The real reason you're seeing dead fish this spring? Canadian weather
A difference in temperature and a canal may be to blame for this smelly start to spring
Videos of dead fish appearing all around Lake Ontario from Mississauga to Toronto may sound concerning at first, but are due to a fish known to die off in the spring, the Alewife.
The Alewife is actually from the Atlantic Ocean, but during the 19th century, the Alewife migrated to the Great Lakes through the Welland Canal. It’s the same canal that brought the ‘Vampire Fish’ to the Great Lakes, which are just some odd-looking fish.
DON'T MISS: Frost alerts cover southern Ontario ahead of a chilly night
A group of dead Alewife seen at Humber Bay Park on May 7, 2025. (David Hodge/The Weather Network)
The Alewife mostly resides in the deeper, colder parts of the lake, but begins to swim to shallow waters to lay their eggs during the spring. While in the warm waters, strong winds pushing up cold water from the bottom of the lake can cause their bodies to rapidly change in temperature.
This can cause them to become sick or kill them outright, as they can’t handle such a temperature change. Winter weather also has a similar devastating effect, weakening their immune system, which can make it harder for them to fight disease.
More dead Alewife mixed in with driftwood at Humber Bay Park on May 7th. (David Hodge/The Weather Network)
The Toronto and Region Conservation Authority, which investigated the pile-up, says that this should all resolve on its own when the warmer weather makes it easier for the Alewife to lay its eggs in shallow waters. They say this event is similar to another pile-up that happened in 2019.
And while this die-off is unpleasant to look at (and smell), birds and other animals should handle most of the clean-up, fulfilling the circle of life.
The thumbnail was originally from the Toronto and Region Conservation Authority