What’s With All the Dead Fish in Lake Ontario?

Dead fish spotted along the shores of Lake Ontario

Over the last several weeks, we’ve gotten numerous messages with the same question: “What’s with all the dead fish?” 

It’s very concerning when you turn up at your favourite spot along the lake to be met with dozens of dead fish. We’re glad many of you have taken the time to report this issue to us. Let’s break it down – what are these fish, and why is this happening?

A fish by any other name: Alewives

Most of the fish you’ve seen along the shores of Lake Ontario and Lake Erie are called Alewives, also known as gaspereau in Atlantic Canada or river herring along the Atlantic coast of the U.S. These small, silvery fish, originally confined to Lake Ontario, are known for their invasion of the Great Lakes through the Welland Canal in the late 1800s, particularly taking hold of Lake Huron and Lake Michigan. 

Changing Seasons

These little fish are very sensitive and unable to handle rapidly changing temperatures. When the weather warms up in the spring, water temperatures rise, making the shallow waters perfect for laying eggs. However, we’ve seen rapid changes in weather, and storms can stir up colder waters from deeper parts of the lake, shocking the systems of these fish and ultimately leading to the die-offs we’ve seen this season. While the die-offs have been more severe this year, this is an annual occurrence and not a major cause for concern. 

What do we do about the dead fish?

While this is unsightly, nature has its own environmental clean up crew. Over the next few weeks, the shorelines will be cleaned up by scavengers, shorebirds, and other wildlife who will make a meal of the alewives.

While the seasonal Alewife die-off is not a cause for concern, we appreciate and encourage community members to document and report mass die-offs of wildlife. In other regions of the Great Lakes watershed, the die-offs have spanned beyond alewives, and may be attributable to a spike in a fish virus. Reporting die offs to local governments and environmental organizations in your region flags these issues for scientists to look into.

To report environmental concerns to Swim Drink Fish, please use our Contact Us page or the Report feature on Swim Guide.