Around 1,000 fish in Dungannon Park Lake died over the past few days, according to a spokesperson for Mid Ulster council.
The “fish kill” however, has affected only Roach stocks.
“Other coarse fish and game fish in the lake remain healthy and unaffected,” they said.
“Water sampling has taken place and toxicology tests are being undertaken to try and determine the cause, which appears to be isolated to a particular species.
“The council is working with DARD’s Fish Disease Unit and 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.”
Local fisherman Tommy Conlon and lake user has called for an external investigation into why the fish are dying.
It seems that as well as the dead Roach, the lake’s Trout have also started to die.