A fish kill on the River Gully in Co Laois is being investigated by the Southern Regional Fisheries Board and the local county council.
Over 2,000 brown trout have died over the past 48 hours on the popular angling tributary of the River Nore near Durrow.
A pollutant introduced accidentally of deliberately is the most likely cause although Laois Co Co say a cause has not been identified.
"Samples from the water have been tested and they have not shown anything that could be identified as a cause," Mr Sean Mullins, acting director of services at the council said.