Thousands of dead fish wash ashore ‘alarming residents’ in Port Credit, Ontario, Canada

Thousands of dead fish have washed up on the shores of Port Credit over the last three days, alarming some nearby residents.

The small, silvery fish, which are about 15 centimetres long, have been spotted all along the shoreline and at the mouth of rivers.   

But though the scene may look like a sure sign of an unfolding ecological disaster, conservationists say it is a natural phenomenon caused by nothing more dangerous than fluctuations in the temperature of Lake Ontario.

The fish is believed to be the alewife, a species that is not native to Lake Ontario.

According to the University of Wisconsin’s Sea Grant Institute website, it is a Atlantic Coast saltwater fish that made its way into the Great Lakes in the late 1940s.

Because the alewife is not well adapted to the freshwater environment in the lakes, it is well known among aquatic biologists for dying off in large numbers, particularly in spring when it moves to shallow waters to mate and is vulnerable to changes in the water temperature.  

Jon Clayton, an aquatic biologist with CVC, speculates that high winds over the last few days could have driven colder water from the middle of Lake Ontario to the shore, and the fish couldn’t cope with the rapid cooling.

He believes that people may have noticed the die-off more this year than in the past as a result of the rapidly growing numbers of alewife fish in the lake. Data from the Ontario Ministry of Natural Resources suggest that there are 681 million alewife in Lake Ontario, a number that has more than doubled since 2012.