A new study has reconstructed the feeding behaviour of 55 long-finned pilot whales in the weeks before they mass stranded on the Isle of Lewis in 2023, offering the first direct evidence of where they feed, while raising questions about the risks that come with it.

It was on a July morning in 2023 that 55 long-finned pilot whales came ashore on the Isle of Lewis in what was the largest mass stranding recorded in Scotland in recent memory. 

While post-mortem examinations confirmed the animals had all been in good health, an investigation by the Scottish Government’s Marine Directorate concluded that it was a combination of biological, behavioural, and environmental factors that had contributed to the event.  

Among those factors was the deeply social group’s apparent decision to follow a female experiencing a difficult birth into dangerously shallow water.

While going some lengths to demystify the scenario, the interpretation that left many questions still unanswered. Chief among those is what had brought the pod to that coastline in the first place? 

This week, a new study – led by the Scottish Marine Animal Stranding Scheme (SMASS) at the University of Glasgow and published in PLOS One – has managed to reconstruct the animals’ movements and foraging behaviour in the weeks before they came ashore.

Deep-water cetaceans like long-finned pilot whales are notoriously difficult to study in the wild, and their feeding histories are rarely documented. However, it’s according to those researchers at the University of Glasgow that stable isotope analysis offers a way in. By examining chemical signatures preserved in skin tissue, scientists can reconstruct what an animal has been eating and where it has likely been foraging in the weeks prior to death.

Analysing tissue samples from all 55 whales, the team found that the pod had been feeding primarily along the continental shelf edge and slope where deeper offshore waters are known to support substantial populations of fish and squid during spring and early summer. 

It is the first direct evidence that long-finned pilot whales use these shelf-slope habitats as important seasonal feeding grounds.

However, despite being in good nutritional condition at the time of death, the whales’ stomachs were found to be empty, raising questions about their foraging activity in the immediate period before stranding.

“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,” said Dr Andrew Brownlow, director of SMASS. “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 shelf-slope habitats where the whales were foraging lie in close proximity to rapidly shallowing coastal zones. The research team has suggested this geography may represent an “inherent risk factor”, placing animals engaged in normal feeding behaviour closer to conditions that can precipitate a stranding.

“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,” said Anna Kebke, PhD researcher at the University of Glasgow and lead author of the study. 

“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, underscoring both the vulnerability of the species in Scottish waters and the urgency of the research now underway.

The study, ‘The application of stable carbon and nitrogen isotopes to assess the feeding ecology of long-finned pilot whales (Globicephala melas) in Scotland’, was supported by the Natural Environment Research Council, the Joint Nature Conservation Committee, and the Portuguese Fundação para a Ciência e Tecnologia.

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