Multi-year acoustic monitoring reveals fine-scale habitat preferences of harbour porpoise (Phocoena phocoena) and species decline within a Special Area of Conservation

Understanding spatio-temporal variation in habitat use is essential in underpinning our ecological understanding of species behaviour and supporting conservation and management decisions. Passive acoustic monitoring (PAM) data, collected from three monitoring locations within a Special Area of Conservation (SAC) for harbour porpoise in southwest Ireland, detected porpoises on average 77% of days throughout 2020 - 2023. General Additive Models (GAMs) were applied to examine the spatio-temporal trends in harbour porpoise occurrence and foraging behaviour. Preferred habitat was identified at the most sheltered site, with twice the acoustic detections observed compared to the other sites. Harbour porpoise occurrence and foraging behaviour were found to be variable across relatively small spatial scales and across diel, tidal, and lunar cycles, suggesting sequential habitat use across the region. Notably, an anomalous increase in porpoise detections was observed during a COVID-19 lockdown, aligning with decreased anthropogenic activity and ambient noise levels in the region during this period. However, we observed an overall 67% decline in acoustic detections between 2020 and 2023. This is of concern given the status of the bay as a SAC for harbour porpoise, and the declining abundance reported in the region prior to the commencement of this study. This study provides insights into fine-scale spatial and temporal patterns in the distribution of harbour porpoise that can inform future monitoring. Long-term PAM, with appropriate spatial coverage, should be integrated into protected area management plans to detect potential decreases or shifts in species occurrence over time. • Harbour porpoises detected on 77% of days in an SAC from 2020–2023 using PAM data. • GAMs revealed diel, tidal, and lunar effects on porpoise occurrence and foraging. • Sheltered habitats had double the acoustic detections compared to other monitoring sites. • COVID-19 lockdown linked to increased porpoise detections amid reduced noise levels. • 67% decline in detections highlights need for long-term PAM in conservation planning.