Five kilometres of the River Clodiagh, a tributary of the Tullamore river in Co Offaly and popular with local anglers, have been devastated in a suspected chemical leak.
Inland Fisheries Ireland (IFI) said it is following a definite line of inquiry and it may take a considerable number of years for the river to recover.
The 3,700 fish deaths, mainly trout but also salmon, lamprey, minnow and stoneloach, suggested the Clodiagh is believed to have recovered from a previous pollution spill which is believed to have been caused by fertilisers.
Water and sludge samples have been taken with a view to a possible prosecution, the IFI said.
Agency director Amanda Mooney said: “This pollution discharge has been a devastating blow to the River Clodiagh.
“The quantities of fish present would indicate that the river had recovered considerably in recent years from a previous eutrophication state, with excellent salmonid spawning and nursery habitat.
Most of the fish kill on the Clodiagh is believed to have started around the Clonaslee area in northern Laois with farming not believed to have been responsible for the pollution.
Prior to the incident the river was said to hold fair stocks of wild brown trout weighing anything from 0.75 to 2lbs.
Source: Belfast Telegraph 21st August 2015