A pod of around 60 pilot whales appeared to have left the sea loch where they risked becoming stranded, said rescuers.
The animals were thought to have moved away from the shallow waters of Loch Carnan in South Uist on Saturday, but returned later on in the day.
However, yesterday marine experts said the whales had not been seen since Saturday evening and were thought to have left the loch.
Their apparent departure followed the death of one of the animals, whose body was discovered on an island in the loch.
A post-mortem examination carried out on the whale suggested it died of an infection.
Dave Jarvis, of the charity British Divers Marine Life Rescue (BDMLR), said: “It appears that what has been witnessed is a group of these extremely social creatures accompanying an ill individual and that the infection may have caused this animal to strand.
“Despite an extensive search, there have been no sightings of the remainder of the pod since last evening.
“Unfortunately, due to the increasingly worsening weather conditions, it has proved impossible to utilise a helicopter to assist in this exercise.”
The animals were first seen around Loch Carnan, in the Western Isles, on Thursday, sparking fears they would die in a mass stranding.
Some of the animals had injuries to their heads and a substantial operation to save them was launched.
Mr Jarvis said the operation was being wound down in light of the latest developments, but that those involved would continue to keep an eye out for the whales.
He said it was not known where the animals have gone.
Scotland’s environment minister, Stewart Stevenson, said he was “pleased” the whales appeared to have moved on.
Pilot whales prefer deep water but come inshore to feed.