Up to 2,000 Atlantic salmon, trout, lamprey and other fish species may have died following a pollution incident on the Aughrim River in Co Wicklow.
Dead fish were observed in a 3km stretch of the river near Coats Bridge, downstream from Aughrim village.
Inland Fisheries Ireland officers have been at the scene since Sunday morning and have collected specimens of dead fish along the river.
The source of the pollution has not yet been identified but water samples have been taken as part of the ongoing investigation and to gather evidence.
Monitoring and assessment of the river will continue for a number of days.
Inland Fisheries Ireland has said it was first notified of the "harmful ecological event" last weekend following reports from the public to its confidential phone line.
The body, which is the State agency responsible for the protection and conservation of freshwater fish and habitats, has continued to urge the public to report instances of fish kills, water pollution or habitat destruction to it.
