Environmental fisheries officers have been at the river where a large number of dead fish have been found over a 4km stretch from Dunleer to the River Dee.
The fish include adult and juvenile salmon and trout, eels, minnow and stoneloach.
The investigation to locate the source of the pollution has been continuing since Friday last when a member of the public alerted the Inland Fisheries Ireland.
Last year the Dee and Glyde Fishing Development Association launched a new project at Dunleer to increase salmon spawning on the east coast of Irealnd.
Salmon stocks in the country are low and this project creates spawning beds using rock and gravel, oxygenating deflectors and removal of silt and weeds.
The project was launched by Minister of State Fergus O’Dowd at Dunleer.
Louth Leader have provided funding for the project. This was the third phase of the project and Louth Leader gave a further €26,000 euro towards it. The Dee and Glyde Fishing Development Association spent a number of years working on this project.