In 2017, ZIP ran a trial in Remutaka Forest Park, which provided strong evidence that even relatively small rivers are an effective barrier to possum migration.

Last year, we began a similar trial in the Perth River valley, to assess whether larger rivers, reinforced with a line of traps, can be an effective barrier to the migration of rats.

The Perth River research area is bordered to the south and west by the Perth and Barlow Rivers – both glacial-fed fast flowing waterways with generally rocky margins, including large boulders and rock faces.

We began bio-marking rats along a 3.2km section of the true left side of the Perth River and Scone Creek in April 2018, using a line of automated lure dispensers (ALDs) containing a food lure (initially peanut butter, which was later changed to mayonnaise), laced with rhodamine B bio-marker. Rhodamine B, when consumed, internally stains carcasses a bright pastel orange. In rats it is also metabolised into growing whiskers for up to 60 days, where it shows up as a solid band of vivid yellow/green fluorescence under a fluorescence microscope. A line of kea-safe A24 rat traps was also installed along the true left side, at 50m spacing, to ‘reinforce’ the river barrier.

We intended to begin trapping rats along the true right of the river, and examining them for the presence of rhodamine B (which would indicate that they had crossed the river), after the completion of a predator removal operation on the true right side. The predator removal was originally scheduled to be carried out during winter 2018, but poor weather in the valley forced us to delay the operation until March 2019. Nonetheless, we decided to run the majority of the river barrier assessment trial as originally planned.

What we did

ALD dispensing rhodamine B-laced egg mayonnaise on the true left side of Scone Creek

DOC200 trap with ALD on the true right of the Perth River near Elizabeth Stream

In August 2018, we established a line of traps along the true right side of the Perth River and Scone Creek, directly across the river from the line of ALDs dispensing bio-marker. This trap line consists of 155 single-set DOC200 traps (in standard wooden box with baffles) at 20m spacing, each of which is lured with egg mayonnaise, dispensed via an ALD. Traps are checked approximately every 10 days, depending on weather.

Ship rat showing internal rhodamine B staining under ultraviolet light

The Perth River valley field team examine rat carcasses for rhodamine B under ultraviolet light

All rat carcasses caught on the true right are examined on site (via field autopsy) for traces of Rhodamine B staining. Whisker samples are collected for analysis under the fluorescence microscope at Lincoln University.

Rhodamine B negative whisker, viewed under fluorescence microscope

Rhodamine B positive whisker, viewed under fluorescence microscope


By 19 February 2019, after more than seven months of continuous bio-marking and trapping, 143 rats had been caught on the true right of the river. Of these, we have confirmed that 142 showed no sign of having ingested rhodamine B, and therefore are unlikely to have crossed the river. Whiskers from a single rat contained ambiguous markings that may or may not indicate the presence of rhodamine B.

As a point of comparison, a total of 82% (32/39) of rats caught and analysed on the true left side of the river in April 2018 were found with traces of rhodamine B, meaning this proportion of the resident population (as a minimum) had been feeding from the automated lure dispensers. This suggests that there is a reasonably high likelihood that any rats that did cross the river would contain signs of the bio-marker.

The trial has now been running across different seasons; including winter where the river levels are lowest but the water is at its coldest, and rat breeding and dispersal activity is likely to be minimal; but also spring/summer when the river levels rise significantly as snow and ice melts, and rat breeding and dispersal activity increases. We are very encouraged by this promising result!

The trial continues…

Because the predator removal operation has not yet been carried out, there is still a sizeable rat population on the true right side of the Perth River and Scone Creek, which means that a large number of rats are able to be caught and analysed for the presence of rhodamine B. While this means that the trial carries a higher labour cost than originally planned, the additional data we are gathering has enabled us to increase our confidence in the effectiveness of these rivers as barriers to rats before carrying out the predator removal operation.

We will continue to run this trial up until the predator removal operation in March 2019. If the operation is successful (i.e. if it completely removes all rats from the operational area), then it will provide us with an opportunity to assess the effectiveness of the rivers as a barrier to protect an otherwise predator-free area against reinvasion.



ZIP has learnt a huge amount about using Rhodamine B, and particularly about identifying Rhodamine B in carcasses and whiskers, from Dr Penny Fisher. Thank you very much, Penny, for sharing your wealth of knowledge and experience with us, and for putting up with our many questions!