Anglers have reported a “heartbreaking” fall in the number of fish in part of the River Eden.
Members of Carlisle Angling Association said the number of catches had fallen off a cliff and they had found suspected sewage fungus in a stretch of the river close to a sewage outlet.
The EA said it took concerns about pollution seriously and investigated where appropriate.
United Utilities, which runs the nearby sewage treatment works, said effluent from its plant was treated to the highest standards.
Ash Temple from the association said he had been fishing part of the river near Carlisle Wastewater Treatment Works since he was young.
He said: “There were salmon getting caught everywhere up and down the river; you’d see salmon jumping everywhere.
“It was fantastic, but [over] the last 10 years it’s kind of fell off a cliff.
“Breaks my heart really. This is where I grew up.”
His friend and fellow angler Gary Byrne said there was a longstanding and popular fishing pool there, known as the Bone Mill.