The otter population of the River Ticino (N Italy) 20 years after its reintroduction

On the River Ticino (Piedmont and Lombardy regions, N Italy), the Eurasian otter Lutra lutra became extinct in the 1980s and was reintroduced in 1997. Since then, the status of the reintroduced population has been assessed only occasionally, in 2008 and 2010. Between 2016 and 2018, we conducted an extensive survey for otter signs along the whole Italian stretch of the River Ticino, following the ‘Standard Method’ for otter surveys. In 2016–2017, we found 101 spraints (mean marking intensity: 0.40 spraints/100 m), spread over a 97-km long stretch of the river. In 2018 only five spraints were collected, the two furthermost marking sites being 32 km apart. Genotyping of nDNA extracted from 21 faecal samples enabled the identification of six different individuals. The surveys led to drawing a reliable picture of otter distribution and population size, with evidence of otter occurrence on a longer than previously recorded stretch of the river. The results of the 2018 survey suggest that stochastic factors may still threaten the survival of reintroduced otters and would suggest a reinforcement of the population is required to increase its genetic diversity.