Almost two years following an unusual fish occurrence at Canada’s biggest nuclear power facility that posed a safety challenge to its reactors, recently obtained documents reveal the exceptional nature of the incident. Experts caution that abnormal ecological circumstances are increasingly posing a threat to nuclear plants globally.
In February 2025, an inundation of gizzard shad fish overwhelmed the cooling systems at Bruce Nuclear Generating Station near Tiverton, Ont., resulting in the shutdown of one reactor, decreased power output from another, and the presence of dead fish littering Lake Huron shores for several days.
CBC News obtained over 1,200 pages of federal and provincial records shedding light on how an environmental phenomenon involving a small, cold-sensitive fish tested the capabilities of Canada’s largest nuclear power plant in unforeseen ways.
Bruce Power, the company managing the plant, declined a broadcast interview and later issued a written statement without addressing specific inquiries from CBC News.
Bruce Power’s internal investigation determined the plant had faced “a novel environmental event.” The Canadian nuclear regulator described an unprecedented quantity of gizzard shad fish overwhelming the plant’s defense mechanisms designed to keep fish out of the cooling water system.
Another investigation by the company in June 2025 concluded that the two fish surges on Feb. 1 and Feb. 13 had a significant impact on reactor safety.
David Novog, a nuclear safety expert from McMaster University, explained that a challenge to reactor safety signifies a reduction in the effectiveness of defense mechanisms, prompting nuclear power stations to shut down.
Former principal scientist and manager at Bruce Power, Douglas Boreham, reassured that the reactors responded as intended when the cooling systems were disrupted, emphasizing that there was no threat to reactor safety.
A report by the Canadian Nuclear Safety Commission warned in April 2026 that the fish die-off incident could have led to the shutdown of all four units at Bruce A.
The scale of the fish event was immense, with Bruce Power estimating that 8.1 million gizzard shad were in the vicinity of Bruce A on a single day in February 2025, surpassing previous public counts.
The report attributed the large fish population to warmer winters leading up to 2025, allowing the fish to thrive before frigid conditions in 2025 made them susceptible to cold shock.
Federal estimates indicated that millions of gizzard shad became trapped and died at Bruce A between Jan. 30 and March 26, 2025, necessitating the use of a crane to remove the carcasses.
Restarting a reactor in Canada after a shutdown requires several days, impacting the electricity grid significantly, according to Novog.
Instances of environmental conditions disrupting cooling systems at nuclear plants have been observed in other countries as well, as plants adapt to a changing climate while being relied upon for low-carbon electricity.
An investigation by Bruce Power identified shortcomings in environmental monitoring and an “organizational failure” to address emerging issues related to changing environmental conditions. The company has since invested nearly $10 million in mitigation measures and research to prevent future fish-related disruptions.
Enhancements include physical barriers, acoustic deterrents, monitoring systems, and AI technology to detect and deter large fish gatherings near the plant’s cooling-water intakes.
Experts emphasize the importance of implementing preventive measures to avoid disruptions at nuclear plants in the future, especially as they play a crucial role in providing clean energy amidst climate challenges.