Ian McMullen from Hertfordshire was reported missing on Thursday evening after failing to return from a fishing trip on the River Oykel in Sutherland.
Police said searches will continue through the weekend for Mr McMullen, who was last seen near Oykel Bridge shortly after 5pm.
Inspector Jamie Wilson said: “Unfortunately Mr McMullen remains missing following extensive search activity on the River Oykel and the surrounding area.
“I would like to take the opportunity to thank the public and all our partner agencies for their support with this search.
“Further activity will continue over the weekend.”
Local coastguard teams, Assynt Mountain Rescue Team, the RNLI, police dogs and the coastguard helicopter have been involved in the search.
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.