Millions of Fish Tested Canada’s Largest Nuclear Plant. The Incident Points to a Growing Climate Risk

In February 2025, an unprecedented event occurred at Canada’s largest nuclear plant, Bruce Nuclear Generating Station, located on the shores of Lake Huron. Millions of gizzard shad, a cold-sensitive fish species, overwhelmed the cooling systems, shutting down one reactor and reducing power at another.

The incident was not an isolated anomaly but rather a symptom of a broader trend: climate change’s unpredictable effects on nuclear power plants worldwide. As David Novog, a professor at McMaster University and Canada Research Chair in Nuclear Safety, noted, “a novel environmental event” tested the plant’s defenses in ways its operators had not anticipated.

The sheer scale of the fish die-off – estimated at between 3.8 million to 4.95 million gizzard shad – was staggering. The carcasses washed ashore along Lake Huron and required a crane to help remove them, highlighting the significant disruption caused by this event.

A reactor shutdown is not just a matter of clearing out dead fish; it can take at least three or four days to restart in Canada, resulting in substantial losses for the grid. Novog pointed out that each nuclear station generates hundreds of megawatts of electricity every hour of the day, making even temporary losses substantial.

The Canadian Nuclear Safety Commission issued a stark warning after the incident: “the potential consequences of this event could have resulted in all [four Bruce A] units being shutdown.” This was not an empty threat; a shutdown would have left millions without power and stretched the grid to its limits. The province’s Independent Electricity System Operator assessed a “medium risk” that another reactor could go offline, emphasizing the need for contingency planning.

The incident at Bruce Nuclear Generating Station serves as a wake-up call for the nuclear industry. Climate change is altering ecosystems in unpredictable ways, posing new challenges for power plants that rely on stable environmental conditions. The warmer winters leading up to 2025 may have contributed to the massive gizzard shad population development, making it more vulnerable to cold shock when temperatures plummeted.

The paradox of nuclear power is clear: while it provides a relatively low-carbon source of energy, its operation relies on predictability and stability in environmental conditions. As climate change continues to reshape ecosystems, nuclear plants will face increasing pressure to adapt or risk catastrophic failures.

To mitigate these risks, the industry must invest in research and development to better understand and adapt to unpredictable environmental conditions. This includes developing more robust cooling systems, enhancing emergency preparedness, and exploring new technologies that can withstand changing ecological conditions.

The consequences of inaction are dire. A single nuclear plant shutdown can send shockwaves through the grid, leaving millions without power. The costs are not just economic; they are also environmental, as backup systems required to compensate for lost capacity can have a significant carbon footprint.

Policymakers and industry leaders must recognize the warning signs and take action before it’s too late. The fish in the water at Bruce Nuclear Generating Station was not an isolated incident but a harbinger of a new reality: climate change’s unpredictable effects on nuclear power. We must adapt to this reality, investing in research and development, contingency planning, and emergency preparedness.

The clock is ticking, and the stakes are high. The fish in the water at Bruce Nuclear Generating Station should serve as a stark reminder of what’s at risk: not just the reliability of our power supply but also the very fabric of our society.