A lesson for planning rodent eradications: interference of invasive slugs during the Gough Island mouse eradication attempt in 2021

Abstract

Context. House mice (Mus musculus) are the main drivers of biodiversity declines on Gough Island (6500 ha; 40°21 0 S, 009°53 0 W), central South Atlantic. A mouse eradication operation was planned, the largest global attempt targeting only this species. Understanding and managing challenges of operating at such scales are crucial for maximising the chance of eradication success. The Gough Island mouse eradication attempt was implemented between June and August 2021, after years of planning and trials. We expected poor weather and negligible non-target bait consumption. Aims. We aimed to assess the impact of expected and unexpected challenges faced during the eradication operation on Gough Island, namely poor weather and rapid bait disappearance. Methods. We set up bait degradation plots across the primary habitats to monitor the impact of expected heavy rain on bait pellets. In contrast, bait availability monitoring and slug laboratory trials were set up ad hoc in response to unexpected observations of high bait consumption by invasive slugs in the lowlands, where both slugs and mice are more abundant. Key results. Bait degradation rates were very different between the highlands and the lowlands, with bait in the highlands lasting about six times longer, despite bait pellets receiving more precipitation and the highlands being persistently under cloud. Bait availability in the lowlands dropped by >80% within a few days of the second and third bait application, down to critically low levels (~2 kg ha −1 ). Importantly, mouse activity was negligible by this time. Non-native slugs appeared to be the main cause of such a sudden drop in rodent bait availability. Conclusions. The expected rainy weather was not a significant direct cause of bait degradation in the short term. In contrast, the unexpected slug interference, overlooked in earlier planning trials, resulted in major adjustments of the baiting strategy. Indeed, the rapid bait disappearance in the lowlands triggered the third bait application over this area, at a higher rate. This was not enough, as mice are still present. Implications. This is the first report of slug interference during aerial rodent eradications. Our results illustrate how interference by non-target species could affect future pest eradications using baits and should, as far as possible, be assessed early during planning.

Detections of house mice on Gough Island approach zero within days of aerial baiting

“…However, it may be related to the unexpectedly fast bait disappearance (i.e. up to 3 kg ha −1 per night) caused by non-native slugs (Samaniego et al 2022). Some mice may not have encountered bait from the first application, rather than rejecting the bait.…”

“…A third application with a target rate of 10-12 kg ha −1 was undertaken on 1-2 August over areas of operational concern (Fig. S1), which included areas perceived to be at highest risk of remaining mice and included much of the lowlands following the discovery of significant interference by slugs in this area (Samaniego et al 2022).…”

“…Lures were not used prior to 30 June so that mice were not distracted from eating toxic bait during the main eradication period. Wax tags and chew cards are widely used in New Zealand and elsewhere to monitor rodent activity (Thomas et al 1999;Sweetapple and Nugent 2011;Samaniego et al 2018).…”

The next frontier: assessing the feasibility of eradicating mammalian pests from Auckland Island

“…A small pool of highly skilled pilots is available in New Zealand with the required skillsets and remote deployment of several months is achievable. New Zealand helicopter pilots have regularly been deployed around the world on remote island eradications lasting several months, including on Campbell Island, Macquarie Island, South Georgia Island, Antipodes Island and, most recently, four pilots for a mouse eradication on remote Gough Island in 2021 (McClelland 2011;Springer 2018;Horn et al 2019;Martin & Richardson 2019;Samaniego et al 2022).…”

A trial on Te Pākeka/Maud Island for reducing aerial baiting sow-rates for the eradication of house mice

“…Bait application rates on other successful larger island eradications where mice were targeted as part of multi-species eradications were on Macquarie Island with bait application rates ranging from 24-44 kg ha −1 (Springer 2016); and parts of South Georgia (range of mice covered 58 000 ha over several discrete blocks isolated by glaciers) where Norway rats (Rattus norvegicus) were also present, and bait application rates of 10 kg ha −1 were used (Martin & Richardson 2019). The mouse eradication project on Gough Island (6500 ha), a project that faced logistical difficulties, used a baiting prescription of two applications of a minimum of 10.5-13.4 kg ha −1 (Samaniego et al 2022).…”