Inland Fisheries Ireland (IFI) is investigating a possible fungal infection outbreak that may have killed up to 1,000 brown trout in the River Blackwater, in Co Cork.
State agency IFI said it is currently assessing the impacted area, which spans an 8km stretch of the river between Mallow and Roskeen Bridge (Lombardstown).
Inspections of the Allow and Minor Awbeg rivers have not detected incidences of infected or dead trout.
The body is liaising with Uisce Éireann, the Environmental Protection Agency (EPA) and Cork County Council, all of whom have had staff on the river throughout the day.
Marine Institute staff are supporting IFI and are undertaking sampling to identify the possible infection.
Anglers are being urged to refrain from fishing in the affected stretch of river.
They are also being asked to wash, clean and dry all equipment, and to avoid crossing catchments with tackle to minimise the risk of spreading any possible infection.
How a potential fungal disease outbreak may have occurred is being investigated by the IFI.
However, investigations, at this stage, have not yielded evidence of pollution.
Monitoring and assessment of the river will continue to be undertaken to help identify the cause of the incident.
The IFI said water levels are very low in the River Blackwater at Mallow, and water temperatures have been recorded between 17-19C.
"Freshwater fish are vulnerable to low water levels and high temperatures," it said.
Adding: "Ireland has been impacted by hot and dry weather spells this summer, and these conditions have led to the periodic closure of fisheries in the west in recent months."
The state agency said it is continuing to appeal to people to report instances of fish kills or suspected water pollution to its 24-hour confidential phoneline, 0818 34 74 24.
