Natural Resources Wales (NRW) are appealing for information on a pollution incident that killed 300 fish over the weekend.
NRW officers are on site investigating the source of the pollution over a 1km stretch of the Afon Dulas tributary of the Tywi River, Carmarthenshire.
Original reports had found only 30 fish had been killed but continued investigation revealed the number to be over 300.
Andrea Winterton, South West Operations Manager for Natural Resources Wales, said:
“Our rivers support many species of plants and animals so its important pollution is dealt with as quickly as possible.
“We have been searching for the cause of the pollution since the report came in on Sunday.
“If you have any information regarding the incident please report them to our incident hotline on 0800 807060.”