Infectious salmon anaemia has been responsible for the slaughter of millions of fish in the past six months and has put a question mark over the pounds 270m salmon farming industry north of the border.
The disease, which weakens the immune system of the fish and kills them through internal bleeding, has been the bane of salmon farms across the North Sea in Norway over the past 15 years.
A Norwegian-owned company, Hydro Seafood GSP, has been at the centre of the Scottish outbreak and recent speculation has focused on the possibility that diseased fish may have been imported from Norway into Scotland.
But yesterday Professor Tony Hawkins, head of the Marine Laboratory in Aberdeen which is investigating the outbreak, said it was also possible that the disease had arisen independently in Scottish fish. "It arose independently in Canadian fish farms and that may have happened here," he said.
"It may be a virus that fish carry which only becomes a disease in conditions of stress, such as thousands of fish being crowded together."
Laboratory teams are investigating 23 different kinds of sea fish - such as herrings and sprats, which visit the sea lochs where the salmon farms are located - to see if they can find the virus.
Scientists have examined the records of all 10 fish farms where the disease has been confirmed, and none shows that Norwegian salmon have been imported.
However, there is another possible import route, via the "well boats", boats with holds open to the sea, which are used to transport salmon from farm to farm.
"It is possible that a well boat which had not been properly sterilised came over from Norway to Scotland, but that would be virtually impossible to establish unless someone comes forward and says so," Professor Hawkins said.Reuse content