by Elizabeth McMeekin
New research, focused on the feeding behaviour of long-finned pilot whales, has shed light on one of Scotland’s largest mass stranding events.
The study, led by the Scottish Marine Animal Stranding Scheme (SMASS) based at the University of Glasgow, used stable isotope analysis to reconstruct the feeding history of 55 long-finned pilot whales that mass stranded in 2023.
The findings have shed light on the animals’ movements and foraging behaviour in the weeks prior to the event, and on the broader factors that may contribute to mass strandings.
In July 2023, 55 long-finned pilot whales mass-stranded on the Isle of Lewis in the largest such event recorded in Scotland in recent memory. Post-mortem examinations undertaken by SMASS confirmed that the animals were in good health. A subsequent investigation report, published recently by the Scottish Government’s Marine Directorate, identified that a combination of biological, behavioural, and environmental factors had contributed to the event. The report found that, notably, the group, a highly social species, appeared to have followed a female experiencing a difficult birth (dystocia) into dangerously shallow water.
While the likely cause of the 2023 Isle of Lewis stranding has been established, this latest study, published in PLOS One, provides important additional evidence on the period leading up to the event.
Deep water pelagic cetaceans such as long-finned pilot whales are notoriously difficult to study in the wild, and their feeding behaviour and movements prior to stranding are rarely documented.
Stable isotope analysis reconstructs feeding history from chemical signatures preserved in skin tissue, and offers a means of recovering dietary information, revealing where these otherwise elusive animals have been and what they have been eating in the weeks before they come ashore.
The isotopic data from the 2023 stranding indicated that the pod had been feeding primarily along the continental shelf edge and slope: deeper offshore waters believed to support substantial fish and squid populations during spring and early summer.
Although the animals were in good nutritional condition at the time of death, their stomachs were empty, raising questions about their foraging activity in the immediate period before stranding. The study is the first direct evidence that long-finned pilot whales use shelf-slope habitats as important seasonal feeding grounds.
The research team say the proximity of these habitats to rapidly shallowing coastal zones may represent an inherent risk factor, placing foraging animals closer to conditions conducive to stranding.
A whale stranded on a remote shore in Scotland, and a whale which was stranded at Dumbarton on June 21, 1905.
Understanding where these animals feed – and how those patterns may be shifting in response to environmental change – is therefore essential to assessing the impact of human activities on the species, and to developing effective management strategies to minimise the risk of future events.
Anna Kebke, PhD researcher at the University of Glasgow and lead author of the study, said: “Understanding the feeding habits of large marine predators such as long-finned pilot whales is critical for the development of conservation strategies. However, dietary data are often lacking.
“Our findings demonstrate the importance of deep-water food sources to long-finned pilot whales, providing valuable insights into their early spring-summer feeding habits.
“These results highlight the value of stable isotope analysis in advancing our understanding of cetacean trophic ecology and better informing marine mammal conservation management.”
Two further mass strandings involving long-finned pilot whales occurred in Scotland in 2024 and 2025, highlighting the urgency of this work.
Dr Andrew Brownlow, Director of SMASS, added: “Post-mortem examination tells us about the animals’ condition at the moment of stranding; stable isotope analysis tells us where they had been and what they had been eating in the weeks before. Together, they allow us to move from asking what happened at the moment of stranding to asking what set these animals on a course towards it.”
The study, ‘The application of stable carbon and nitrogen isotopes to assess the feeding ecology of long-finned pilot whales (Globicephala melas) in Scotland’ is published in PLOS One. The work was supported by the Natural Environment Research Council, the Joint Nature Conservation Committee, and the Portuguese Fundação para a Ciência e Tecnologia.
A full copy of the study can be found here, following publication: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0346340