Otters appear to have declined throughout Ireland amid increased pressures on water quality but remain ubiquitous

Governments must report on the conservation status of designated species. We assessed changes in the Eurasian otter (Lutra lutra) population throughout Ireland (the whole island) from the 1980s to the 2020s using four National Otter Surveys (n=6,060 sites surveyed) and citizen science data (n=10,980 records). In 2023-24, otters were recorded at 65% [95% CI 62-68%] of sites in the Republic of Ireland (ROI) and 66% [58-74%] in Northern Ireland (NI). Rivers, lakes and coasts were included. Long-term trends in the population estimate indicated a decline of -24% to -27% (or -3% to -18% after correction for negative methodological biases). Short-term trends -7% to +2% in ROI and -26% to -27% in NI. Otters remain widespread, present in 95% of hectads (10km squares), with no range reduction. They prefer large, fast-flowing rivers with rocky substrates, unmaintained banks, and riparian woodlands (trees and shrubs), positively associated with angling (likely due to salmonid prey) but negatively with peatlands. Eleven pressures were identified, notably site disturbance including agricultural pollution, siltation and water abstraction, which increased over time. No single pressure threatens otters nationally, but cumulative effects on water quality and ecosystems likely contributed to decline. Future research should account for potential methodological negative biases disaggregating detection from occupancy probability to more accurately capture the magnitude of any change while future surveys should adopt techniques less vulnerable to biases e.g. environmental DNA detection in water samples.