NatureScot Research Report 1364 - Hebridean Cetacean Research Programme, 2021-2024.
Abstract
This report presents findings from the Hebridean Whale and Dolphin Trust (HWDT) Cetacean Research Programme, focusing on three years of surveys conducted between April 2021 and March 2024 off the west coast of Scotland. The programme, which has been running for over 20 years, collects data on marine megafauna through visual and acoustic line transect surveys. Key findings include the completion of 22,645 km of survey effort, resulting in 3089 sightings of ten mammal species and two fish species. Notably, harbour porpoises (Phocoena phocoena) were the most frequently recorded species (n=801). Sighting rates for minke whales (Balaenoptera acutorostrata) and common dolphins (Delphinus delphis) were the highest recorded in 20 years, with rates of 1.08 and 2.31 sightings per 100 km in 2022, and 1.57 and 2.86 sightings per 100 km in 2023, respectively. In contrast, sighting rates for basking sharks (Cetorhinus maximus) were the lowest recorded in 20 years, with rates of 0.068 and 0.069 sightings per 100 km in 2022 and 2023, respectively. Most basking shark sightings occurred within the Sea of the Hebrides Nature Conservation Marine Protected Area (NCMPA). Acoustic surveys generated 1,766 hours of full bandwidth acoustic recordings, from which 2777 acoustic detections of harbour porpoise were identified. The report also notes the cessation of Acoustic Deterrent Devices (ADDs) use around salmon farms in the research area since 2022. The programme trained 294 volunteers in survey protocols, highlighting the importance of long-term monitoring for understanding distribution, abundance, and habitat preferences of key species such as harbour porpoises, minke whales, common dolphins, and basking sharks.