ZIP’s most powerful tool for possums is our prototype automated reporting leghold trap system.

As ground birds (in our case weka) are present at Bottle Rock peninsula, all ZIP’s leghold traps are tree-mounted, at a minimum height of 1.3 m.

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 (Thomson et al 2001).

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.

Ramps were attached to half the visually-lured leghold trap platforms in the system (alternating with non-ramped), angled at 60˚ to the horizontal, to prevent weka from accessing the trap.

Traps were mounted on a prototype platform of our own design, with a recess for the trap and chain to minimise disturbance by possums approaching the trap plate.

All leghold traps at Bottle Rock are remotely inspected in line with MPI guidelines.

Over a period of approximately 350,000 trap nights, 399 possums were caught on leghold traps in the virtual barrier. Of this number, 263 were caught on ramped traps (during approximately 200,000 trap nights) and 136 were caught on traps without ramps (during approximately 150,000 trap nights).

As a result of this trial we now believe that ramped platforms could be as much as 20% more effective than those without ramps.

What we don’t know is why!

It is possible that possums are weaker (or perhaps lazier) climbers than generally believed, and prefer to walk up a gentle slope than scale a tree.

Another possibility is that the ramp acts as an additional visual cue, appealing to possums’ curiosity and leading them to more closely investigate traps. Conversely, raised platforms without ramps could be less visible to possums moving along the ground.


C. Thomson, B. Warburton & L. Moran (2001) Weka- and kiwi-safe possum trap sets, DOC Science Internal Series 24, Department of Conservation, Wellington, 16 p.

‘Try, Sense, Respond’

 For the purpose of this trial, i.e. understanding how best to maximise our possum leghold catch rate, we were interested to learn whether ramped legholds perform better than unramped ones, rather than why one option might be superior.

This is a good example of ZIP’s ‘try, sense and respond’ approach to research and development.

There is often a balance to be struck between understanding a complex dynamic or problem, and rapidly developing a new tool or technique in order to maximise our progress.

Typically, ZIP’s approach is to prototype a design and field test it as quickly as possible.

When trials are underway, we analyse the performance data. If something appears to be working well, we amplify it, and conversely we do not hesitate to drop any developments that do not appear to be promising (as soon as our analyst Nick deems that the data is sufficiently robust).

This adaptive approach, while common in technical innovation, differs from traditional scientific methodology.