Findings
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.
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.
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.
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.
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.
Between 2011 and 2014 a trial was conducted as part of the Poutiri Ao ō Tāne project, a restoration project located at the Maungaharuru-Tutira catchment, 60 km north of Napier.
This trial sought to compare the efficacy of run-through tunnel traps and standard single-entry trap boxes for ship rats, stoats and ferrets.
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.
