Almost two years after an unusual fish incident at Canada’s largest nuclear facility challenged reactor safety, recently acquired documents shed light on the extraordinary nature of the event. Experts caution that atypical ecological circumstances are posing an increasing challenge for nuclear plants globally.
In February 2025, a massive influx of gizzard shad overwhelmed the cooling systems at Bruce Nuclear Generating Station in Tiverton, Ont., causing the shutdown of one reactor, a power reduction in another, and leading to dead fish littering the shores of Lake Huron for several days.
Through the examination of over 1,200 pages of federal and provincial records, CBC News revealed how a unique environmental phenomenon involving a small, cold-sensitive fish tested the capabilities of Canada’s largest nuclear power plant in unexpected ways.
Despite Bruce Power declining a broadcast interview, the private company operating the plant later issued a written statement but did not address specific queries from CBC News.
Bruce Power’s internal probe determined that the plant faced “a novel environmental event.” The Canadian nuclear regulator described an “uncharacterized and unanticipated volume of gizzard shad” that overwhelmed the defenses designed to prevent fish from entering the plant’s cooling water system.
Another investigation by the company in June 2025 concluded that the fish incidents on February 1 and February 13 “challenged reactor safety.”
David Novog, a professor at McMaster University specializing in Nuclear Safety, explained that a challenge to reactor safety implies a reduction in the effectiveness of one of the plant’s defense layers, necessitating a shutdown as a precautionary measure.
Douglas Boreham, a former principal scientist and manager at Bruce Power and currently a professor at Northern Ontario School of Medicine University, stated that the reactors responded as intended when their cooling systems were disrupted, emphasizing that there was no safety issue.
The potential repercussions were outlined in an April 2026 report by the Canadian Nuclear Safety Commission (CNSC) prepared for the International Atomic Energy Agency, detailing how the fish die-off disrupted cooling systems serving two operational reactors.
Bruce Power estimated that a staggering 8.1 million gizzard shad were present near Bruce A on a single day in February 2025. The fish congregated in the warmer waters discharged by the nuclear plant due to severe winter conditions causing cold shock among the species poorly adapted to cold water.
Ryan Lauzon, a fisheries biologist from Saugeen Ojibway Nation, co-authored a report on the 2025 fish kill, attributing the large fish population to warmer winters preceding 2025.
The investigation at Bruce Power identified gaps in real-time environmental monitoring and an “organizational failure” to recognize emerging issues associated with changing environmental conditions and implement design changes to safeguard the plant from cooling water blockages.
Since the incident, Bruce Power has invested nearly $10 million in mitigation measures and research to prevent similar fish events from disrupting the plant in the future. These measures include physical barriers, acoustic deterrents, monitoring systems, and artificial intelligence for early fish detection.
Experts emphasize the importance of implementing preventive measures to avoid disruptions as nuclear plants play a crucial role in providing low-carbon electricity amid the challenges posed by changing environmental conditions.
The incident at Bruce Power is part of a growing trend where environmental conditions are impacting the cooling systems of nuclear plants worldwide, underscoring the need for plants to adapt to a changing climate while continuing to support efforts in combating climate change.
