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

Abstract Governments must report on the conservation status of species designated as protected. We assessed changes in the Near Threatened Eurasian otter Lutra lutra population throughout Ireland (the whole island) from the 1980s to the 2020s using four National Otter Surveys (6,060 sites surveyed) and citizen science data (10,980 records). In 2023–2024, otters were recorded at 65% (95% CI 62–68%) of sites in the Republic of Ireland and 66% (95% CI 58–74%) of sites in Northern Ireland. Rivers, lakes and coasts were included in the surveys. Long-term trends in the population estimate indicate a decline of 24−27% (or 3−18% after correction for negative methodological biases). Short-term trends indicated changes of −7% to +2% in the Republic of Ireland and −26% to −27% in Northern Ireland. Otters remain widespread, having been recorded in 95% of hectads (10 × 10 km squares), with no range reduction. They prefer wide, fast-flowing rivers with rocky substrates, unmaintained banks, and riparian woodlands (trees and shrubs), and are positively associated with angling (probably because of 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 the decline. Future research should account for potential negative methodological biases, disaggregating detection from occupancy probability to capture the magnitude of any change more accurately, and future surveys should adopt techniques less vulnerable to biases (e.g. environmental DNA detection in water samples).