It is feared an algal bloom may have killed thousands of fish in County Cork.
The fish were found on Monday evening by an angler on a minor tributary of the Bandon River, near Curraghalicky Lake.
Inland Fisheries Ireland (IFI) has been investigating and believes the fish kill was caused by significant algal bloom on the lake was the cause of the fish kill.
The agency said the number of dead fish is in the hundreds and possibly thousands.
Cork County Council has also taken water samples from the lake and river.
IFI spokesperson Sean Lambe said it appears the algal bloom was the main cause.
“There is no source of any pollution,” he said. “There is nothing agricultural, as in slurry or anything like, that in the water
“The only thing that we can take from it is that the algae bloom, which is very evident in the lake and which flowed into the stream, was the cause of the fish kill.
The species of fish worst affected were trout, juvenile salmon and bream.
“We carried out an aerial survey of the lake and we put a boat out there as well,” said Mr Lambe.
“We found mainly dead bream on the actual lake itself.
“In the river coming out of the lake for about a kilometre and a half from the lake itself, we found dead brown trout and some juvenile salmon.
“The main casualties have been bream.”
Algae are microscopic marine plants that can prosper when there are too many nutrients – typically nitrogen and phosphorous – in the water.
Large growths of algae are known as algal blooms. Because they can reduce or even eliminate the oxygen in the water, they can result in fish kills.
They are typically green, yellow-brown, red or blue-green in colour and can sometimes be harmful to humans.
They are commonly caused by runoff from agriculture and discharges from storm sewers or wastewater treatment plants.