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.
Earlier this evening, the Minister of State at the Department of Agriculture, Food, Fisheries and the Marine, met with local anglers to see firsthand the extent of the fish kill and to hear their concerns.
Anglers said they believe the fish kill levels could be even higher than the figures confirmed by the IFI.
They are calling on the statutory body to give them directions on how to manage the affected stretch of the river until the cause of the fish kill is known.
