There is some anecdotal evidence to suggest that possum migration across landscapes is slowed by natural features, such as rivers and high mountain ranges. If we were able to confirm that these features really do halt or greatly slow down the migration of possums, then this new knowledge would broaden the range of sites where the Remove and Protect model of possum management could be applied across New Zealand. Natural barriers may also be cheaper to maintain than a virtual barrier.

Between August and October 2017, ZIP ran a trial in the Orongorongo Valley (Rimutaka Forest Park), to determine how effectively this relatively small river prevents possum migration.

What we did

We became aware that a 1080 operation was being planned across both sides of the Orongorongo River in Rimutaka Forest Park, in order to control possums (which pass on bovine tuberculosis). This operation presented an opportunity to measure how effectively a relatively small river prevents possum migration, and so we proposed to OSPRI and the Department of Conservation that a 250ha exclusion zone be established within the 1080 treatment zone as part of a trial to do this. The exclusion zone (an area without 1080 baiting) covered a 4km stretch of land on the true right side of the Orongorongo River.

OSPRI and DOC agreed to our proposal, and so the 250ha zone was excluded from the aerial 1080 operation carried out on 30 July 2017.

Our trial was carried out over a period of nine weeks. During this time, we (i) used a non-toxic bio-marker called pyranine to mark possums within the exclusion zone, and (ii) captured possums using kill traps on the ridge top boundary of the exclusion zone and leg-hold traps on the ‘other’ (true left) side the river.

When consumed by animals, pyranine stains the stomach and intestinal tract and appears as a vibrant green fluorescence (or ‘glow’) when viewed under ultra-violet (UV) light (see Figure 1).

Figure 1: Pyranine glow (under UV light) of the intestines of a possum

Figure 1: Pyranine glow (under UV light) of the intestines of a possum

Each week, a zone in the lower half of the exclusion block (shown in light green in Figure 2, below) was aerially sown with non-toxic cereal pellets dyed with pyranine, and the same pyranine bait was also deployed by hand along the true right edge of the Orongorongo River, to ensure that possums received continuous exposure to the biomarker.

We installed possum kill traps at 20m spacing along the north, west and southern boundaries of the exclusion zone, to gain a baseline measurement for possum movement across land from the pyranine sowing area. This line of traps (known as the ‘containment line’) was checked weekly.

We also installed a network of automated reporting leghold traps at 20m spacing along the 4WD track on the true left side of the Orongorongo River, to intercept any possums that had crossed the river from the exclusion zone (or had survived the aerial 1080 operation on the true left side of the river). This network was operated in accordance with the MPI Guidelines for good practice when remotely monitoring live capture traps for vertebrates.

Each possum caught in the traps was examined externally and internally for the presence of pyranine using a UV black light.

What were the results?

We caught 82 possums marked with pyranine on the upper edge of the containment line (see Figure 2). In order to have been marked with pyranine, all of these ‘glowing’ possums must have moved uphill for at least 300 metres (i.e. from the nearest edge of the zone sowed with pyranine).

Figure 2: Comparison of glowing and non-glowing possum catches on the top section of the containment line, and the leghold line across the river. The river flows from right to left.

Figure 2: Comparison of glowing and non-glowing possum catches on the top section of the containment line, and the leghold line across the river. The river flows from right to left.

None of the 44 possums we caught on the line of leg-hold traps on the true left side of the river were marked with pyranine. The location where these animals were caught suggests that they survived the 1080 treatment because they were mostly in small exclusion zones established around huts.

This result provides strong indication that the Orongorongo River was acting as a barrier to possum movement during the course of this trial.

What did we learn?

Even though the Orongorongo River is not a large, deep, or fast flowing river, it proved to be a barrier for possums during our trial. The results of this trial support the historical anecdotal evidence that rivers are indeed a barrier to possum migration.

This result has encouraged us to consider rivers as components of barrier systems when identifying and evaluating large-scale sites to demonstrate the Remove and Protect model of possum management.


We’d like to thank staff from OSPRI and DOC for their support of this research project. Landcare Research (via Peter Berben) allowed our rangers to stay in the Landcare Research field base in the Valley – thank you. Phil Cowan (retired, from Landcare Research) helpfully answered our questions and provided us with information that supported our project. We would especially like to thank Roger Pope, the DOC caretaker at Catchpool Visitor Centre, for being the daily contact person for ZIP rangers working at the research site.


A note on pyranine bio-marker

During this project, field staff identified fluorescent green external markings (i.e. ‘glows’) on two possums caught on the leghold trapline. Strangely, neither possum showed any internal pyranine marking.

Consequently, we undertook another trial to assess the external effects of pyranine on possums exposed to high levels of the bio-marker compared with possums that had not been exposed to it, at our Lincoln facility.

We discovered that external fluorescence can be attributed to possum urine, or simply pale hairs around the whiskers and paws that appear to glow green under UV light – i.e. not the result of pyranine staining.

Our rangers also noted the presence of fungi in the Orongorongo Valley that glowed blue/green when viewed under a UV light, which suggests that naturally occurring environmental sources could mark possum fur.

As a result of this work, we concluded that pyranine can only be used as a reliable bio-marker for possums when it stains their internal organs.