2 March 2022

We’ve recently finished seasonal predator monitoring in the parks and reserves where we’re trapping. Seasonal monitoring is important because it shows us how effective the traps are and gives insight into local possum populations.

Possums are monitored using wax-tags – plastic tags holding a hunk of wax. The tags are spaced at intervals along “lines ” in the bush to give us good coverage of where animals might be moving through. Predators, particularly possums, gnaw on the wax and leave tooth marks.

Looking at which tags get chewed gives us a rough idea of where animals are still present.

The tags are left out for a week and then collected and checked for chews. Once all the tags have been identified and reported, it’s time for some mammalian maths!

Dividing the number of chews by the total number of wax-tag lines gives us what’s known as residual trap catch index (RTCI) – an estimation of the number of catches we’re still getting against the number of traps we have.

Our goal is to suppress possums in these reserves to below 3% residual trap catch index. This means in the future we’ll aim to get catches down to only 3% of our traps.

We have now completed a full year of seasonal monitoring and have some interesting insights.

All of our reserves had less chews this summer! The Botanic Garden was down from 8 to 2 (0.3% RTCI) and the Town Belt dropped from 36 to 19 (3.1% RTCI).

This is good news and likely means possum numbers are lower than before we started trapping in these areas last year – makes sense!

Town Belt behind Otago Boys High School showing markers where wax-tags have been chewed at different times. Green markers = Autumn 2021. Blue markers = Winter 2021. Red markers = Summer 2022.

Some of the wax-tag lines detected possums during all times of the year while others have not detected possums in summer. This might mean that possums prefer to live in certain areas of the Town Belt in different seasons, perhaps because of shelter and what food is available.

In areas where we’ve had sustained possum presence, we might need to add more traps or change the type of trap we’re using. In locations with low catches and no wax-tag chews, we can decrease the number of traps or only set them at certain times of the year.