Detecting detectability: identifying and correcting bias in binary wildlife surveys demonstrates their potential impact on conservation assessments

Presence of spraint at bridges as an effective monitoring tool to assess current Eurasian fish otter distribution in Austria

Monitoring carnivore populations requires sensitive and trustworthy assessment methods to make reasonable and effective management decisions. The Eurasian fish otter Lutra lutra experienced a dramatic population decline throughout Europe during the twentieth century but is currently recovering in both distribution range and population size. In Austria, most assessments on otter distribution have applied a modified version of the so-called “British” or “standard” method utilizing point-wise surveys for otter spraints at predefined monitoring bridges. In this study, we synthesize several recent statewide assessments to compile the current otter distribution in Austria and evaluate the efficiency and sensitivity of the “monitoring bridge” approach in comparison to the “standard” method. The otter shows an almost comprehensive distribution throughout eastern and central Austria, while more western areas (Tyrol and Vorarlberg) are only partially inhabited, likely due to a still ongoing westward expansion. Furthermore, the bridge monitoring method utilizing presence/absence information on otter spraints reveals itself to be a time- and cost- effective monitoring tool with a tolerable loss of sensitivity for large-scale otter distribution assessments. Count data of spraints seem to be prone to observer bias or environmental influences like weather or flooding events making them less suitable for quantitative analyses.

Marking behaviour and census of Eurasian otters (Lutra lutra) in riverine habitats: what can scat abundances and non-invasive genetic sampling tell us about otter numbers?

Growing human-wildlife conflicts and legal conservation obligations increased the need for precise information on Eurasian otter (Lutra lutra) population parameters for species status assessment and wildlife management measures. Scat surveys have become the method of choice to monitor species distribution range, abundance and habitat use. Although methodological concerns exist, scat abundance is often used as main indicator for otter population estimates and trends. To evaluate whether scat counts serve as good proxy for otter densities in linear stream habitats, we modelled the relationship between old/fresh scat abundance and otter numbers on two spatial scales, including also seasonal effects. Actual otter densities and marking behaviour were ascertained by genotyping of otter faeces collected at 218 marking sites along 132.6 km of four salmonid streams. Otter densities in our study areas ranged from 0.16 to 0.28 otters per stream kilometre. The age of scats and the level of spatial scale were determining factors when evaluating the suitability of scat abundance as index for otter densities. Covering large parts of an otter’s territory, fresh scat abundance increased with otter density, irrespectively of season. On the spatial scale of single marking sites, the relationship was not well supported, and no relationship could be found when using old scats in the models. Otter marking behaviour and differences in marking site use intensity, as well as seasonal differences in traceability and accumulation of old scats were discussed to bias the relationship between scat abundance and otter numbers.