Our intensive possum detection and response effort in the Perth River valley field site over the last 12 months indicates that we have now achieved possum freedom in the site. This is a major milestone, which has not (to our knowledge) ever been achieved at this scale on the New Zealand mainland.

Possum freedom means that we have removed all of the possums we knew to be present, and therefore we are confident that if any possums do remain they will be so few that they are unable to re-establish a viable population.

Since the aerial spot treatment, aimed at rats, in March 2020, we have been carrying out ground-based detection with our network of 143 lured trail cameras, and then targeting our response actions to those locations where possums are detected. We have now been searching for over 14,000 detection nights, and have removed the three possums detected during that time.

How we got there

Predator removal operation (Phase 1)

13 April 2019

After Phase 1 of the predator removal operation, an estimated 8-10 possums were detected in the field site. We live-caught one of these, fitted it with a VHF collar and monitored it through to Phase 2 – when it was removed.

 

Predator removal operation (Phase 2)

23 July 2019

In the 11 months following Phase 2, an estimated 5-6 possums were detected in the field site.

Decision to delay targeted ground-based removal of survivors

We began to detect a small number of possums in the field site 36 days after Phase 2 of the predator removal operation. However, we did not begin work to remove these possums until December 2019, primarily because the 1080 caution period following Phase 2 meant that Pepper, the possum dog, was unable to enter the site for several months. Possums are relatively slow breeders, so we considered that we had time on our side in which to effectively target possums before the population became unrecoverable.

During this time, we also began to reassess our approach to detecting and responding to surviving or invading possums. We had originally planned to deploy a permanent lean network of ZIP PosStop possum traps that included a ‘SafeLock’ mechanism to enable the traps to be locked during daylight hours, when kea are most active. A trial between December 2018 and July 2019 found that, while the SafeLock mechanism was successful in preventing unwanted trigger events, over time kea learned to interfere with the trap and compromise its capture effectiveness.

Then, in December 2019, we discovered evidence that a small number of rats had survived the predator removal operation and begun to breed within the field site. Removing these rats before the emerging population could spread became our most urgent priority in the site.

Spot treatment to target rat breeding

5 March 2020

We carried out a targeted aerial ‘spot treatment’ operation to remove the emerging population of rats before it could spread. Although rats were the target, we believe that this spot treatment was responsible for eliminating 2-3 of the surviving possums, with no possums detected in the Lower Barlow area, or in the Perth/Barlow confluence, after the operation.

On 19 May, a ranger discovered a partially decomposed possum carcass near the Perth River. This left us with an estimated 2-4 possums to remove.

Three possums successfully removed by detection and targeted response

19 May, 11 June and 29 June 2020

The detection and targeted response strategy has successfully removed the remaining three possums detected in the block.

The first of these is thought to have been the sole invader to date, as it was not detected in the field site until 8 March, roughly eight months after the predator removal operation. The detection location – near the head of the Barlow River – is an area we consider to be at increased risk of invasion, due to the lower flow of the river in this area. This individual was a 3kg female possum, caught on 19 May in a cage trap after being tracked by Pepper and Mike. Camera detections were used to narrow the initial search area.

The 2kg male possum caught in the Teichelmann area on 11 June

Pepper the possum dog - one half of our impressive possum-hunting team in the Perth River valley

The second is assumed to have been a survivor of the predator removal operation, which was first detected in the Teichelmann/Upper Perth area on 24 August 2019. This possum was a 2kg sub-adult male, caught on 11 June in an automated cage trap after being seen in that location on the camera network.

The third possum is also believed to have been a survivor of the predator removal operation, and was first detected in the Teichelmann/Upper Perth area on 11 September 2019. Again, trail cameras enabled us to hone in on a targeted area, where we livened automated reporting cage traps and set Pepper and Mike to work. The possum was caught on camera several times next to, and even on top of, the automated cage traps, only increasing the determination of the response team to catch it. On 29 June, our persistence paid off, and the possum was finally caught in an automated cage trap. The possum was a 3.3kg female, with no evidence of pregnancy, which is great news.

While we were already confident that possums were functionally extinct in the field site, catching up with this last known individual really builds confidence in our ground-based toolset and approach. We know that the job is not over, and we need to keep protecting the Perth River valley site from the ever-present risk of possum invasion. We will continue to systematically survey the field site for possums, using Pepper and the camera network, to keep building our confidence in the rivers as effective barriers to possum reinvasion. And, just in case, our response tools remain at the ready!

Our work in the Perth River valley is made possible by the support of the Department of Conservation, NEXT Foundation, and Predator Free 2050 Limited.