Thousands of brown trout and salmon died on a 12-mile stretch of the Ballymartin River and Sixmilewater between Mallusk and Antrim after a pollutant entered the watercourse on Friday.
EHS staff were tasked to the Sixmilewater on Friday evening following reports of a heavy foam moving down the river near Templepatrick.
On Saturday morning the extent of the damage to the watercourses was becoming apparent, with hundreds of dead fish being reported by members of Antrim & District Angling Association (ADAA).
Fishery Manager Allan Kirkpatrick commented: "I counted 1,837 dead fish myself and that was only on part of the river. Over the 12-mile stretch we're talking many thousands of dead fish, not to mention the effect this will have on all the wildlife such as the otters and kingfishers that live along the river.
Investigation into Fish Kill Continues
UK - The Environment and Heritage Service (EHS) is continuing its investigation into a major fish kill on two local rivers at the weekend.