Camera-trapping for abundance estimation of otters in seasonal rivers: a field evaluation

Abstract

Cryptic mammals with semi-aquatic habits, like otters, are notoriously difficult to study. Elusive behaviour, low population density, nocturnal habits and the type of continental habitat that characterize this group of species make the estimation of their population density an arduous task. Passive infrared-triggered camera devices are a non-invasive methodology that may present key advantages for the study of otter density. We have evaluated its use for the detection and estimation of the density of the Eurasian otter (Lutra lutra) in seasonal intermittent Mediterranean rivers, where the presence of dry riverbed sections between pools is usual, due to the discontinuity of flow during the summer drought. Through six camera-trapping surveys carried out in the Yeguas River (South Spain), we evaluated the efficiency of camera-trapping in different scenarios of methodological design, including a comparison with the visual census method. The results indicated that camera-trapping can be an optimal method to estimate otter abundance if some conditions are achieved, being the most important to use camera devices with fast trigger speed (0.1–0.2 s), deployed in key places over the river bed. In the case of our best-designed surveys, related to effort and efficiency of camera devices, we found that detection probability could reach asymptotic values in a relatively short time of 1 week, with negligible effects of imperfect detection. Considering the robustness of the collected data from camera-trapping (larger sample size and sampling periods), the density estimation from camera trap method (0.43 otters/km and day) was more accurate than estimation from visual census (0.12 otters/km and day). Camera traps can provide valuable information not only for population size and trends but also for other key demographic parameters, such as sex-ratio and reproduction and activity rhythms.