Thousands of wild Atlantic salmon returning to Scotland's rivers have been infected by a mysterious parasite which has left them with bleeding wounds.

The Fisheries Research Service (FRS) in Scotland has revealed that evidence of the infection has been found in salmon in more than 50 rivers across the country.

The unprecedented infection is also widespread in England, Wales and Ireland.

But Scotland's salmon fishery boards yesterday stressed the appearance of the infection - known as "red vent syndrome" - was having no effect on a boom year across the country for catches of the king of fish.

A spokesman for the FRS said: "There have been reports from across Scotland of wild Atlantic salmon returning with inflamed vents. These symptoms seem to be restricted to the wild Atlantic salmon populations.

"By September FRS had received notification of over 50 rivers across Scotland with returning Atlantic salmon that were showing signs of swollen, bleeding vents. These fish appear to be in good physical condition apart from the red vents, and have only recently returned from the sea to rivers."

Andrew Graham Stewart, a spokesman for the Association of Salmon Fishery Boards in Scotland, told The Scotsman: "The evidence seems to be that the wounds heal after some time in fresh water so the likely effect on spawning won't be too bad.

"But many thousands of fish have been infected. This infection has been seen before, but never to this extent."