05/03/2026
Took a while but the report into the 2023 long-finned pilot whale mass stranding event at Tolsta, Isle of Lewis in July 2023 is now published:
https://www.gov.scot/publications/investigation-long-finned-pilot-whale-globicephala-melas-mass-stranding-event-tolsta-na-h-eileanan-siar-july-2023/documents/
This was a large and multidisciplinary investigation which concluded that the event was not caused by trauma, infectious disease, or acute anthropogenic acoustic disturbance. Instead, the findings indicate a convergence of biological, behavioural, and environmental factors.
Comprehensive pathological and toxicological analyses, alongside data from analysis of underwater noise recorders, established that the pod was in generally good health prior to the stranding but suggest that the animals moved into shallow water in association with a single compromised female.
Evidence from necropsy data identified one adult female whale experiencing a prolonged and difficult birth- this was a possible biological trigger. Observers on the preceding day reported “social milling” behaviour close to shore, in which individuals cluster tightly, often around a vulnerable group member. In highly cohesive species such as pilot whales, this affiliative behaviour is likely central to survival in the deep ocean but can lead to problems in shallow coastal environments.
If a member of the pod was in distress, this species’ well-documented social cohesion would have led others to aggregate closely in a protective response. In this instance, that behaviour appears to have drawn the group into the shallow, sandy bathymetry of Tràigh Mhòr, where the bay’s gently sloping seabed and suspended fine sediments may have created an ‘acoustic trap’, attenuating echolocation signals and diminishing the capacity for the group to safely navigate back to deeper water.
In addition to investigating the proximate causes of the Tolsta stranding, the investigation advanced the application of genetic profiling and stable isotope dietary markers to characterise pod structure and kinship. These methodological developments will help strengthen future forensic investigations of mass strandings.
A real strength long term strandings monitoring programmes such as those in the UK is the ability to look trends over time. Looking back over the past three decades show that mass stranding events in UK waters are increasing in both scale and frequency. The Tolsta stranding in 2023 was followed almost exactly one year later by a larger mass stranding event on Sanday, Orkney, among the largest recorded in the UK. We hope to have the report on this 2024 MSE event out in the next few months.
The Tolsta event is a reminder that mass strandings are rarely the result of a single cause, rather, they emerge at the intersection of individual physiology, group social behaviour, and external marine environmental conditions. Understanding how these factors interact is essential if we are to improve our capacity to anticipate, interpret, and, ideally then mitigate human impacts on an already changing ocean.
Many thanks to the very many of you who were involved in this effort, from the strandings response itself to the necropsy, logistics or the investigation. We literally could not have achieved any of this without this help.
