Natural Resources Wales (NRW) is appealing for information following a report of dead fish in the rivers Tarell and Usk near Brecon.
Officers continue to investigate the incident after a concerned person reported seeing at least 40 dead adult fish in the river. They reported seeing adult salmon, trout and coarse fish.
NRW Officers have been to the rivers and identified the possible source of pollution and taken water samples.
Steve Morgan, Operations Manager for Natural Resources Wales, said:
“Our rivers provide a home to rich, diverse and valuable species of plants and animals so it’s important to deal with pollution as quickly as possible.
“We investigate every report of dead fish in our rivers and this investigation is on-going.
“There is no longer any obvious sign of pollution and the water now appears clear, but we’ll continue to investigate to attempt to confirm the cause.
"If you have any information or see fish in distress, we urge you to report it on 0300 065 3000, and press 1 (24-hour service).”