Duncan Ferguson, director of the Spey Fishery Board, said it was one of the largest pollution incidents he had seen in 36 years of working on the river.
“It’s a tragic event,” he said. “It’s a really bad outcome and it didn’t have to happen.”
Ferguson said the salmon population could face a five-year period of recovery.
He added: “There are no invertebrates left, thousands of fish have died. This part of the river is now basically inert. It’s a tragedy and it can’t be repeated.”
The chemical is thought to be industrially linked, he said.
The spill comes during the peak period for fly fishing on the Spey.
A Sepa spokesman said: “We are investigating a potential pollution incident in a tributary of the River Spey and are working to identify the source and impacts.”
