London — A dozen whales that initially survived a mass stranding this week off the northeast coast of Scotland had to be euthanized, a British marine life rescue charity has said. A total of 77 long-finned pilot whales were found washed ashore Thursday, 65 of them already dead, by the British Divers Marine Life Rescue (BDMLR) organization on the Sanday island of Scotland’s Orkney archipelago.

“Sadly the remaining 12 pilot whales have been euthanized due to their condition deteriorating from the many hours they have spent stranded on the beach,” the BDMLR said in a statement late Thursday.

Having been at the beach for a long time led to “crush injury from their own weight and the high likelihood that they have inhaled water with the incoming tide.”

They had also sunk deeper into the sand when the tide washed over them and were unable to refloat themselves, the charity added.

“There are no obvious indications as to why they all stranded,” the statement said, adding that the organization would try to recover as many of the whales as possible for post-mortem examinations.

When the BDMLR initially found the whales after a report early on Thursday, they had already been stranded for several hours.

Such mass strandings are not uncommon among pilot whales, which tend to travel in tightly-knit groups.

“Pilot whales are a really social species. They really rely on their family bonds. So, it might have been that just one of them got into difficulty and the rest of the pod just stranded with it because they stick together,” BDMLR rescue and community coordinator Molly Brown told the Reuters news agency. “In moments of need, they never leave each other’s side.”

“This is definitely one of the larger mass strandings but not necessarily the largest,” BDMLR told AFP.

Last year, 55 pilot whales washed ashore on the island of Lewis, off northwest Scotland.

Much bigger strandings have taken place in other parts of the world, with the largest in parts of New Zealand and Australia.