Dozens of fish have been found dead in Hillsborough Forest lake, however the Environment Agency has ruled out pollution as the cause.
The find comes just weeks after a dog became sick after swimming in the lake and visitors expressed concern about the algae on the surface.
Staff from the NIEA along with fisheries officers from the Department of Culture, Arts and Leisure, which is responsible for the lake, have been examining the area.
“Fish mortalities currently comprising hundreds of dead sticklebacks and a small number of trout have been confirmed, and more trout are under stress,” a NIEA spokesperson said.
At the lake, there are signs warning about the water quality as there is a naturally occurring toxic algae present that multiplies in warm weather. It can reduce oxygen levels in the water and may be one reason why the fish are dying.