Angling groups have called for an urgent dredging of the River Shannon after the discovery of 26 million Asian Clams at the top of Lough Ree.
Inland Fisheries Ireland confirmed that the shellfish (Corbicula fluminea) are spawning at an alarming rate in a stretch of hot water being discharged from the ESB Lough Ree power plant at Lanesborough, Co Longford.
The clams live on the same food stuff as fish and have the ability to wipe out coarse fish stocks.
A significant number of the clams have also been found in a navigation channel at the top of Lough Ree, which is one of Ireland's best known coarse fishing lakes.
Brian Tabiner of the Lanesborough Pike Angling Club described the situation as extremely urgent.
He said Inland Fisheries Ireland has done an excellent job in identifying the problem but unless dredging was carried out within a very short period of time the stretch of river where the clams are would be flooded for the winter.
Mr Tabiner said spawning would multiply their number into many more millions, which would threaten the future of the fishery before next spring.