Millions of fish tested Canada's largest nuclear plant. The incident points to a growing climate risk

In February 2025, millions of gizzard shad overwhelmed cooling systems at Bruce Nuclear Generating Station near Tiverton, Ont., shutting down one reactor, reducing power at another and leaving dead fish washing ashore along Lake Huron for days.

CBC News obtained more than 1,200 pages of federal and provincial records that reveal how an environmental phenomenon involving a small, cold-sensitive fish tested Canada's largest nuclear power plant in ways its operators had not anticipated.

Bruce Power, the private company that operates the plant, declined a broadcast interview. The company later provided a written statement, but did not answer CBC News's specific questions.

Bruce Power's own root-cause investigation concluded the plant had encountered "a novel environmental event." Canada's nuclear regulator described an "uncharacterized and unanticipated volume of gizzard shad" — enough to overwhelm defences meant to keep fish out of the plant's cooling water system.