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.

中文翻译：

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相机捕捉季节性河流中水獭的丰度：现场评估

众所周知，具有半水生习性的隐性哺乳动物（如水獭）很难研究。难以捉摸的行为，较低的种群密度，夜间习性以及这组物种所特有的大陆生境类型使估算种群密度成为一项艰巨的任务。被动式红外触发相机设备是一种非侵入性方法，可以为水獭密度研究提供关键优势。我们评估了其在检测和估计欧亚水獭（Lutra lutra）在季节性的间歇性地中海河流中，由于夏季干旱期间水流不连续，因此水池之间通常存在干river的河床段。通过在Yeguas河（西班牙南部）进行的六次相机诱捕调查，我们评估了在不同方法设计场景中的相机诱捕效率，包括与视觉普查方法的比较。结果表明，如果达到某些条件，相机诱捕可能是估计水獭丰度的最佳方法，这对于使用触发速度快（0.1-0.2 s）的相机设备最为重要，该设备部署在河床上方的关键位置。就我们设计最佳的调查而言，涉及摄像头设备的工作量和效率，我们发现，在1周的较短时间内，检测概率可以达到渐近值，而检测不完善的影响可忽略不计。考虑到摄像头捕获的数据的鲁棒性（更大的样本量和采样时间），摄像头捕获方法（0.43 otters / km和天）的密度估计比视觉普查（0.12 otters / km和天）的估计更准确）。相机陷阱不仅可以为人口规模和趋势提供有价值的信息，而且还可以为其他关键的人口统计参数（例如性别比，生殖和活动节奏）提供有价值的信息。43獭/公里/天）比根据视觉普查（0.12獭/公里和天）的估算更为准确。相机陷阱不仅可以为人口规模和趋势提供有价值的信息，而且还可以为其他关键的人口统计参数（例如性别比，生殖和活动节奏）提供有价值的信息。43獭/公里/天）比根据视觉普查（0.12獭/公里和天）的估算更为准确。相机陷阱不仅可以为人口规模和趋势提供有价值的信息，而且还可以为其他关键的人口统计参数（例如性别比，生殖和活动节奏）提供有价值的信息。