Four years ago on the north coast of Scotland, 39 long-finned pilot whales stranded themselves as the tide was going out. Rescue teams managed to save just over half of the cetaceans, but what caused this mass stranding event has been the center of speculation ever since. Most attention has, however, been focused on the Royal Navy testing ground located in the area.

Now, a long-awaited report conducted by British governmental scientists has been released, in which the authors finally reach a conclusion over what caused the stranding event. The day before the beaching, the Navy detonated three 454-kilogram (1,000-pound) bombs in the region, and another 113-kilogram (250-pound) bomb after the strandings began.

They write of the first three bombs that it is possible "that the magnitude and frequency of successive underwater detonations […] would have had a significant detrimental effect on the hearing and therefore navigational competence of any cetaceans in proximity,” and that the fourth explosion “might have served to drive the animals further inland.”

Shortly after the first three bomb detonations in Cape Wrath – the largest live bombing range in Europe – on July 22, 2011, a pod of 70 pilot whales swam into a shallow tidal inlet nearby. Despite best efforts to try and herd the whales out to deeper water, 39 were left stranded as the tide went out. Volunteers and experts tried to refloat as many of the mammals as possible, but 19 subsequently died.

The report, conducted by the Department for Environment Food and Rural Affairs, showed that at the time of death, the whales were apparently healthy, showing no signs of obvious disease or illness. Whilst happening in a region close to a busy shipping lane and high fishing activity, they found that none of these sufficiently explained the beaching. The only notable difference identified over those two days was the occurrence of the military tests.

Around 4.5 kilometers (2.8 miles) from the inlet in which the whales were stranded is the island of Garvie. The island is used as target practice during military exercises, and when they miss and the bombs sink to the bottom of the ocean, the Navy then have to go in and manually explode them. It is thought that the noise from these explosions damaged the hairs in the whales' ears making them “functionally deaf.” It seems this confusion is what led them to swim into the inlet in the first place, and then reluctant to swim out again.   

The report concludes: “Following an extensive investigation into a range of factors, munitions disposal operations conducted around Garvie Island the day before and during the [mass stranding event], was the only external event with the potential to cause the [mass stranding event].”