If you’ve seen dead fish along the shore of Lake Ontario recently, you’re not alone. Toronto and Region Conservation Authority (TRCA) aquatic staff have investigated the reports and confirmed that the fish are Alewife, a small, silver species that commonly experiences large die-offs in the spring.
Alewife (Alosa pseudoharengus) are not native to the Great Lakes. They came from the Atlantic Ocean in the late 1800s through the Welland Canal.
Since then, mass die-offs have happened many times, including in Lake Ontario. These events can be upsetting to see (and smell), but they are a normal occurrence in the lake’s ecosystem.
The main reason Alewife die off is because of sudden changes in water temperature.
In the spring, they swim from the deep, cold parts of the lake to the warmer, shallow areas near the shore to spawn (lay eggs). If the weather suddenly changes, often due to strong storms or wind, cold water from the bottom of the lake can rise quickly to the surface. This rapid temperature drop can shock the Alewife, making them sick or even killing them.
Cold winter weather can also weaken their immune systems, making it harder for them to fight off disease. So when spring brings fast-changing conditions, it can be too much for them to handle. A similar die-off happened in 2019, and this year’s event appears to be the same.
As the water warms and conditions stabilize, these events typically resolve on their own. Nature has its own cleanup crew: waves and currents may carry the fish back into the lake, while scavengers like gulls and other wildlife take advantage of the sudden food source.
Die-offs like this, while unpleasant, are part of natural cycles that help recycle nutrients back into the ecosystem.
Other Examples of Fish Die-Offs Due to Natural Events
Wetlands and ponds often experience a natural phenomenon known as “winter fish kill” after prolonged winters, especially when thick ice forms beneath heavy snow cover, as occurred this winter.
Thick ice and heavy snow can block sunlight, reducing oxygen levels in the water. In shallow wetlands and ponds, ice may even reach the bottom, making survival difficult for fish.
When ice melts, these natural winter losses become visible. While it may look concerning, winter fish kills are a normal seasonal event, with scavengers like raccoons and turkey vultures eventually consuming these fish.
If you also notice dead fish in rivers or streams during the fall, it may not be a cause for concern. Many Pacific salmon species, such as Chinook and Coho, die naturally after they spawn.
This is a normal part of their life cycle. After laying their eggs, these salmon complete their journey and die, providing a vital food source for other wildlife. Their decomposing bodies also return important nutrients to the water, supporting the river ecosystem.
By contrast, Atlantic Salmon do not die after spawning. They can survive and return to spawn again in future years, making them unique among salmon species found in Ontario’s waters.
You can learn more about both spring and fall Salmon migration HERE.
How to Report Dead Fish in Toronto
If you come across dead fish on the beach, here’s what you can do:
- Call the Ministry of Natural Resources: 1-800-387-7011. Use this line to report large fish die-offs.
- Call the Spills Action Centre (MECP): 1-800-268-6060. Call if you think pollution or a chemical spill may be involved.
When reporting, try to include the location, number of dead fish, and anything unusual you notice, like odd smells or signs of pollution.
Scientific data collection and research informs decisions affecting the natural areas and watercourses within our region. Learn more!