Hundreds of fish possibly as many as 1,000 have died on a Perthshire river.
The cause of the incident on the River Ericht, just north of Blairgowrie, is under investigation but is understood to be a natural phenomenon.
The temperature of the water and the large number of fish heading upstream at the same time are believed to be factors, combined with possible disease.
The Scottish Environment Protection Agency (Sepa) said its officers would be looking into the deaths and a PhD veterinary student from Stirling University has been taking samples.
Salmon are prone to spreading disease when they are caught in close proximity and may have been funnelled together on their journey upstream to breed.
“If fish get injured then disease gets in through the wound if there are a lot of fish together,” said Ronnie Marshall, a ghillie on the Ericht at Blairgowrie.
With the widespread practice of catch and release, some salmon suffer injuries at this stage, making it more likely that they succumb to disease.
Dead fish had been seen as far down as the bridge over the river in Blairgowrie but most were to be found on the beat a couple of miles upstream.
Dr David Summers, fisheries director of the Tay District Salmon Fisheries Board, said this type of incident has happened on the Ericht before.
“I suspect temperatures have been quite low and with the water low, it is more difficult to ascend some of the obstacles,” he said.
The conditions were also perfect for the spread of the fungal infection saprolegnia.
The river, which runs through Blairgowrie and Rattray, leads into the River Isla.
The impact the incident will have on angling remains to be seen.