A serious fish kill in the Six Mile Water river is being investigated as anglers fear thousands of fish have died.
The cause of their deaths is not yet known and an investigation by the Northern Ireland Environment Agency (NIEA) has not been able to identify a source or symptoms of pollution.
Anglers are angry about damage to the river's fish stock and harm to the spawning of brown trout and dolloghan from Loch Neagh. Serious fish kills also occurred in the river in 2011 and 2008.
Bill Robinson, chairman of the Ballynure Angling Club, said the river is plagued by these small-scale pollution incidents that are never resolved.
He added: "The latest kill is a slug of pollution that has come into the river, killed fish and now gone down stream, so there's no trace of it.
"I don't believe we will ever get to the bottom of it."