Northern Ireland Environment Agency is investigating after pollution caused a "major fish kill" at Dundrum Bay.
More than sixteen hundred dead fish have been counted so far.
Northern Ireland Environment Agency traced the pollution source to premises where a chemical has escaped into the river.
The investigation's ongoing and water samples have been collected.
A spokesperson for the Department of Agriculture the Environment and Rural Affairs (DAERA) said the samples will serve as "evidence for an enforcement action."
A clean up is underway.
The agency was notified of the incident at Carrig River, known for its shellfish production, on their pollution hotline on Saturday.
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