Hundreds of brown trout have been killed in what is believed to have been a major pollution incident.
The Northern Ireland Environment Agency (NIEA) said it received a report of a fish kill at the Enler River near Comber on Friday afternoon.
It is believed that at least 300 trout are dead.
In 2013, nearly 5,000 dead brown and sea trout were found after a fish kill.
A farmer was later fined £6,000 for three separate pollution offences on the Enler.
"We are devastated, especially after the last fish kill in 2013," Ian Kittle, secretary of the Inler Angling Club, said.
"The river was starting to recover and we had been busy restocking it. This incident is not as bad. But at the same time, we have lost juvenile fish and more. Our resident brown trout are dead and we also found a large sea trout that had been coming up from the lough to spawn."
The NIEA said an inspector visited the area after the incident was reported to investigate and determine the environmental impact. "The inspector is following a definite line of inquiry," a spokesman said.