An investigation is under way after hundreds of fish were found dead in the Tolka River near Damastown in Dublin.
Among the species of fish found were brown trout, eel, stickleback, lamprey and minnow, according to Inland Fisheries Ireland. They were found over a 2km stretch of the river.
The organisation said early indications suggest that more than 500 dead fish were found.
The IFI said that the alarm was first raised yesterday and samples have been taken from the river for analysis.
It said that an interagency group with representatives from Fingal County Council, the Environmental Protection Agency and the National Parks and Wildlife Service, was convened.
Since then, a definite line of enquiry was established and the group’s role has been completed, according to the IFI.
Members of the public are being urged to report any instances of fish kills, water pollution, habitat destruction or illegal fishing to its confidential number 0818 34 74 24.