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.
Murder investigation launched following death of man in Coleraine
Man arrested over threats to elected representatives
Teachers’ unions welcome pay increase while pledging to tackle workload
Minister to explore if he can enforce use of Ulster Banner at Commonwealth Games