The State's body for the conservation of freshwater fish is following a definite line of inquiry in relation to a major fish kill in Inishowen in Co Donegal.
Inland Fisheries Ireland said it is investigating the fish kill incident which happened in the Glenagannon River.
Environmental and fisheries officers from the North-Western River Basin District were alerted to the incident by a member of the public who called the agency's 24-hour confidential hotline number on Friday.
Patrols were carried out along the river on Friday and Saturday and it was confirmed that over 2,250 juvenile brown trout and Atlantic salmon were killed.
IFI said the fish kill covered approximately four kilometres of the river and comprised several year classes of fish.
The dead fish were removed from the river.
A statement from IFI said it is following a definite line of inquiry to determine the cause of the fish kill and this may result in legal proceedings being initiated.
Director of the North-Western River Basin District Milton Matthews acknowledged the ongoing support of the public in reporting suspected cases of water pollution and fish kills.
"We would like to thank the member of the public who alerted us promptly to this serious incident through our 24-hour confidential hotline last Friday," he said.
"This enabled our environmental team in Letterkenny to instigate our investigations without delay. This can be critical in identifying and confirming the underlying cause of fish kill incidents such as this one on the Glenagannon River."
To report fish kills or water pollution, members of the public are encouraged to call Inland Fisheries Ireland's hotline.
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Noel Carr, Secretary of the Federation of Irish Salmon and Sea Trout Anglers, said the fish kill was "heartbreaking".
Speaking to RTÉ News, he said these trout are very scarce wild fish in Donegal and along the west coast of Ireland.
To rehabilitate the fishery would take two life cycles - up to ten years. As well as the heartbreak, Mr Carr said it would impact local fishermen in the area and was also a very poor result for angling visitors going to the area.