Fish deaths in Olway Brook, Llansoy, are investigated
An investigation is under way after more than a thousand fish were found dead in a Monmouthshire stream.
The dead fish found in the Olway Brook near Llansoy include trout, eel and lamprey.
Natural Resources Wales said a suspected source of pollution had been "identified and stopped".
But a spokesman said low oxygen levels in the water following the incident meant other fish remained "in some distress".
"We are continuing to monitor the environmental impact of this incident and are currently aerating the brook to prevent further fish deaths," he said.
"We'll be monitoring the river closely over the coming days to ensure that the oxygen levels in the water are high enough and that no more fish are killed."
Anyone finding fish in distress is asked to contact the agency's 24 hour pollution reporting line on 0800 80 70 60.