Newly uncovered documents shed light on an extraordinary fish event that disrupted operations at Canada’s largest nuclear power plant almost two years ago. In February 2025, millions of gizzard shad overwhelmed the cooling systems at the Bruce Nuclear Generating Station in Ontario, leading to the shutdown of one reactor and reduced power at another. The incident highlighted the challenges posed by unusual ecological conditions for nuclear plants worldwide.
Revealed through over 1,200 pages of federal and provincial records obtained by CBC News, the environmental phenomenon involving the cold-sensitive gizzard shad tested the capabilities of the plant in unforeseen ways. Bruce Power, the company operating the plant, declined a broadcast interview but later issued a written statement without addressing specific queries from CBC News.
Following its own investigation, Bruce Power concluded that the plant faced “a novel environmental event.” The nuclear regulator in Canada described the event as an “uncharacterized and unanticipated volume of gizzard shad,” which overwhelmed the plant’s defense mechanisms designed to prevent fish from entering the cooling water system.
A separate company investigation in June 2025 determined that the fish influx on two occasions had “challenged reactor safety.” Experts like David Novog, a professor specializing in nuclear safety, explained that such challenges could compromise the effectiveness of the safety protocols, leading to mandatory shutdowns of nuclear power stations.
Despite concerns raised by the incident, former principal scientist and manager at Bruce Power, Douglas Boreham, reassured that the reactors responded appropriately to the cooling system disruptions, ensuring there was no risk to reactor safety. He emphasized that such shutdowns were a part of the plant’s design to maintain optimal operations.
The Canadian Nuclear Safety Commission (CNSC) highlighted potential consequences of the event in an April 2026 report, indicating how the mass die-off of cold-water fish impacted the cooling systems serving the reactors, resulting in shutdowns and power reductions.
The magnitude of the fish event was staggering, with an estimated 8.1 million gizzard shad present near the plant on a single day in February 2025. Severe winter conditions and ecological factors led to the congregation of fish seeking warmer waters discharged by the nuclear plant, causing a significant portion to perish.
The incident underscored the need for nuclear plants to adapt to changing environmental conditions globally. Novog pointed out similar events in other countries, such as Romania and France, where environmental disruptions affected cooling systems, necessitating reactor shutdowns.
An investigation by Bruce Power post-event identified deficiencies in environmental monitoring and an “organizational failure” to address emerging challenges associated with changing environmental conditions. Subsequent mitigation efforts included investing nearly $10 million in preventive measures and research to avert future disruptions caused by mass fish events.
Enhancements in the plant’s defenses post-incident include physical barriers, acoustic deterrents, monitoring systems, and AI technology to detect and prevent large fish congregations near cooling-water intakes. These measures aim to ensure uninterrupted operations as nuclear plants play a critical role in providing low-carbon electricity amidst the challenges posed by climate change.