Inland Fisheries Ireland has said they now believe up to 10,000 wild brown trout died in a fish kill first reported last week by local anglers along a stretch of the River Blackwater at Mallow, Co Cork.
It will be up to three weeks before they will know the actual cause of death, as they await the results of diagnostic testing carried out by the Marine Institute four days ago.
The dead fish were recorded along a 18km stretch of the river around 6km downstream of Mallow near Killavullen and up to 12km upstream near Banteer Bridge.
Diseased fish have also been seen on the Awbeg river, and the Clyda river, which join the Blackwater west of Mallow.
Sean Long, Director of the IFI’s South-Western River Basis District said: “Disease outbreaks can occur in wild fish populations and are influenced by numerous factors including environmental change.
“No evidence of a polluting or harmful discharge has been detected to date in the river.”
The Marine Institute’s Fish Health Unit has advised that the removal of dead fish from the water is not necessary.
Meanwhile, IFI officers are continuing to monitor the watercourse.