Estimating the relative abundance of American mink Mustela vison on lowland rivers:: evaluation and comparison of two techniques

Monitoring elusive species, which are 'difficult to study', often relies on the use of indirect indices to estimate relative abundance. It is important to know the accuracy of such indices and factors affecting it. For the American mink Mustela vison (an invasive species of conservation concern in the UK), we compare two indices of relative abundance: sign surveys (based on the detection of scats) and a new method based on the use of tracking plates on rafts. We found that raft surveys consistently performed better than did sign surveys and that estimates of relative abundance derived from both raft and sign surveys were linearly related to the number of individuals captured (but statistically significantly so only for raft surveys). Although both indices were highly correlated, there was considerable unexplained variation in the relationship between them. No statistically significant seasonal effects were detected for either method. Costs of the two methods were comparable after the first 2 years (based on four surveys per year), although raft surveys were more economical than were sign surveys in the longer term. In areas where polecats M. putorius are sympatric with mink, it will be important to develop methods to minimise confusion between the tracks of polecats and mink.