Nearly two years following an unusual fish incident at Canada’s largest nuclear power facility that “challenged reactor safety,” newly acquired documents illustrate just how remarkable the occurrence was—while experts caution that atypical ecological situations are increasingly posing risks to nuclear power plants globally.
In February 2025, an overwhelming number of gizzard shad inundated the cooling systems at the Bruce Nuclear Generating Station located near Tiverton, Ontario, resulting in the shutdown of one reactor, a reduction in power at another, and dead fish washing up on the shores of Lake Huron for several days.
CBC News has obtained over 1,200 pages of federal and provincial documents that detail how an environmental phenomenon involving this small, cold-sensitive fish tested Canada’s largest nuclear facility in ways its operators had not anticipated.
Bruce Power, the private entity responsible for running the plant, declined to participate in a broadcast interview. While the company later issued a written statement, it did not address the specific inquiries posed by CBC News.
Dead fish ‘challenged reactor safety’
Bruce Power’s internal root-cause analysis found that the plant experienced “a novel environmental event.” Canada’s nuclear regulatory body characterized it as an “uncharacterized and unanticipated volume of gizzard shad”—sufficient to overwhelm the defenses intended to keep fish out of the cooling water system.
Gizzard shad gather beneath the ice in the warmer waters of Lake Huron by the millions near Bruce Nuclear Generating Station. The cold-sensitive fish seek out warm water refuges in winter, including the thermal discharge from the plant. (Zack Melnick/Inspired Planet Productions)
Another investigation conducted by the company concluded in June 2025 that the two significant fish runs, which occurred on February 1 and February 13, “challenged reactor safety.”
David Novog, a professor at McMaster University and Canada Research Chair in Nuclear Safety, explained that this phrase carries a specific connotation in an industry where plants are safeguarded by several layers of defense.
“When we refer to a challenge to reactor safety, it usually indicates that one of those defenses has either diminished in effectiveness or has been taken offline. In such cases, nuclear power stations must shut down,” he stated.
There was no risk to reactor safety, in my opinion.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 served as a principal scientist and manager at Bruce Power, remarked that the reactors functioned as intended when their cooling systems faced disruption.
“Perhaps the language in that document should have been slightly adjusted to indicate that operations were impacted, but I believe there was no risk to reactor safety,” he noted.
“There was no safety concern. It is designed to respond this way; operations cease when the system is not functioning optimally.”
Nuclear regulator alerted to worst-case scenario
The potential implications are outlined in an April 2026 report by the Canadian Nuclear Safety Commission (CNSC) prepared for the International Atomic Energy Agency.
The forebay at Bruce Power holds water drawn from Lake Huron before it enters the plant’s cooling water system. Millions of gizzard shad congregated around Bruce A in February of 2025, with large numbers that became trapped and died in the intake system. (Bruce Power)
On February 13, 2025, the massive die-off of cold-water fish disrupted the cooling systems for two operational reactors. Bruce Power had to cease operations at Unit 2 and lower output at Unit 1.
Just under two weeks prior, on February 1, the fish event also impacted cooling at Unit 4, which was already offline for refurbishment. Even when a reactor is offline, its fuel continues generating heat and requires cooling.
In formal regulatory correspondence sent to Bruce Power on May 5, 2025, the CNSC cautioned that “the potential consequences of this event could have led to all [four Bruce A] units being shut down.”
8.1 million fish in a single day
The magnitude of the fish incident was staggering.
In a report issued in March 2026 to Fisheries and Oceans Canada, Bruce Power estimated that 8.1 million gizzard shad were present near Bruce A on one day in February 2025—far surpassing any previously released estimations.
A school of gizzard shad gathers in the warm water near Bruce Nuclear Generating Station in Lake Huron, near Tiverton, Ont. (Inspired Planet)
The report indicated that harsh winter conditions, featuring heavy snowfall and a sharp decline in air temperature, drove water temperatures near freezing, causing cold shock to the fish. The remaining gizzard shad, a species ill-suited to cold water, clustered around the warmer water expelled by the nuclear facility.
Ryan Lauzon, a fisheries biologist with the nearby Saugeen Ojibway Nation, whose traditional territory encompasses the Bruce Peninsula and parts of the Lake Huron shoreline, co-authored a report on the 2025 fish die-off. He mentioned that warmer winters preceding 2025—including unprecedentedly low ice levels in 2024—might explain the growth of such a large population.
“It is a plausible hypothesis,” he stated. “It may have allowed these fish to form larger populations” before the colder conditions in 2025 exposed vast numbers of fish to risk.
A dead gizzard shad lies on a Lake Huron beach. Large numbers of dead gizzard shad washing ashore in 2025 raised concerns about whether disease could be involved. (Lee Brintell/Facebook)
Federal records estimate that between 3.8 million and 4.95 million gizzard shad became trapped and perished at Bruce A from January 30 to March 26, 2025—several times the previous record of approximately 800,000 established in 1977.
The numbers were so significant that Bruce Power employed a crane to assist in their removal, according to emails obtained by CBC News from Ontario’s Ministry of the Environment, Conservation and Parks. The remains were disposed of at a landfill.
Reactor shutdown ‘a big hit to the grid’
Novog pointed out that a reactor shutdown is not merely a matter of clearing away the dead fish and flipping a switch. Restarting a reactor in Canada typically requires at least three to four days.
An aerial image of the Bruce Power site located near Tiverton, Ont. (Submitted by Chad Richards)
“That’s not trivial. Each nuclear facility generates hundreds of megawatts of electricity every hour, so losing them for several days is a significant blow to the grid,” Novog explained.
Internal documents acquired by CBC News reveal that Ontario’s Independent Electricity System Operator indicated the province’s electricity supply was “tight” while Unit 2 remained offline. It assessed a “medium risk” that another Bruce reactor might also go offline, but noted that the grid could still maintain reliability through electricity imports and alternative domestic generation.
Nuclear power’s climate paradox
The situation at Bruce Power is not the only recent instance of environmental conditions hindering the cooling systems that nuclear plants rely upon.
The Cernavoda Nuclear Power Plant in Romania, seen here next to Danube-Black Sea channel on September 3, 2025. Its Candu reactors depend on water from the river for cooling. In July 2026, severe drought brought the Danube to unprecedentedly low levels, forcing the plant to shutdown Unit 1 and later Unit 2 as a precaution. (Daniel Mihailescu/AFP via Getty Images)
Novog noted that he has witnessed such occurrences becoming more frequent throughout his career, referencing events that disrupted nuclear cooling systems in Romania and France.
In the summer of 2026, reactors were taken offline due to record-low water levels on the Danube River in Romania. At France’s Gravelines nuclear facility, significant jellyfish influxes clogged cooling systems for two consecutive summers, necessitating reactor shutdowns in both 2025 and 2026.
Nuclear facilities must adapt to a shifting climate, even as governments increasingly depend on them to combat climate change.
“It appears to me that over time, such events have been on the rise,” Novog concluded.
‘Organizational failure’ to adapt
Bruce Power’s investigation identified insufficient real-time environmental monitoring and what it termed an “organizational failure” to acknowledge emerging issues tied to changing environmental conditions, as well as the need for design modifications to prevent cooling water blockages.
A few jellyfish lie on a beach near the Gravelines nuclear plant in northern France. In August, a massive influx of jellyfish clogged filters in the plant’s seawater pumping stations, automatically shutting down four reactors. (Reuters)
Since the incident, Bruce Power has reported spending nearly $10 million on mitigation strategies and research aimed at preventing another mass fish occurrence from affecting the plant.
In a statement received after the CBC News deadline, Bruce Power indicated that it has bolstered its defenses since the fish event and is preparing for new risks linked to climate change, acknowledging that “we cannot predict every future event.”
The initiatives include new physical barriers to deter fish from entering the cooling-water intakes, underwater acoustic deterrents, sonar and video monitoring systems, as well as an artificial intelligence platform designed to provide earlier warnings when large numbers of fish approach.
Novog emphasized that preventing such disruptions will be increasingly critical as nuclear plants are relied upon for low-carbon electricity generation.
“The optimal solution is to prevent fish from gathering in those areas, allowing the units to operate continuously, which is why I believe we will see greater focus on mitigation and preventive actions at Bruce and other facilities worldwide in the years ahead.”