Mustelid mugshots: photographing facial masks of European polecats (Mustela putorius) for individual recognition and density estimation using camera traps

Small carnivores play an important role in ecosystems, but their often hidden lifestyle makes them generally hard to observe. Camera-trapping methodology shows great promise as a standardized monitoring tool for small carnivores. The European polecat (Mustela putorius) is a species of conservation concern, listed on the Annex V of the EU Council’s Directive on the conservation of natural habitats and of wild fauna and flora. Because of this, EU member states have to report the status of their polecat populations regularly. However, standardized methods to estimate the population size of polecat populations are currently lacking. We present a tube-based camera-trapping system, the Polecam, designed to obtain photographs of the facial masks of individual polecats. We tested the system in four study areas in southern Sweden during Spring and Autumn 2021. We obtained 52 observations of polecats, out of which we were able to identify 38 (73%) to individual. From these observations, we estimated area and season specific estimates of density for five season-area combinations using a multi-session spatial capture-recapture model. Density estimates ranged from 0.84 to 2.7 individuals/1000 ha, similar to the estimates from similar landscapes in other countries in Europe. We also obtained observations of six other species of small carnivores, suggesting that the Polecam could be used for multi-species monitoring, as well as density estimation of European polecats.