Snow Tracking: A Relevant Method for Estimating Otter Lutra lutra Populations

Risto Sulkava

《Wildlife Biology》 13卷2期 (2007/06) Pp. 208-218

並列摘要

Densities of otter Lutra lutra populations are difficult to measure. However, otters have to move between ice-free feeding areas in winter, and for this reason finding otter tracks is easy during the winter season. Snow tracking has been used for estimating the total population of otters in a 1,650 km^2 study area in central Finland since 1985 by the use of three methods. Using the home-range mapping method (HMM), all rivers, especially rapids and other possible feeding areas for otters, were studied carefully. Otter individuals were identified by size, age and course direction of the tracks, and the census was controlled by two revisits to possible feeding areas a few days after the first tracking. After the revisits, no further individuals were found in any of the 16 river systems. HMM gives exact estimates of the total population of otters, and in 2002/03, 52 otters (including 11 litters and 16 cubs altogether) lived in the study area. Litters are usually found while snow tracking, and the method therefore also supplies information on the productivity of the population. Using the segment method (SM), all shorelines of rivers and streamlets were divided into segments, and each segment was searched for otter tracks. The population estimate was mean number of animals determined from observed fresh signs in the sampled segments, and extrapolated across all segments in the study area. When the population was estimated by the positive segments found in SM, 59 otters were found; almost the same number as was estimated by using the HMM (52 otters). I also used a faster sampling method; the one-visit census (OVC). Only the most easily reachable otter sites of all possible home ranges were investigated within a few days. The number of permanent OVC sites was 111, and all sites were visited two to four days after the last snowfall. Using the OVC, about 50% of otters estimated by using the HMM were found in every sampling. The OVC method is cheap and reliable, and is therefore a useful tool for monitoring otter populations in large northern areas, such as for example in Finland, Sweden, Russia and Canada, given that infrastructures, i.e. roads, exist.

並列關鍵字

census ； Lutra lutra ； monitoring ； otter ； population ； reproduction ； snow tracks