SDLP MLA Daniel McCrossan has written to the Stormont Environment minister Edwin Poots calling for those responsible for a large fish kill in Co Tyrone to be "identified quickly".
Mr McCrossan said a "full investigation" is needed after thousands of fish were killed in the Aughlish River. It happened along a 3.1 mile stretch of the Aghlisk River between Dromore and Fintona.
Species including brown trout and juvenile salmon were affected.
The Loughs' Agency said it was told of the "extensive pollution" at around 6.30pm on Sunday. It said samples have been taken from a "suspected agricultural source" and sent for testing.
Mr McCrossan said he had written to Mr Poots, the Agriculture, Environment and Rural Affairs (DAERA) minister, about the fish kill to "ensure that a full investigation takes place".
"The significant fish kill that has taken place in the Aughlish River in Co Tyrone is a disaster for our local environment and biodiversity," he said.
"At this stage it is believed that thousands of brown trout and juvenile salmon may have been killed after the Loughs Agency received a report of extensive pollution on Sunday evening.
"I have written to Minister Edwin Poots this evening to ensure that a full investigation takes place and that those responsible are identified quickly.
"People the length and breadth of Tyrone not only appreciate our local environment and biodiversity, many rely on it for their jobs.
"It is important that this incident is addressed swiftly and robustly."
