The Shannon Regional Fisheries Board has confirmed that it is investigating a significant fish kill, mainly of trout and young salmon, on the campus grounds of the University of Limerick.
Up to 600 fish have been found dead in the Plassy Millrace which flows into the Shannon on the southern bank of the campus.
The kill was reported to the fisheries board by local anglers last weekend, who noticed dead fish on the water.
The majority were trout, but they also included salmon parr, as the young fish are known, and sea lamprey, both of which are protected species.
A spokesman for the fisheries board said yesterday that a suspected source of the pollution had been identified, and that legal proceedings could follow.
"We have a definite line of inquiry in our investigations, and we are working to make sure that this type of incident doesn't happen again.
"It is particularly sad when this happens, but when it does we have to take whatever action is necessary," said the fisheries board spokesman.