An investigation is underway after a number of dead fish were reported at The Lough in Cork.
Initial investigations by Inland Fisheries Ireland indicate the cause of death to be a fish health issue with a bacterial or fungal infection suspected of causing the mortalities.
They estimate that in the region of 200 carp have been infected in this outbreak.
Anglers are requested to suspend all fishing activity at the lake until further notice.
Any anglers who have been fishing the venue in the last month are advised to carry out appropriate disinfection of their landing nets, fish mats, footwear and other gear that may have become contaminated to prevent the spread of the disease to other fisheries.
"A small number of live fish have been securely transported to a specialist fish health unit to identify the infectious agent. All dead fish that have been collected are being held in cold storage at an Inland Fisheries Ireland facility pending the outcome of the tests to determine the exact cause of death," the organisation said.
The results of tests on dead fish at The Lough are due back within days.
Members of the Carp Angling Association notified Cork City Council on Thursday. As a result, the council notified Inland Fisheries Ireland and the Marine Institute of Ireland. A spokesman for the council said the Marine Institute are carrying out tests on water samples, to establish if there is any contaminant in it which led to the deaths.
He added: "We expect the results in the middle of the week. The water levels are high and the water quality is good. We are waiting now for the experts to come back to us on it."
In 2009, several swans and ducks died at The Lough after an outbreak of botulism. Five carp also died in that outbreak.
The Lough is very popular with anglers and is stocked with carp by the Coarse Angling Club. Up to 2,000 carp can populate the lake at a time and the area is one of the most popular amenities in the country for carp fishing.