Temporal variation and diel occurrence patterns and behaviour in harbour porpoises (Phocoena phocoena)

Harbour porpoises are small cetaceans that inhabit temperate and subpolar waters of the Northern Hemisphere, mainly in shallow, nearshore areas. The Iberian harbour porpoise (Phocoena phocoena meridionalis) is a genetically distinct population from other North Atlantic populations and is classified as “Critically Endangered”, having experienced a significant decline since the mid-20th century. This study presents the first assessment of harbour porpoise occurrence and behaviour along Portugal's northern coast, near the Douro River estuary, based on acoustic data collected from the C-PODs. These passive acoustic monitoring devices detect porpoises through their characteristic echolocation clicks. Harbour porpoises produce Narrow Band High Frequency (NBHF) clicks for navigation, foraging, and communication. Over a period of nearly 28 days of recording, from December 2024 to January 2025, porpoises were detected on 78.6% of days, with travelling being the predominant behaviour. A Generalised Additive Model (GAM) was used to examine the patterns of occurrence based on Detection Positive Minutes (DPM), and the behaviour based on the minimum Inter-Click Interval (ICI). Results indicate that porpoise presence at the Douro River mouth is influenced by the year, the day and the temperature, with most detections occurring between 13 and 14 °C. Behavioural analysis showed that the year and tides influence the behaviour of the harbour porpoise. These findings confirm the regular presence of harbour porpoises along Portugal’s northern coast, which have been detected since 2017 by visual monitoring, and highlight the importance of long-term acoustic monitoring for guiding future conservation efforts for this endangered population.