ZIP’s automated reporting leghold traps have proven to be the key to a highly effective – and efficient – possum ‘virtual barrier’ at our Bottle Rock field site, stopping over 95% of invading possums. But what about the few individuals that do get through the barrier, and into the protected, possum free area..?

Previous studies (by Landcare Research and OSPRI) have shown that possums, once isolated, roam considerable distances (in the order of 50-100 hectares), presumably looking for other possums. They are also notably curious, and therefore when they encounter a ‘novel’ object in the environment there is a reasonable likelihood they will interact with it.

These factors, combined with the slow breeding rate of possums, suggested to us that the traditional intensive detection networks of devices deployed throughout the area may be excessive to detect a ‘lonely’ possum in an otherwise possum-free area – before a population has time to establish. Rather, perhaps we could manage Remove and Protect sites with something much leaner.

We tested this hypothesis at Bottle Rock by deploying eight automated reporting leghold traps within the ‘protected’ area on the peninsula, at a ‘lean’ density of one trap every 50 hectares. Trap locations were selected for either good visibility of the trap to approaching possums, or in preferred habitat or terrain features.

In the 18 months between October 2016 and May 2018, these traps have successfully prevented possums from re-establishing on the peninsula. During this time, 66 possums have entered the peninsula through the virtual barrier, and all were caught in the lean detection system within a few months of arrival. 

Bottle Rock lean detection map.PNG

The traps that have had the highest catch rates are on the northern side of the peninsula, adjacent to Ship Cove. The habitat on this side of Bottle Rock is mature podocarp/broadleaf forest, dominated by tawa, kohekohe, and kamahi, and is preferred habitat for both possums and rats.

Thanks to the intensive detection efforts of our Bottle Rock field team (servicing a background network of chew cards every few weeks), we are confident that the system is working well, and that that there is no breeding possum population on the peninsula.

We will soon be installing a modified lean detection system in the Perth River Valley to confirm, and then maintain, possum freedom across an area of 12,000 hectares. In order to maximise our chance of success, leghold traps will be placed at an approximate density of one trap every 21 hectares (making for a network that is roughly twice as dense as the one on Bottle Rock).

In parallel with this work, we will continue to test and refine the lean detection network at Bottle Rock. 

This winter, we will examine the time taken for an individual possum to encounter, and then be caught by, the lean detection network on the peninsula. We will do this by GPS-collaring possums, one at a time, and releasing them on to the peninsula, then tracking their movements. We will also simulate the Perth River Valley network with motion-activated cameras, to compare the densities of the two detection networks. These trials will give us more insight into where, and how far, an individual possum goes when it enters an area with no other possums present.

This will not be the first time we have attempted to GPS-track ‘lonely’ possums.

We ran similar trials on Bottle Rock between May and June 2017, releasing two female possums – ‘Mauline Hanson’ and ‘Schappelle Clawby’ – on to the peninsula and attempting to catch them using the lean detection network. 

Unfortunately, Mauline Hanson’s GPS collar failed after 9 days, so we were unable to learn much from her travels around Bottle Rock Peninsula. Schappelle Clawby roamed the peninsula for 32 days, and in that time covered a total area of 89 hectares. Neither possum was caught in the lean detection network during their time on the peninsula; however, given the overall performance of the lean detection network we are confident that they would have been caught, had we allowed the trial to run longer.

In September 2017 we sought to measure the spatial ‘footprint’ of a lonely possum in the absence of traps by GPS-collaring two possums, a male dubbed ‘Steve Furwin’ and a female dubbed ‘Julia Killard’, and releasing them into the Jackson-Arawhata trial site. Again, GPS failures with the collars limited the learning we could gather from that work.

We are confident we have rectified these technology issues, so we are expecting better performance from the collars this time around!

Watch this space…

Want to learn more?

If you’d like to learn more about ZIP’s automated reporting leghold system, check out ZIP Technical Report #4.

If you’d like to learn more about our previous trials with GPS-collared possums, check out ZIP Technical Report #5.