“This is not happening in a lake, it’s happening in a fast-flowing river,” Nadeau said. “These fish die and then get carried down by the current so if there is a point source like a chemical spill you need to get there as fast as you can.”

The first dead fish were found by local residents and fishing guides in the Ottawa River near the small hamlet of Cumberland. Further investigation determined the fish were coming from the Lievre River, which flows into the Ottawa River east of the city. The first incident was recorded July 8, with three more on July 19, July 29 and July 31.