Population trends of house mice during tussock mast seeding on Auckland Island

: House mice ( Mus musculus ) are an invasive species on Auckland Island in the New Zealand subantarctic and planning for their eradication is underway. Mast seeding events cause rodent populations to irrupt, though little is known about this phenomenon in snow tussock grass ( Chionochloa spp.) systems on Southern Ocean islands. The aim of this study was to understand population fluctuations of mouse abundance on Auckland Island for the 2 years following a mast event, and with which tools to monitor abundance, to inform planning of bait application for eradication. Mouse populations were studied using kill and live trapping at two sites on Auckland Island, and mouse density was estimated using spatially explicit capture-recapture models. Mouse population density was highest during summer mast seeding of Chionochloa antarctica and then declined the following winter and subsequently remained low for the following year. Breeding remained seasonal, with a pulse in early summer and a very low level continuing through winter in both years, regardless of mast conditions. These results are similar to those from other cool temperate Southern Ocean islands where seasonal resource availability appears to drive breeding. Throughout the study the capture probability of mice was generally higher when population density was lower, which highlights that conclusions about population trends could be misleading if abundance indices are not calibrated to measures of population density. Mouse eradication should preferentially take place outside of a mast event but would likely still succeed during and following a mast event. Our work fills a key knowledge gap about rodent population trends during mast events for Southern Ocean islands, which is particularly important where eradications are planned.