Fish kill along River Moyola tributary
There has been a fish kill along a tributary of the River Moyola near Draperstown in County Londonderry.
It is understood that the numbers involved could be "significant".
The Northern Ireland Environment Agency (NIEA) investigation has traced the source of the problem to nearby premises but said it was not agricultural.
Investigators will work over the weekend to establish the actual extent of the impact.
Norman Henderson from the NIEA described it as a "fairly serious fish kill".
"It's a healthy wee river and it has been quite badly impacted," he said.
"Obviously we're in the early stages of the investigation because this came in on Friday and we're in darkness and we can't investigate at this time of the night but we'll be out again tomorrow, colleagues from Inland Fisheries will be out again tomorrow and they'll quantify the number of fish and the precise length of river affected."
Jack Chapman from Moyola Angling Association said the fish kill had destroyed a lot of good work that had been done on the river.
"We have lost some fish, fish can be replaced, but our concern is for the invertebrate and insect life in the river," he said.
"We can't replace that and that's the food for the fish of the future.
"Also the club has done a lot of enhancement work around the river putting in spawning beds for the fish to spawn on, those may also have been destroyed in this incident."
An estimated two-mile stretch of river has been affected.
The Moyola is a game fishery with salmon, brown trout and Dollaghan trout.