Environment Agency scientists in Northern Ireland are investigating three major fish kills in the space of a week.
Hundreds of dead pike, perch and eels were found in the Quoile River, near Downpatrick, in Co Down, on Friday morning.
It came after "significant numbers" of dead pike and other coarse fish were found in numbers in two separate incidents in the Portadown area on Tuesday and Wednesday.
Water quality inspectors and officers from the Fisheries Conservancy Board are investigating all three incidents.
The Northern Ireland Environment Agency said the kill on the Quoile was caused by "an increase of natural organic matter" reducing oxygen levels.Heavy rainfall can stir up the bottom of shallow waters, releasing rotting plant matter which chokes fish.
Aerators gave been deployed to boost oxygen levels in affected stretches.