How to Calculate Range and Population Size for the Otter? The Irish Approach as a Case Study

All EU Member States are obliged to submit reports to the EU Commission every 6 years, detailing the conservation status of species and habitats listed on the Habitats Directive. The otter (Lutra lutra) is one such species. Despite a number of national surveys that showed that the otter was widespread across the country, in Ireland’s 2007 conservation status assessment the otter was considered to be in unfavourable condition. While the Range, Habitat and Future Prospects categories were all considered favourable, Population was deemed to be unfavourable.This paper examines the data behind the 2007 assessment by Ireland, which included three national otter surveys and a series of radio-tracking studies. Range was mapped and calculated based on the results of national distribution surveys together with records submitted from the public. Population size was estimated by calculating the extent of available habitats (rivers, lakes and coasts), dividing that by the typical home range size and then multiplying the result by the proportion of positive sites in the most recent national survey.While the Range of the otter in Ireland did not decrease between the 1980/81 and the 2004/05 surveys, Population trend was calculated as -23.7%. As a consequence, the most recent national Red Data List for Ireland lists the species as Near Threatened (Marnell et al., 2009).