Millions of fish tested Canada's largest nuclear plant.
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Millions → test → plant
The incident points to a growing climate risk
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.
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conditions → point → 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.
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millions → overwhelm → 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.
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operators → obtain → ways
Bruce Power, the private company that operates the plant, declined a broadcast interview.
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that → operate → interview
The company later provided a written statement, but did not answer CBC News's specific questions.
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company → provide → questions
Dead fish 'challenged reactor safety'
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fish → challenge → safety
Bruce Power's own root-cause investigation concluded the plant had encountered "a novel environmental event."
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plant → conclude → 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.
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regulator → describe → system
Another company investigation concluded in June 2025 that the two fish runs, on Feb. 1 and Feb. 13, "challenged reactor safety."
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runs → conclude → 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.
"
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plants → say → 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.
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one → talk → effectiveness
And when that happens, nuclear power stations are required to shut down," he said.
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he → happen → ?
Douglas Boreham, former principal scientist and manager at Bruce Power
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.
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systems → work → Power
"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.
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he → modify → opinion
I mean, it's designed to do that.
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it → mean → that
You shut things down when the system isn't optimal.
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system → shut → things
"
Nuclear regulator warned of worst-case scenario
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.
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consequences → warn → Agency
On Feb. 13, 2025, the mass cold-water fish die-off disrupted cooling systems serving two operating reactors.
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off → disrupt → reactors
Bruce Power was forced to shut down Unit 2 and reduce power at Unit 1.
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Power → force → Unit
Less than two weeks earlier, on Feb. 1, the fish event affected cooling at Unit 4, which was already shut down for refurbishment.
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which → affect → refurbishment
Even after a reactor is shut down, its fuel continues producing heat and must be kept cool.
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fuel → shut → heat
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."
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units → send → event
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.
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shad → estimate → 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.
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conditions → say → fish
The surviving gizzard shad, a species poorly adapted to cold water, congregated around the warmer water discharged by the nuclear plant.
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shad → survive → 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.
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territory → include → 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.
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population → say → 2024
"It is a reasonable hypothesis," he said.
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he → say → ?
"It potentially allowed these fish to develop larger populations" before colder conditions in 2025 left large numbers of fish vulnerable.
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conditions → allow → fish
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.
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shad → estimate → 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.
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Power → bring → Environment
The carcasses were hauled to a landfill.
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carcasses → haul → landfill
Novog said a reactor shutdown isn't simply a matter of clearing out the dead fish and flicking a switch.
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shutdown → say → switch
Restarting a reactor in Canada would take at least three or four days.
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Restarting → restart → days
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.
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Novog → generate → grid
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.
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Unit → obtain → News
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.
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grid → assess → imports
Nuclear power's climate paradox
What happened at Bruce Power is not the only recent case of environmental conditions disrupting the cooling systems nuclear plants depend on.
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plants → happen → systems
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.
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that → say → Romania
…and 10 more, not listed.
The Aporia
