A healthy functioning forest absolutely underpins that. Which is why we're totally committed to that kaupapa."

Logan Skinner says Predator Free 2050 holds promise for his region, and his industry. "We don't have amazing cities, art or history," he says. "Visitors come to the West Coast for our natural beauty, and if we can restore our forests and birdlife back to pre-predator times, this is going to appeal to a significant tourism niche. It'll also build some creditably for our 100% Pure tourism brand."

Like Makaawhio and Skinner, says Al Bramley, "We're trying to find a way out of 1080, and this is our first step towards that. We understand that they don't like our methods: in truth, we'd rather not use it either, but in the present regulatory and technical environment, it's the only tool we have."

If Remove and Protect works here in Whataroa, he says, "we won't need to use 1080 at landscape scale in that valley again. And that would be the first time on the New Zealand mainland that we might contemplate that."