An enormous fish die-off tested Canada's largest nuclear plant. A changing climate could test many more | CBC News

Newly obtained records reveal the 2025 event 'challenged reactor safety' when fish swamped plant defences

Almost two years after a freak fish event at Canada's largest nuclear power plant "challenged reactor safety," newly obtained documents show how extraordinary the event was — as experts warn unusual ecological conditions are becoming a growing challenge for nuclear plants around the world.

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.

Another company investigation concluded in June 2025 that the two fish runs, on Feb. 1 and Feb. 13, "challenged reactor safety."

David Novog, a professor at McMaster University and Canada Research Chair in Nuclear Safety, said the phrase has a specific meaning in an industry where plants are protected by multiple layers of defence.

"When we talk about a challenge to reactor safety, what it usually implies is that one of those defences has been reduced in effectiveness or taken offline. And when that happens, nuclear power stations are required to shut down," he said.

Douglas Boreham, a professor of medical sciences at Northern Ontario School of Medicine University who previously worked as a principal scientist and manager at Bruce Power, said the reactors responded as designed when their cooling systems were disrupted.

"Maybe the wording in that document should have been slightly modified to say it was operations that were impacted, but there was no risk to reactor safety, in my opinion," he said.

"There was no safety issue. I mean, it's designed to do that. You shut things down when the system isn't optimal."