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A pilot project for the eradication of beavers (Castor canadensis) in Tierra del Fuego started as part of a bi-national agreement, signed between Argentina and Chile, to restore the affected environments. The project covers nine pilot areas of different landscapes and land tenures in the Argentinian part of Isla Grande de Tierra del Fuego. We report on the results from operations in the fi rst of the pilot areas. From October 2016 to January 2017, ten trappers (named restorers for advocacy purposes) used body-grip traps, snares and an air rifle, in a first phase, which included 2,237 trapping nights and 1,168 trap-sets. Shooting eff orts were not monitored. Traps were set for 1,401 trapping-nights and caught 175 beavers at a success rate of 12.5% (captures per trap night). Snares were set for 936 snare-nights and caught 22 beavers at a success rate of 2.3%. Seven beavers were shot. Most beavers (65%) were removed during the fi rst week of trapping in the different watercourse sections. Stopping trapping for a week or more did not increase efficiency. From March to May 2017 restorers removed 24 survivors and/or reinvaders, including 10 from two previously untrapped colonies. Capture efficiency for this removal period was low for body-gripping traps but not for snares. The sex ratio of catches was 47% females to 53% males. The age structure of catches was 15% kits, 29% yearlings, 51% adults, with 4% not aged. An estimated total of 41 colonies was trapped, giving an average of 5.6 animals per colony. After nominal eradication was declared by restorers, 154 camera trapping nights were deployed to assess eradication success. Nine cameras (of 26 cameras used) detected beavers. Therefore, eradication was not achieved using the methods and eff orts in the first part of the pilot study. This highlights the need for more eff ort or the application of different techniques or trapping strategies. For example, daily checking of traps may cause the animals to be cautious so, the next step in the programme will involve exploring alternative trapping methods to reduce disturbance.

House mice are significant invasive pests, particularly on islands without native mammalian predators. As part of a multi-institutional project aimed at suppressing invasive mouse populations on islands, we aim to create heavily male-biased sex ratios with the goal of causing the populations to crash. Effective implementation of this approach will depend on engineered F1 wild-lab males being effective secondary invaders that can mate successfully. As a first step in assessing this possibility, we are characterising genetic and behavioural differences between Mus musculus strains in terms of mating and fecundity using wild house mice derived from an invasive population on the Farallon Islands (MmF), a laboratory strain C57BL/6/129 (tw2), and F1 wild-lab off spring. Mice with the ‘t allele’ (tw2) have a naturally occurring gene drive system. To assess fertility in F1 wild-lab crosses, tw2 males were paired with wild-derived females from the Farallon Islands (MmF). Results of these matings indicate litter sizes are comparable but that weaned pup and adult wild-lab mice are heavier in mass. Next, we initiated tests of male competitiveness using larger (3 m2) enclosures with enrichment. We introduced both an MmF and a tw2-bearing male to two MmF females to assess mating outcomes. Preliminary results of these experiments show none of the off spring carried the t-allele. However, performing the same experiment with F1 wild-lab males instead of a full lab background resulted in 70% of off spring carrying the tw2 allele. This indicates that F1 wild-lab males may be able to successfully compete and secondarily invade. It will be important in subsequent experiments to determine what characteristics contribute to secondary invasion success. More generally, a better understanding of characteristics contributing to overall success in increasingly complex and naturalistic environments will be critical in determining the potential of a gene drive-based eradication approach for invasive mice on islands.