A major search is under way for Ian McMullen, who was reported missing on Thursday evening after failing to return from a fishing expedition on the River Oykel near Oykel Bridge in Sutherland.
Mr McMullen, who is in his 80s and from Hertfordshire, was last seen on the river shortly after 5pm.
Searches were carried out along the river on Thursday evening and resumed on Friday.
Local coastguard teams, Assynt Mountain Rescue Team, the RNLI, police dogs and the coastguard helicopter have been involved in the search.
Police are appealing for anyone with information to contact them.
Inspector Jamie Wilson said: “As times passes we are naturally increasingly concerned for Mr McMullen’s safety.
“Extensive search activity is ongoing on the river in the area where he was last seen on Thursday afternoon and downstream.
“I would ask anyone who has information on his whereabouts or believes they have seen him to contact Police Scotland on 101 as soon as you can.”
Mr McMullen is around 5ft 9in and of large build, with a distinctive brown mark on his left check.
When last seen he was wearing grey, full length waders and possibly a grey life jacket.