City cleanup crews were working Saturday to remove hundreds of dead fish found floating in a pond in Toronto’s High Park after what is being described by conservation officials as a die-off.

Residents who frequent the popular park began to notice the dead fish along the shore of Grenadier Pond last week.

According to the Ontario Ministry of Natural Resources the die-off happened on July 28. Ninety-five per cent of the dead fish are sunfish.

Lenka Holubec, a member of a volunteer-led park committee, told CP24 she first noticed a few dead fish on Monday. By Tuesday, the numbers of dead fish along the shoreline had increased, she said, and the resulting stench was overwhelming. Holubec added she’s never seen such a large die-off at Grenadier Pond.

“Some small die-offs have occurred but nothing to this kind of magnitude.”

Officials from Natural Resources and the Ministry of Environment are trying to determine what caused the die-off, and have collected fish and water samples for testing. Investigators will look at water quality and weather as possible contributing factors.

With a report from CP24’s Jackie Crandles