Newly obtained documents have revealed the extraordinary nature of a fish event that disrupted operations at Canada’s largest nuclear power plant almost two years ago, posing new challenges for nuclear facilities globally. In February 2025, millions of gizzard shad overwhelmed the cooling systems at Bruce Nuclear Generating Station near Tiverton, Ont., leading to the shutdown of one reactor and reduced power at another. The event left dead fish along Lake Huron’s shores for several days.
The obtained federal and provincial records, totaling over 1,200 pages, shed light on how this unique environmental phenomenon involving a cold-sensitive fish tested the plant in unforeseen ways. Bruce Power, the plant operator, declined a broadcast interview but later issued a written statement without addressing specific questions from CBC News.
An investigation by Bruce Power determined that the plant faced “a novel environmental event,” while the Canadian nuclear regulator described an unexpected influx of gizzard shad that overwhelmed the plant’s fish deterrent systems. Another company report in June 2025 highlighted that the fish runs in February had “challenged reactor safety,” according to David Novog, a professor at McMaster University specializing in nuclear safety.
In a report prepared for the International Atomic Energy Agency in April 2026, the Canadian Nuclear Safety Commission highlighted the disruptions caused by a mass die-off of cold-water fish on Feb. 13, 2025, leading to the shutdown of Unit 2 and reduced power at Unit 1. The report warned of the potential consequences, including a scenario where all four units of Bruce A could be shut down.
The scale of the fish event was massive, with Bruce Power estimating that 8.1 million gizzard shad were present near the plant on a single day in February 2025. The severe winter conditions and warmer waters discharged by the plant attracted the fish, resulting in a significant die-off. The event raised concerns about the plant’s operational impact and the environmental implications of such incidents.
Following the disruption, Bruce Power initiated mitigation measures and invested nearly $10 million in research to prevent similar incidents in the future. The company strengthened its defenses, incorporating physical barriers, underwater deterrents, monitoring technologies, and an artificial intelligence system to anticipate and prevent fish-related disruptions. These efforts underline the need for nuclear plants to adapt to changing environmental conditions and ensure uninterrupted low-carbon electricity generation.