Nearly two years following an unusual fish incident at Canada’s largest nuclear facility that posed challenges to reactor safety, newly accessed documents illustrate the remarkable nature of the occurrence — as specialists caution that atypical ecological conditions are increasingly problematic for nuclear facilities globally.
In February 2025, millions of gizzard shad inundated the cooling systems at Bruce Nuclear Generating Station located near Tiverton, Ontario, resulting in the shutdown of one reactor, a power reduction at another, and a prolonged presence of deceased fish along the shores of Lake Huron.
CBC News has secured over 1,200 pages of federal and provincial documents that detail how this environmental anomaly involving a small, cold-sensitive fish tested Canada’s largest nuclear facility in unexpected ways.
Bruce Power, the private entity managing the plant, declined to participate in a televised interview. The organization later issued a written statement but did not address specific inquiries from CBC News.
Dead fish ‘challenged reactor safety’
An investigation by Bruce Power determined that the plant faced “a novel environmental event.” Canada’s nuclear oversight body characterized the situation as an “uncharacterized and unanticipated volume of gizzard shad”, which overwhelmed the barriers intended to prevent fish from entering the plant’s cooling water system.
Gizzard shad congregating beneath the ice in the warmer waters of Lake Huron near Bruce Nuclear Generating Station. The cold-sensitive fish seek warm water spots during winter, including the thermal discharge from the plant. (Zack Melnick/Inspired Planet Productions)
Another investigation concluded in June 2025 that the two fish migrations 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 the term carries particular significance in an industry where plants are equipped with multiple layers of safeguards.
“When we refer to a challenge to reactor safety, it typically means that one of those safeguards has been less effective or taken offline. In such cases, nuclear power facilities 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 medical sciences professor at Northern Ontario School of Medicine University, who previously served as a principal scientist and manager at Bruce Power, noted that the reactors functioned as intended when their cooling systems were compromised.
“Perhaps the language in that document could have been adjusted to clarify that operations were affected, but I believe there was no risk to reactor safety,” he stated.
“There was no safety concern. The system is designed to respond this way. Operations are halted when conditions are not optimal.”
Nuclear regulator warned of worst-case scenario
An April 2026 report by the Canadian Nuclear Safety Commission (CNSC) prepared for the International Atomic Energy Agency outlines the potential repercussions.
The forebay at Bruce Power contains water sourced from Lake Huron before entering the plant’s cooling system. Millions of gizzard shad gathered around Bruce A in February 2025, with many becoming trapped and dying in the intake system. (Bruce Power)
On February 13, 2025, the massive die-off of cold-water fish disrupted the cooling systems for two active reactors. Bruce Power had to shut down Unit 2 and decrease output at Unit 1.
Just under two weeks prior, on February 1, the fish incident impacted cooling for Unit 4, which was already offline for refurbishment. Even when a reactor is shut down, its fuel continues generating heat and requires cooling.
In official regulatory correspondence sent to Bruce Power on May 5, 2025, the CNSC cautioned that “the potential repercussions of this event could have resulted in all [four Bruce A] units being shut down.”
8.1 million fish in a single day
The magnitude of the fish occurrence was staggering.
In a March 2026 report to Fisheries and Oceans Canada, Bruce Power estimated that on a single day in February 2025, 8.1 million gizzard shad were present near Bruce A — a figure significantly higher than any previously disclosed counts.
A school of gizzard shad gathers in warm water near Bruce Nuclear Generating Station in Lake Huron, near Tiverton, Ont. (Inspired Planet)
The report indicated that harsh winter conditions, including heavy snowfall and a sharp decline in air temperature, caused water temperatures to approach freezing, resulting in cold shock among the fish. The surviving gizzard shad, a species ill-suited to cold water, flocked to the warmer water discharged by the nuclear facility.
Ryan Lauzon, a fisheries biologist with the nearby Saugeen Ojibway Nation, whose traditional territory encompasses the Bruce Peninsula and adjacent Lake Huron shores, co-authored a report on the 2025 fish mortality. He mentioned that warmer winters in the years leading up to 2025 — including record-low ice levels in 2024 — could shed light on how such a large population formed.
“It is a plausible hypothesis,” he said. “It may have allowed these fish to establish larger populations” before the colder conditions in 2025 rendered many fish susceptible.
A dead gizzard shad rests on a Lake Huron beach. The substantial number of deceased gizzard shad appearing onshore in 2025 raised concerns about the possible involvement of disease. (Lee Brintell/Facebook)
Federal documents 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 prior record of roughly 800,000 set in 1977.
The scale of the situation prompted Bruce Power to bring in 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 bodies were transported to a landfill.
Reactor shutdown ‘a big hit to the grid’
Novog indicated that shutting down a reactor involves more than just clearing away the dead fish and turning a switch back on. Restarting a reactor in Canada requires a minimum of three to four days.
An aerial view of the Bruce Power site near Tiverton, Ont. (Submitted by Chad Richards)
“This is not a trivial matter. Each nuclear facility generates hundreds of megawatts of electricity every hour, so losing them for multiple days is a significant blow to the grid,” Novog remarked.
Internal provincial documents acquired by CBC News indicated that Ontario’s Independent Electricity System Operator reported a “tight” electricity supply while Unit 2 was offline. It evaluated a “medium risk” that another Bruce reactor could go offline, but noted that the grid could stay reliable through electricity imports and other local generation.
Nuclear power’s climate paradox
The incident at Bruce Power is not the only recent example of environmental factors disrupting nuclear plant cooling systems.
The Cernavoda Nuclear Power Plant in Romania, visible here next to the Danube-Black Sea channel on September 3, 2025. Its Candu reactors rely on river water for cooling. In July 2026, severe drought caused the Danube to drop to unprecedented low levels, leading to the shutdown of Unit 1 and later Unit 2 as a precaution. (Daniel Mihailescu/AFP via Getty Images)
Novog has observed that such occurrences have increased throughout his career, referencing incidents that disrupted nuclear cooling systems in Romania and France.
In the summer of 2026, reactors were taken offline due to record-low water levels in the Danube River in Romania. At France’s Gravelines nuclear facility, massive jellyfish invasions clogged cooling systems for two consecutive summers, necessitating reactor shutdowns in 2025 and 2026.
Nuclear facilities are compelled to adapt to a changing climate, even as governments increasingly depend on them to aid in combating it.
“It appears to me that these types of events have been on the rise over time,” Novog remarked.
‘Organizational failure’ to adapt
Bruce Power’s investigation identified insufficient real-time environmental monitoring and what it termed an “organizational failure” to recognize emerging issues linked to shifting environmental conditions and to implement design modifications to safeguard the plant from cooling water obstructions.
A few jellyfish on a beach near the Gravelines nuclear plant in northern France. In August, a large influx of jellyfish clogged the filters at the plant’s seawater pumping stations, resulting in the automatic shutdown of four reactors. (Reuters)
Since the incident, Bruce Power has indicated that it has invested nearly $10 million in mitigation strategies and research aimed at averting future mass fish occurrences from disrupting operations.
In a statement received after the deadline for CBC News, Bruce Power noted that it has enhanced its defenses since the fish incident and is preparing for emerging risks linked to climate change, recognizing that “we cannot foresee every future event.”
These measures include new physical barriers to prevent fish from entering the cooling-water intakes, underwater acoustic deterrents, sonar and video surveillance, along with an artificial intelligence system designed to provide advance warnings when large groups of fish approach.
Novog emphasized that preventing such disturbances will become increasingly critical as nuclear plants are relied upon to provide low-carbon electricity.
“The ideal solution is to prevent fish from gathering there, allowing the units to operate continuously, and that is why I believe you will see more emphasis on mitigation and prevention measures at Bruce and other facilities worldwide in the coming years.”