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.
Man charged with murder of Amy Doherty
New medical reports on Eleanor Donaldson submitted, court hears
MLAs set to be recalled over cancellation of special school summer schemes
Murder investigation launched after death of man in Portadown
Minister looking at ‘contingency plans’ for special school summer scheme