Comparison of track and direct observation estimations for assessing abundance of the Eurasian otter, Lutra lutra

The population density of the Eurasian otter, as in many other carnivores, is difficult to estimate in the wild. Spraint (otter scat) counts are usually used as an indirect indicator of abundance but its reliability is poorly known. In this work two methods of estimation (direct observations, DO, and track sampling, MNT) are compared in Central Spain. A new approach is applied to correct inherent biases to track sampling. Furthermore, the influence of sample size on MNT estimations was tested. The results indicate a highly significant relationship between the estimations of abundance derived from DO and MNT methods, although MNT could underestimate the density of otters when it is under 0.01 otters/km. The application of the new track sampling method could result in a successful reduction or removal of the biases. On the basis of current knowledge, it is argued that both MNT and DO could provide a realistic picture of the otter populations and facilitate their estimation and monitoring with sufficient reliability.