On a beach on Lewis Island, Scotland, 55 pilot whales ran aground in 2023 in circumstances that have long remained mysterious. A scientific investigation today suggests the reasons for this tragedy.
In July 2023, an unfortunate scene took place on an isolated beach in Scotland: 55 pilot whales ran aground almost simultaneously. An exceptional event that raised many questions among scientists. Illness, noise pollution or trauma had first been considered to explain this disaster. But an in-depth investigation now reveals a very different cause, linked to the particularly close-med social behavior of these cetaceans.
A strach linked to a difficult birth
The massive stranding occurred on Tràigh Mhòr beach, near Tolsta, on Lewis Island. Soon after the discovery of the animals, the scientific teams tried to understand what could have pushed such a large group to dangerously approach the shore.
According to a report by the Scottish Government’s Marine Directorate, several biological and environmental factors were combined. The analyses showed in particular that the black globicephals (Globicephala melas) were in good health before the incident, which rules out the hypothesis of an epidemic or a generalized physiological problem.
Researchers now believe that the group was following a female in great difficulty when it was time to give forth. The post-mortem examination suggests that she was undergoing a long and complicated delivery, which would have triggered a collective movement towards shallow waters.
In this context, the social cohesion of pilot dolphins, a species known for its very strong links between individuals, would have played a decisive role. As explained by scientist Andrew Brownlow, who led the investigation of the Scottish Marine Animal Stranding Scheme, « the Tolsta event reminds us that massive stranding is rarely the result of a single cause. Rather, they appear at the intersection of individual physiology, group social behavior and external marine environmental conditions. »
Before the stranding, the animals were observed turning near the shore. This behavior could correspond to a collective attempt to help or protect the female in difficulty.
Social behavior that can become fatal near the coasts
Among pilot whales, solidarity is an essential strategy for survival in the open sea. Individuals in the same group often gather around a weakened or injured limb to protect it. Andrew Brownlow explains that « if a member of the group was in distress, the well-documented social cohesion of this species would have led the others to group closely together in a protective response ».
But this protective mechanism can become dangerous when the group gets too close to the coast. In the case of Tràigh Mhòr Bay, the seabed configuration would have aggravated the situation. Scientists mention in particular a possible « sound trap ».
According to Andrew Brownlow, « in this case, this behavior appears to have attracted the group to the shallow and sandy bathymetry of Tràgh Mhòr, where the gently sloping seabed and the fine sediments in suspension could have created an acoustic trap, attenuating echolocation signals and reducing the group’s ability to safely navigate deeper waters. »
Deprived of reliable acoustic landmarks and unable to find the exit of the bay, the cetacans finally found themselves trapped on the beach. Unable to return to the sea, they had to be euthanized in order to avoid prolonged suffering.
The researchers continue their investigations, especially after another spectacular stranding that occurred almost a year later in the Orcads, where 77 pilot whales were found on a beach in Sanday. The data accumulated by the scientists also show that the massive stranding of whales and dolphins in Scottish waters has increased sharply in recent decades.
For Andrew Brownlow, a better understanding of these events remains crucial: « Understanding how these factors interact is essential if we want to improve our ability to anticipate, interpret and, when possible, mitigate the impacts of a changing ocean, » he said in the report.
source : GEO
