The Northern Ireland Water Services yesterday admitted responsibility for one of the North's worst fish kills.
Several thousand young trout, salmon and dollaghan were killed when effluent poured into the Mullagh river, Co Derry.
The Mullagh is the main tributary of the Moyola, one of the North's best salmon rivers.
"It's our fault and we're obviously sorry that it has happened", said Mr William Duddy, operations manager of the Northern Water Board.
"One of our people opened the wrong valve at one of our treatment stations and the sludge poured into the river. As soon as the error was discovered, we immediately took remedial action to try to limit the damage.
The main thing now is to ensure the spillage doesn't make its way into the main Moyola river.
"It was a bad mistake on our part; it was human error," he said. "We will work closely with local anglers and with the Fisheries Conservancy Board to do everything we can to restore the Mullagh tributary to full health."
Mr Tom Maguire, chairman of the Moyola Angling Association, said thousands of young fish had been killed.
"Members have collected thousands of dead fish from the river. The spill covers a three-mile stretch. We recently received almost £75,000 from the Department of Environment in a river enhancement scheme. Now another branch of the department has destroyed years of good work.
"We reckon it will take at least five to six years to clean things up. We have 600 members in the association and since the spillage was discovered on Sunday night, I've seen dozens of members in tears.
"Five years' stock has been wiped out," he said. Mr Brian Finn, of the Fisheries Conservancy Board, said the spillage was a disaster.
"It's a disaster for the local anglers and it's a bad day for the medium and long-term future of the river and its stock."