Why are there no sharks in Lake Ontario?

Answer 1

Sharks have evolved to survive in salt water and are usually found in the ocean. Lake Ontario is fresh water. And Niagara falls prevents them from traveling to the lake.

Actually, Niagara Falls is on the Niagara River which empties into Ontario from the west. The lake connects to the ocean via the St. Lawrence River on the east, so sharks could, in theory, enter Lake Ontario. They could get into the other lakes through the Welland Canal between Erie and Ontario or through canals from the Mississippi, but none have been reported much further north then St. Louis which is 1000 miles downriver.

The lakes are young, formed when glaciers retreated about 12,000 years ago, which would limit shark adaptation to their environment.

Given the cold temperature of the Great Lakes they wouldn't be a good habitat for sharks. The bull shark, which can tolerate fresh water, is primarily tropical. Bull sharks have been recorded as far north as Massachusetts which is roughly 200 miles east of Lake Ontario but would have to enter the lakes through the Gulf Of St. Lawrence which is very cold year-round.

I would be very surprised if a bull shark or any other species would survive very long in Lake Ontario or the Great Lakes. One or two pups might get into the water from the ballast water of ocean-going vessels (although they now have to purge before entering the lakes) or be released from a home aquarium, but they wouldn't last very long.