A pollution investigation is underway after more than 2,000 fish died in a river in Co Derry.
An incident at Muff River in the Eglinton area last Friday has since been confirmed a a "major fish kill" by the Northern Ireland Environment Agency (NIEA), which said a "range of species" had been affected.
The suspected pollution spill has been described by a local councillor as a "significant blow to the local environment".
It follows a fish kill in a tributary of the Callan River in Co Armagh last month, in which hundreds of juvenile salmon died as a result of a slurry spill.
Speaking of last week's incident at Eglinton, Derry City and Strabane SDLP councillor Declan Norris said it had caused "significant concern in the local area".
"A number of fish of all kinds have been killed as a result of pollution and we need to get to the bottom of exactly what happened and who was behind this," he said.
“I will be contacting the Northern Ireland Environment Agency and the Loughs Agency for an update on their investigation and to ascertain what information has been obtained about this fish kill so far, and to ensure that everything is being done to find those responsible.
“Pollution causes serious harm to our environment and this fish kill is being treated as a major incident. We need to send a clear message that pollution will not be tolerated, especially when it has such a harmful impact on wildlife and our local area.”
DUP councilor Jule Middleton said it was feared the river would take "some years for it to fully recover".
"With a range of species and ages affected there will also a wider effect on the environment too," she said.
"There is a particular fear that sea trout who return to the river to spawn may be impacted by the incident."
She said she hoped a probe into the fish kill will "quickly identify the source of this incident and ensure that enforcement action is taken".
A NIEA spokesperson said the agency received a report of the incident on Friday morning, indicating a "significant pollution incident" in the waterway.
"NIEA deployed a Water Quality Inspector to the area to confirm the report and assess the environmental impact. A joint investigation with Loughs Agency was initiated, and a major fish kill has been confirmed," the spokesperson said.
"The Loughs Agency has established that over 2,000 fish across a range of species and ages have been killed in respect the incident. A specific line of enquiry has been established and statutory samples have been collected with a view to initiating enforcement proceedings."
The spokesperson added: "As this is a live investigation, NIEA and Loughs Agency are unable to provide any further details in relation to the source as any attempt to identify a business or individual risks compromising any subsequent action. The joint investigation remains ongoing."