‘1080 TO ZERO’ TRIAL IN SOUTH WESTLAND

‘1080 TO ZERO’ TRIAL IN SOUTH WESTLAND

ZIP is developing a modified technique for the aerial application of 1080 to completely remove possums and rats from large mainland areas.  If we are successful, and we also successfully develop techniques to prevent possum and rat invaders from re-establishing, then the large-scale repeated application of aerial 1080 may no longer be necessary to protect New Zealand’s biodiversity.

In 2017, we trialled this technique at a 2,300 hectare site in South Westland.

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1080 TO ZERO: A MODIFIED TECHNIQUE FOR THE COMPLETE REMOVAL OF POSSUMS AND RATS

1080 TO ZERO: A MODIFIED TECHNIQUE FOR THE COMPLETE REMOVAL OF POSSUMS AND RATS

The impact of invasive predators on native biodiversity is well documented, with an estimated 25 million native birds lost to predation every year.

New Zealand is a world leader in completely removing possums, rats and stoats – the three species generally understood to do the most damage – from islands and fenced sanctuaries, through the aerial application of the toxin brodifacoum. However, aerially applied brodifacoum is not registered as a technique on the mainland, so we need to find another option as we push toward a predator free New Zealand by 2050.

Aerially applied 1080 is successfully used on the New Zealand mainland to ‘control’ possums and rats for conservation and disease management purposes, by suppressing populations to very low numbers. However, because standard 1080 operations do not remove all target individuals, and reinvasion back into those sites is not managed, predator populations recover and these operations must be repeated on a cyclical and ongoing basis (typically every 3-5 years) to sustain the benefits of the predator control.

This context led ZIP to conclude that it is a high priority to develop an aerially-based technique for the complete removal of possums and rats from a treatment area. We decided to test whether 1080 could form the foundation of that technique.

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ARE RIVERS AN EFFECTIVE BARRIER TO POSSUM MIGRATION?

ARE RIVERS AN EFFECTIVE BARRIER TO POSSUM MIGRATION?

There is some anecdotal evidence to suggest that possum migration across landscapes is slowed by natural features, such as rivers and high mountain ranges. If we were able to confirm that these features really do halt or greatly slow down the migration of possums, then this new knowledge would broaden the range of sites where the Remove and Protect model of possum management could be applied across New Zealand. Natural barriers may also be cheaper to maintain than a virtual barrier.

Between August and October 2017, ZIP ran a trial in the Orongorongo Valley (Rimutaka Forest Park), to determine how effectively this relatively small river prevents possum migration.

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OPTIMISING SPACING OF TUN200 TRAPS IN THE VIRTUAL BARRIER SYSTEM

OPTIMISING SPACING OF TUN200 TRAPS IN THE VIRTUAL BARRIER SYSTEM

The TUN200 trap box is our most effective tool for rats. Our usual spacing between traps along each of our virtual barrier defence lines is 10 metres. During 2015 and 2016, we ran trials to assess whether TUN200s could be placed at different intervals along the first defence line in our 'virtual barrier' at Bottle Rock, to enhance both the effectiveness and efficiency of the Remove and Protect system.

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TESTING THE 'OPEN' ARCHITECTURE TUN200

TESTING THE 'OPEN' ARCHITECTURE TUN200

ZIP is always looking for ways to improve the effectiveness of the tools we use in the virtual barrier. With this in mind, a trial was run to determine whether a TUN200 trap box with a more ‘open’ entrance architecture might kill significantly more rats than a TUN200 with a standard 7.2 cm diameter entrance hole.

We were surprised by the result...

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CATCHING MORE POSSUMS USING LEGHOLD PLATFORMS WITH RAMPS

CATCHING MORE POSSUMS USING LEGHOLD PLATFORMS WITH RAMPS

Studies have indicated that tree-mounted possum leghold traps may be most effective when paired with a sloping board that acts as a ramp from the ground to the trap.

During 2016, we ran a trial at Bottle Rock to assess the effectiveness of possum leghold traps with ramps in comparison with those without ramps.

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SCENT TRAILS AND THE 'FUNNEL' EFFECT FOR POSSUMS

SCENT TRAILS AND THE 'FUNNEL' EFFECT FOR POSSUMS

When a leghold trap in our system catches a possum, like all possum trappers we are required to clear this trap within 12 hours of sunrise. Interestingly, our capture data shows that there is a one in three likelihood that the next possum to be caught on that line will be caught in the same trap, even though there are over 250 traps for that possum to choose from.

We now believe that this 'funnel' effect is likely to be the result of some form of ground-based scent trail left by possums.

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DETECTING INVADING STOATS AT BOTTLE ROCK PENINSULA

DETECTING INVADING STOATS AT BOTTLE ROCK PENINSULA

When the virtual barrier was installed at Bottle Rock in November 2014, stoat defences were not included as we did not believe that the 400 ha peninsula was large enough in relation stoats' speed or 'typical' home range size (60 – 200 ha) to confirm the area was free of stoats at any point in time. Our suite of control tools and available monitoring tools for stoats at the time were also extremely limited.

However, the virtual barrier system caught a surprisingly large number of stoats (25) during the 2014 beech mast (lured with mice caught in mouse traps inside TUN200 boxes), and another 11 following the rat and possum removal. This gave us some confidence that it would be worthwhile to begin trialing stoat defences at Bottle Rock peninsula.

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USING VISUAL LURES TO ATTRACT POSSUMS TO TRAPS

USING VISUAL LURES TO ATTRACT POSSUMS TO TRAPS

The leghold trap is by far our most effective tool for invading possums, intercepting approximately 60% of those that attempt to pass through each defence line.

When ZIP began operations at Bottle Rock in late 2014, our leghold traps were lured using a 90x90mm white corflute chew card baited with aniseed possum lure, refreshed every 6 weeks.

However, when we observed catch data over time, it became clear that refreshing the lures did little to improve trap effectiveness, and catch rates did not reduce between services.

We now believe that the visual ‘flag’ created by the white corflute square is acting as the attractant, and is itself sufficient to cause possums to investigate traps.

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TARGETING THE FIRST GENERATION OF INVADING SHIP RATS

TARGETING THE FIRST GENERATION OF INVADING SHIP RATS

Following the ‘lonely’ rat releases on Bottle Rock between July and September 2015, we began to reconsider our approach to detecting invaders in the Remove and Protect system.

These releases demonstrated to us that the ‘footprint’ of an individual Ship rat in the landscape can be very small and therefore extremely difficult to detect using current methods.

We are now looking to determine whether the dispersal footprint of a first generation breeding event is both detectable and manageable using currently available tools.

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OBSERVING THE LONELY RAT AT BOTTLE ROCK PENINSULA

OBSERVING THE LONELY RAT AT BOTTLE ROCK PENINSULA

To help us design our detection system at Bottle Rock, we wanted to learn more about what a ‘lonely’ rat does, and where it goes, when it enters an area with no other rats. In other words, we wanted to better understand the ‘footprint’ of a rat invasion in space and time, to help ensure early detection and removal.

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MINIMISING MOUSE INTERFERENCE

MINIMISING MOUSE INTERFERENCE

Because our detection and removal devices are not designed to target mice, we need to be able to exclude them from these devices to ensure optimal performance of the system.

Here are just a few of the mouse-related challenges we’ve overcome since we began our operations.

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