Northern Ireland's Environmental Agency is tonight investigating the third major fish kill in a week.
Hundreds of dead fish were found in the Quoile River, Downpatrick, in Co Down at 9am.
The news comes after separate fish kills were reported in the Portadown area on Tuesday and Wednesday.
The Northern Ireland Environment Agency (NIEA) today said Water Quality Inspectors and officers from the Fisheries Conservancy Board were immediately sent to begin an investigation in Downpatrick.
An NIEA spokesperson said: “The incident has claimed the lives of hundreds of fish — mainly perch, pike and some eels — with hundreds of fish also distressed.
“It would appear this fish kill was caused by an increase of natural organic matter which is causing a higher that normal pressure on the oxygen levels in the water.”
The news comes after a significant number of coarse fish and pike were found dead at Shillington’s Bridge in Portadown only a day after hundreds of fish were killed at Moneypenny’s Lock on the other side of the Co Armagh town.
The Northern Ireland Environment Agency (NIEA) said there was no evidence of a link between the two incidents.
The kill at Moneypenny’s Lock off the Newry Canal is believed to have been caused by the recent adverse weather conditions.
Officers have not yet determined the cause of the kill at Shillington’s bridge.