Up to 400 dead fish have been discovered in the River Gryffe in Renfrewshire, between Bridge of Weir and Crosslee.
And the Scottish Environmental Protection Agency have been called in to probe the problem.
Bridge of Weir Angling Club president John Blair said: “There are an awful lot of dead fish. They seem to have some sort of infection and have what look like acid burns.
“We don’t know what has caused it — but it could be pollution from a burn.
“The water is clear so it is possible they are bringing this in from the sea. But there is nothing like this at the nearby River Cart at Johnstone. I’ve never seen anything like it before, not on this scale.” SEPA said investigations were continuing into the salmon deaths.
A spokesman said: “We have received a number of calls about this over the last week and our officers have attended on every occasion.
“We are still investigating so it would not be appropriate to provide further information at this time.”
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