The virus does not pose a risk to human health and is not a food-safety concern.

"The disease is most often seen when water temperatures increase and fish are crowded especially during spawning," ministry spokesperson Jolanta Kowalski said in an email.

Residents living around the horseshoe-shaped millpond noticed juvenile carp floating dead at the shoreline in mid-August. In the days that followed, residents saw larger, more mature carp dying as well.

Mark Servos, a biology professor at the University of Waterloo who visited the lake during the die-off, said those observations fit the pattern for the virus.

"It affected the younger fish first and then the older fish, so it's the exact pattern for koi herpesvirus. It's exactly what you would expect," he said, adding there's very little that can be done about it.

Fish with the virus can have symptoms including discoloured skin; swollen, pale, and rotting gills; sunken eyes and a notch on the nose; excess mucus; appetite loss; and erratic swimming, according to the province.

The virus has been found in wild carp in a number of different lakes in south-central Ontario.