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

Context. House mice (Mus musculus) are the main drivers of biodiversity declines on Gough Island (6500 ha; 40 degrees 21'S, 009 degrees 53'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 (similar to 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.