A spokesperson for the council said that until the pollutant could be established, it was advised not to swim or fish along the river.

“It is very important that until further information is available about the pollutant that caused this fish kill, that the public should not touch the dead fish and the water should not be ingested. Parents should ensure that children do not play with the fish or swim in the water of the Six Mile Water,” the spokesperson said.

Meanwhile, local politicians have called for urgent action to prevent further fish kills.

Alliance councillor David Ford said he had put proposals to the Department to tackle the problem and is applying pressure on them to implement his suggestions quickly.

“I have put proposals to tackle this devastating problem to the Department of the Environment following the last major incident. These proposals were put to me by a local angler and would involve creating settlement ponds near Mallusk industrial estate that would capture and dilute pollutants before they reach the Ballymartin Water and Six Mile Water. I am encouraged that these plans are being considering by the Department; however, action needs to be taken urgently,” Cllr. Ford said.

He added: “Serious incidents regarding pollution are happening with depressing regularity in this area. Whilst robust action against polluters is welcome, our main concern is protecting water quality and fish in the rivers. Prosecution of offenders after the damage has been done is simply not enough. There are real problems when the courts fail to hand down meaningful penalties.”

His comments were echoed by party colleague Alan Lawther, who commented that all animal life in the river seemed to have been killed by the latest spill.

“Even the hardiest species like eels are dying. I was informed that the effects are spreading into the Six Mile River and fish as far down as Dunadry are dying. Just last March, there was a serious spill into the same river, and local people believe that much of the pollution come from firms operating in Mallusk,” Cllr. Lawther said.

He added: ““I am extremely worried about where these chemicals go from here, as the Six Mile Water feeds into Lough Neagh, our major drinking water source. I hope that the Rivers Agency and Environment and Heritage Service will perform a rigorous investigation to get to those responsible and prosecute them.”

The DUP’s William McCrea also condemned the pollution saying it was an “environmental catastrophe which would take decades to repair”.

“The strongest possible measures must be taken against those responsible,” he said.

Speaking after visiting the river with local anglers Mr McCrea said: “It is not just local anglers who use this river, but it was also an important tourist attraction with many fishermen coming to this area on angling holidays.