Mallow Trout Anglers Club predicts around 40,000 fish could be killed in the River Blackwater by the end of this week.
Inland Fisheries Ireland and The Environmental Protection Agency are investigating the fish kill on a stretch of the river from Lombardstown to Mallow.
Chairman of Mallow Trout Anglers Club John Ruby told RedFM News it's a devastating blow to the area:
"With the fish still dying as we speak, by the end of this week we could be looking at close to 40,000. Loss of tourism, we always have people coming from different parts of the world to fish the Blackwater. One of the top rivers in Europe for trout and salmon. The river is very quiet, eerily quite at the moment."
