Only a single animal was able to be saved after the pod of adults and calves was found at Traigh Mhor beach near North Tolsta.
Reports of the stranding first arose at 7am on Sunday, at which time 15 of the animals were still alive.
Marine Mammal Medics from British Divers Marine Life Rescue, Fire and Rescue, Scottish Marine Animal Stranding Scheme (SMASS) and the coastguard attended.
Two whales were successfully refloated. However, one became restranded and subsequently died.
At about 3.30pm, with only eight adults and four calves still alive, a decision was made to euthanise the remaining animals on welfare grounds.
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A vaginal prolapse on one of the animals suggests the stranding was caused by one female giving birth.
The strong social bonds between pilot whales, which are actually the second largest species in the dolphin family rather than whales, often leads a pod to follow if one gets into difficulties. Consequently they are the most stranded whale.
Further analysis and post-mortem examinations of the whales is being conducted by SMASS before their removal.
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