The mass stranding and death of 55 whales on the Isle of Lewis in 2023 was caused by the mammals’ loyalty to their pod, a report has concluded.
It had been thought that the unusually large incident on Tràigh Mhòr beach, Tolsta, could have been caused by trauma, disease or acoustic disturbance from military or industrially generated noise.
However, the report, from the Scottish government’s Marine Directorate, cited “a convergence of biological, behavioural and environmental factors”, suggesting the long-finned pilot whales, a highly social species, died because the group had been following a female in the throes of a difficult birth.
Dr Andrew Brownlow, the lead scientist of the investigation by the Scottish Marine Animal Stranding Scheme (Smass), said: “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, where possible, mitigate the impacts of a changing ocean.”
The whales had been in good health before the stranding, the report said, but they appeared to move into shallow water, following “a single compromised female”. Postmortem examination suggested the female whale had been experiencing a prolonged and difficult birth, which acted as a trigger for the pod’s fatal move into the shallow bay.
The whales were seen milling near to the shore before they were stranded. Such behaviour – in which a pod comes together to support a sick or injured member – could be crucial to survival offshore as a means of defence from predators, said Brownlow.
“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,” he said.
“In this instance, that behaviour appears to have drawn the group into the shallow, sandy bathymetry [the water depths] of Tràgh 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.”
Unable to return to the water, the whales had to be euthanised on the shore to prevent them suffering further distress.
The scientists’ findings are particularly crucial in their attempt to understand the stranding of a further 77 animals of the same species – one of the largest recorded on UK shores – almost a year later on Sanday beach, Orkney. That incident is still under investigation by Smass.
The organisation’s long-term monitoring data indicates that mass strandings of whales and dolphins in Scottish waters have increased in scale and frequency by up to 300% in the past 30 years. Other recent reports on pilot whale strandings support the conclusion by Smass that human-created sound was not a factor in the strandings.
However, 10 sperm whales were beached in January and February in locations including Cornwall, Denmark and Germany. The unusual strandings of the large-toothed whales have raised concerns from scientists that military or industrial sound pollution may be driving deep-diving whales into shallow waters where they cannot feed.
Additional reporting by Jeroen Hoekendijk
