More than 1,200 fish have been killed in a suspected slurry spill.
Inland Fisheries Ireland (IFI) said it was following a direct line of inquiry for the source of the large discharge of pollution into the Owentaraglin River in north-west Co Cork, a tributary of the Blackwater.
The fish were killed along a 2km stretch near the village of Kiskeam.
Fisheries officers described the river as an important spawning grounds with stocks of salmon, brown trout, eel and stickleback affected.
And they warned that they were particularly concerned that significant numbers of hen salmon carrying eggs were killed in the incident.
The slurry has also done significant damage to the spawning beds and wider aquatic habitat, which will have implications for the future fish population in the area, the IFI said.
Sean Long, director of the South Western River Basin District, said: "It will take years for River Owentaraglin to recover to its former condition as a result of this pollution.
"Inland Fisheries Ireland is reminding farmers of the importance of complying with EU regulations on the storage or movement of slurry."
Recreational angling is said to be worth more than €800m to the economy every year and supports 11,000 jobs.