Thousands of fish have been killed following a severe water pollution incident in the Enler River near Comber.
A local farmer has admitted the pollution was from dirty water which came from his property and has apologised for the incident and promised to do all he can to make amends.
The alarm was raised through an anonymous report from a member of the public to the Water Pollution Hotline that large numbers of dead fish had been seen in the river upstream of Comber town.
The Enler River is a tributary of Strangford Lough.
Farmer Martin Hamilton has apologised after admitting the polluting water came from his fields.
He said water used for washing crops had leaked into the Enler River, near Comber.
He estimated about or 25,000 litres of the dirty water, which kills the oxygen in the river had left the farm.
Mr Hamilton said: "We have a massive run-off on one of the fields and we have got ourselves into a real mess. A total apology from ourselves to everyone involved."
He said: "It is a situation that we are deeply embarrassed by. We plan to throw every resource at it to recover it.
"We are giving cast iron guarantees that we will be replacing any stock that is lost."
Describing the water pollution as "high severity", the Department of the Environment said evidence is being collected from a premises with a view to prosecution.
A joint investigation is being carried out by the Northern Ireland Environment Agency, part of the DoE, and the Inland Waterways and Fisheries branch of the Department for Culture, Arts and Leisure.
Councillor Robert Gibson said thousands of fish have been lost in the incident.
The local council had been aware of pollution on the river, and a smell in the town, for some time, he added.
Eddie Donnelly from the Inler Angling Club said it was the worst fish kill he had ever seen.
"The whole life has been taken out of the river," he said.