The salmon and trout died after chemicals from a water treatment works in the Mourne Mountains leaked into the Upper Bann. Fish along an 18-mile stretch between Rathfriland and Banbridge were affected. Anglers said that it could be up to 15 years before the river was properly restocked.
The leak has been traced to an underwater pipe at the Fofanny water treatment works near the Spelga dam reservoir in the Mournes.
Preliminary scientific analysis has still to be completed, but the Department of the Environment has promised that an immediate and detailed inquiry will be launched.
The chief executive of the water service, Henry Plester, said: "We plan to carry out a full investigation of the water- treatment works and its performance, and to take the necessary steps to avoid a recurrence of any problem which might be identified."
The Northern Ireland Fishery Conservancy Board reacted angrily to the disaster. Bill Smyth, the chief executive of the board, said: "Fish of several generations, particularly trout, have been lost. We will have to restock from other sources and it is going to take five to ten years for the river to recover."
Two other rivers in North- ern Ireland, the Mourne near Omagh, Co Tyrone, and the Bush at Portballintrae, Co Antrim, have been devastated in earlier killings of fish stocks caused by commercial pollution.Reuse content