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Otter Survey of Ireland 2023-24
Reid, Neil Dingerkus, Karina Stone, Richard Colhoun, Kendrew Maddox, Lisa Marnell, Ferdia
Reid, Neil
Dingerkus, Karina
Stone, Richard
Colhoun, Kendrew
Maddox, Lisa
Marnell, Ferdia
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Reid, N., Dingerkus, K., Stone, R., Colhoun, K., Maddox, L. & Marnell, F. (2025). Otter Survey of Ireland 2023-24. Irish Wildlife Manuals, No. 161. National Parks and Wildlife Service, Department of Housing, Local Government and Heritage, Ireland
Abstract
It is a statutory responsibility of government to report regularly on the conservation status of designated species, such as the Eurasian Otter (Lutra lutra). Otter surveys were conducted throughout the Republic of Ireland (ROI) and Northern Ireland (NI) during 2023/24. The change in the status of otters throughout Ireland was assessed over the 40-year period from the 1980s to 2020s using data from four national surveys (n = 6,060 sites) plus species records from data repositories and citizen science appeals (n = 10,980 records). During 2023-24, a total of 1,063 sites were surveyed with otters present at 65% [95%CI 62 -68%] of sites in ROI and 66% [58 - 74%] of sites in NI. The otter remains widespread throughout Ireland yet site occupancy has likely declined since the 1980s. Most change appears to have occurred between the early 1990s and mid-2000s. The Standard Otter Survey method is subject to various methodological biases including variation in detection between surveyors and survey teams and negative impact of flooding due to rainfall prior to surveys which washes away tracks and signs. Estimates of the otter population size (numbers of adult females) in ROI declined over the long-term since the 1980s (somewhere between the naive estimate, as observed in the field, of -27% and the estimate corrected for potential negative biases of -7% and more likely closer to the latter) yet showed no change since the last survey in 2010-11. In contrast, estimates of otter population size in NI declined similarly over the long-term (somewhere between the naive estimate of -24% and the corrected estimate of -18%, again more likely closer to the later) and also over the short term (somewhere between the naive estimate of -27% and the corrected estimate of -26%). The divergence in trends between ROI and NI since the last survey seems likely accounted for by the combination of the experience, skill and expertise of the surveyor during the previous survey and an apparent significant decline in the Neagh-Bann River Basin District during the current survey coincident with a Harmful Algal Bloom in Lough Neagh. Otter population density varied spatially being highest in the south and north-west, and, lowest in the more human-modified east and less suitable central peatlands. Nevertheless, otters remain ubiquitous throughout Ireland having been recorded in 95% of hectads (10 km squares) with no change in distribution or range since the 1980s. Otters were more common at flowing freshwaters than at lakes or the coast and were associated with large, fast flowing rivers, rocky substrates, unmaintained (wild) river banks, riparian woodland (trees and shrubs) and areas with evidence of angling (likely reflecting salmonid prey availability) but negatively associated with peatlands. Eleven candidate pressures were identified with otters negatively associated with disturbance, notably canalisation or resectioning. Siltation, pollution both agricultural and domestic, and water abstraction increased in prevalence since the 1980s being more widespread now than ever. While individual pressures were local in extent, and none was judged an existential threat to otters at the national scale, it seems likely the combination of pressures on water quality and freshwater and coastal ecosystems is implicated in population decline. Ireland remains a stronghold for the otter despite an apparent decline since the 1980s with widespread suitable habitat despite apparent increases in pressures related to water quality. Future work should focus on disaggregating detection from occupancy probability to account for negative methodological biases to more accurately capture the magnitude of any change, while future surveys should consider survey techniques less vulnerable to such biases e.g. environmental DNA detection in water samples.
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2025-11-18
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National Parks and Wildlife Service. Department of Housing, Local Government and Heritage
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IWM161.pdf
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Irish Wildlife Manuals 161