The fishing vessel which sank off Mingulay this morning has been named locally as the 'Louisa', a 16-metre vivier crabber owned by Duncan and Murdo Kennedy, from Marvig, South Lochs.
One crew member has been rescued and taken to hospital, but three remain missing.
An emergency distress signal was picked up from south of Mingulay, 12 miles south of Barra, just after 3am.
The Stornoway Coastguard helicopter was tasked to the scene, where one man was airlifted and taken to Western Isles Hospital.
Additional assistance in the search is being provided by the Barra RNLI.