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.”
The potential consequences are laid out in an April 2026 report by the Canadian Nuclear Safety Commission (CNSC) prepared for the International Atomic Energy Agency.
On Feb. 13, 2025, the mass cold-water fish die-off disrupted cooling systems serving two operating reactors. Bruce Power was forced to shut down Unit 2 and reduce power at Unit 1.
Less than two weeks earlier, on Feb. 1, the fish event affected cooling at Unit 4, which was already shut down for refurbishment. Even after a reactor is shut down, its fuel continues producing heat and must be kept cool.
In formal regulatory correspondence sent to Bruce Power on May 5, 2025, the CNSC warned that “the potential consequences of this event could have resulted in all [four Bruce A] units being shutdown.”
The scale of the fish event was enormous.
In a March 2026 report to Fisheries and Oceans Canada, Bruce Power estimated that 8.1 million gizzard shad were in the vicinity of Bruce A on a single day in February 2025 — far exceeding any counts made public at the time.
The report said severe winter conditions, including significant snowfall and a sharp drop in air temperature, pushed water temperatures close to freezing, causing cold shock among the fish. The surviving gizzard shad, a species poorly adapted to cold water, congregated around the warmer water discharged by the nuclear plant.
Ryan Lauzon, a fisheries biologist with nearby Saugeen Ojibway Nation, whose traditional territory includes the Bruce Peninsula and surrounding Lake Huron shoreline, co-authored a report on the 2025 fish kill. He said warmer winters in the years leading up to 2025 — including record-low ice levels in 2024 — may help explain how such a large population developed.
“It is a reasonable hypothesis,” he said. “It potentially allowed these fish to develop larger populations” before colder conditions in 2025 left large numbers of fish vulnerable.
Federal records estimate between 3.8 million and 4.95 million gizzard shad became trapped and died at Bruce A between Jan. 30 and March 26, 2025 — several times the previous record of roughly 800,000 set in 1977.
The number was so enormous Bruce Power brought in a crane ito help remove them, according to emails obtained by CBC News from Ontario’s Ministry of the Environment, Conservation and Parks. The carcasses were hauled to a landfill.
Novog said a reactor shutdown isn’t simply a matter of clearing out the dead fish and flicking a switch. Restarting a reactor in Canada would take at least three or four days.
“That’s not trivial. Each nuclear station is generating hundreds of megawatts of electricity every hour of the day, so to lose them for multiple days even, is a big hit to the grid,” Novog said.
Internal provincial records obtained by CBC News show Ontario’s Independent Electricity System Operator said the province’s electricity supply was “tight” while Unit 2 remained offline. It assessed a “medium risk” that another Bruce reactor could go offline, but said the grid could remain reliable using electricity imports and other domestic generation.
What happened at Bruce Power is not the only recent case of environmental conditions disrupting the cooling systems nuclear plants depend on.
Novog said he has seen such events become more common over the course of his career, pointing to events that disrupted nuclear cooling systems in Romania and France.
In the summer of 2026, reactors were forced offline by record-low water levels on the Danube River in Romania. At France’s Gravelines nuclear plant, massive influxes of jellyfish clogged cooling systems for two consecutive summers, forcing reactors offline in 2025 and 2026.
Nuclear plants are being forced to adapt to a changing climate, even as governments increasingly rely on them to help fight it.
“It just appears to me that over time, these kind of events have been increasing,” Novog said.
Bruce Power’s investigation identified inadequate real-time environmental monitoring and what it called an “organizational failure” to recognize emerging problems associated with changing environmental conditions and make design changes to protect the plant from cooling water blockages.
Since the event, Bruce Power has said it spent nearly $10 million on mitigation measures and research aimed at preventing another mass fish event from disrupting the plant.
In a statement received after CBC News’ deadline, Bruce Power said it has strengthened its defences since the fish event and is preparing for emerging risks associated with climate change, acknowledging that “we cannot anticipate every future event.”
The measures include new physical barriers to keep fish away from the cooling-water intakes, underwater acoustic deterrents, sonar and video monitoring and an artificial intelligence system intended to provide earlier warning when large numbers of fish approach.
Novog said preventing those disruptions will become increasingly important as nuclear plants are relied upon to provide low-carbon electricity.
“The best solution is to have no fish congregate there and keep the units running and so that’s why I think over the years to come, you’ll see more on mitigation and prevention measures at Bruce and other stations around the world.”
