Around 1,000 fish in Dungannon Park Lake have died over the past few days.
The “fish kill” however has affected only roach stocks.
“Other coarse fish in the lake remain healthy and unaffected,” said Mid Ulster council. “Tests are being undertaken to try and determine the cause, which appears to be isolated to a particular species.
“Purely as a precaution, fishing is not taking place at the lake at the moment.
“We estimate around 1,000 roach have died, but this is out of tens of thousands of coarse fish in the lake.”