Mass Stranding of 55 Whales Linked to Pod Loyalty and Difficult Birth
The mass stranding and subsequent euthanasia of 55 pilot whales on the Isle of Lewis in 2023 has been attributed to the animals’ loyalty to their pod, according to a recent report.
Initially, the large-scale incident at Tràigh Mhòr beach, Tolsta, was suspected to have resulted from trauma, disease, or acoustic disturbances caused by military or industrial noise. However, the Scottish government’s Marine Directorate identified a combination of biological, behavioural, and environmental factors as the cause.
The long-finned pilot whales, a species known for strong social bonds, were believed to have followed a female whale undergoing a difficult birth, which triggered the fatal stranding event.
Investigation Findings by SMASS
Dr Andrew Brownlow, lead scientist of the investigation conducted by the Scottish Marine Animal Stranding Scheme (SMASS), explained the complexity of mass strandings:
“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 report indicated that the whales were in good health prior to the stranding but appeared to enter shallow waters while following a single compromised female. Postmortem examinations suggested this female had experienced a prolonged and difficult birth, which likely prompted the pod’s movement into the shallow bay.
Witnesses observed the whales milling near the shore before becoming stranded. This behaviour, where a pod congregates to support a sick or injured member, is considered vital for survival offshore as a defense mechanism against predators, according to Dr 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.
He further explained that this social behaviour may have inadvertently led the group into the shallow, sandy bathymetry of Tràigh Mhòr. The bay’s gently sloping seabed and suspended fine sediments likely created an ‘acoustic trap,’ which attenuated echolocation signals and reduced the whales’ ability to navigate safely back to deeper waters.
Unable to return to the sea, the stranded whales had to be euthanised on the shore to prevent further suffering.
Significance and Broader Context
The findings are particularly important for understanding another large stranding event nearly a year later on Sanday beach, Orkney, which remains under investigation by SMASS.
Long-term monitoring data from SMASS indicates that mass strandings of whales and dolphins in Scottish waters have increased in both scale and frequency by approximately 300% over the past 30 years.
Additional studies on pilot whale strandings support SMASS’s conclusion that human-generated sound was not a contributing factor in these particular strandings.
However, there have been unusual strandings of 10 sperm whales in January and February across locations including Cornwall, the Netherlands, and Germany. These deep-diving large-toothed whales’ strandings have raised concerns among scientists that military or industrial sound pollution may be forcing them into shallow waters where they cannot feed.
Additional reporting by Jeroen Hoekendijk
