Targeted Mop up and Robust Response Tools Can Achieve and Maintain Possum Freedom on the Mainland

Abstract

Unfenced sites on mainland New Zealand have long been considered impossible to defend from reinvasion by possums, and are thus unsuitable for eradication. In July 2019, we began eliminating possums from 11,642 ha (including approximately 8700 ha of suitable possum habitat) in South Westland, using alpine rivers and high alpine ranges to minimise reinvasion. Two aerial 1080 (sodium fluoroacetate) applications, each with two pre-feeds, were used. Here, we detail the effort to mop up existing possums and subsequent invaders in the 13 months following the aerial operation. Possums were detected and caught using a motion-activated camera network, traps equipped with automated reporting and a possum search dog. The last probable survivor was eliminated on 29 June 2020, 11 months after the initial removal operation. Subsequently, possums entered the site at a rate of 4 per year. These were detected and removed using the same methods. The initial elimination cost NZD 163.75/ha and ongoing detection and response NZD 15.70/ha annually. We compare costs with possum eradications on islands and ongoing suppression on the mainland.

Keywords: New Zealand; brushtail possum; detection; elimination; natural barriers; reinvasion; response.

Figure 1

Study site showing trail camera locations, protective river boundaries and the supplementary aerial treatment coverage.

Figure 2

Possum detections in the Perth Valley from 25 August 2019 (first detection following site wide aerial 1080) to 31 August 2020. Shaded squares indicate the discrete areas of the site discussed in the text.

Figure 3

Possum detections by area over time. Crossed circles represent kills. * The Perth-Barlow confluence possum detection represented on the figure after the supplementary aerial operation was a possum scat, and the body found near the same location is believed to have died from that toxin operation.

Figure 4

Kill locations of possums in the Perth Valley from 5 May (first body found) to 31 August 2020.

Figure 5

Detection, estimated home ranges and kill location for survivors in the Upper Perth Valley (area enlarged from Figure 4): (a) Detection locations 23 July 2019–14 April 2020; (b) detection and capture locations 15 April–29 June 2020. The number of detection locations differs from the number of detections stated during these periods, as camera sites were visited multiple times.

Figure 5

Detection, estimated home ranges and kill location for survivors in the Upper Perth Valley (area enlarged from Figure 4): (a) Detection locations 23 July 2019–14 April 2020; (b) detection and capture locations 15 April–29 June 2020. The number of detection locations differs from the number of detections stated during these periods, as camera sites were visited multiple times.