An evaluation of field and noninvasive genetic methods for estimating Eurasian otter population size

Successful conservation and management of rare and elusive species requires reliable estimates of population size, but acquisition of such data is often challenging. We compare the two most frequently used methods of assessing abundance of Eurasian otter (Lutra lutra) populations, noninvasive genetic sampling (NGS) based on genotyping of faeces and field surveys using snow tracking. In a 100-km² oligotrophic otter habitat with linear water bodies, both methods yielded very similar estimates (10–12 individuals). However, in a 100-km² fishpond area, consisting of a complex network of rivers, fishponds, channels and marshes, genotyping of faeces revealed the presence of a higher number of individuals (46–50 genotypes) than the snow survey (38 individuals). NGS data analysed by capture-mark-recapture (CMR)-based software CAPWIRE provided even higher estimates, being twice the number assessed through snow tracking (76–81 individuals, CI_95% = 49–96 and 55–89). Our results suggest that the performance of both NGS and snow tracking is comparable in simple linear habitats, but in complex habitats with very high otter density a combination of genetic and field methods, or CMR analysis using genetic data, is recommended. We emphasise that to obtain reliable estimates using NGS it is necessary to follow strict protocols for detection and elimination of genotyping errors. Based on a literature review and our experience, we suggest improvements that may increase the success rate and efficiency of NGS for otters.