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 concludes.
It was thought that the unusually large incident on Tràigh Mhòr beach, Tolsta, could have been caused by trauma, illness or an acoustic disturbance caused by noise generated by the military or industry.
However, the report, from the Scottish Government’s Marine Directorate, cited “a convergence of biological, behavioral and environmental factors”, suggesting that the long-finned pilot whales, a highly social species, died because the group had been following a female in the middle of a difficult birth.
Dr Andrew Brownlow, lead research scientist at the Scottish Marine Animal Stranding Program (Smass), said: “The Tolsta event is a reminder that mass strandings are rarely the result of a single cause. Rather, they arise at the intersection of individual physiology, group social behavior and external marine environmental conditions.
“Understanding how these factors interact is essential if we are to improve our ability to anticipate, interpret and, where possible, mitigate the impacts of a changing ocean.”
The whales were in good health before the stranding, according to the report, but appeared to move into shallow waters, following “a single committed female.” The post-mortem examination suggested that the female whale had been experiencing a prolonged and difficult birth, which acted as a trigger for the pod’s fatal movement into the shallow bay.
The whales were seen swarming near the coast before becoming stranded. This behavior – in which a pod gathers to support a sick or injured member – could be crucial to survival on the high seas as a means of defense against predators, Brownlow said.
“If one member of the group were endangered, the well-documented social cohesion of this species would have led others to band together closely in a protective response,” he said.
“In this case, such behavior appears to have attracted the group towards the shallow sandy bathymetry (water depths) of Tràgh Mhòr, where the bay’s gently sloping seafloor and fine suspended sediments may have created an ‘acoustic trap’, attenuating echolocation signals and diminishing the group’s ability to navigate safely back to deeper waters.”
Unable to return to the water, the whales had to be euthanized on shore to prevent further suffering.
The scientists’ findings are particularly crucial as they try 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. SMSs is still investigating that incident.
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 last 30 years. Other recent reports of pilot whale strandings support Smass’s conclusion that man-made sound was not a factor in the strandings.
However, 10 sperm whales were stranded in January and February in places including Cornwall, Denmark and Germany. The unusual strandings of large-toothed whales have raised concerns among scientists that military or industrial noise pollution may be pushing deep-diving whales into shallow waters where they cannot feed.
Additional reports by Jeroen Hoekendijk
