Since it is connected to the sea via the St Lawrence waterway, is it possible there are sharks in there?
Sharks don’t like fresh water or Niagara Falls, so… No.
pwn3d!
Just kidding.
I can’t tell if you’re joking or implying that sharks could live in fresh water if it was tolerable. Is that the case?
There are several species of sharks that are quite at home in fresh water. However the falls may present a problem.
Bulls sharks can live in freshwater
Wait a minute I did not realize the falls were in the way. What is the point of the waterway then? Ships can’t make it past the falls,
I bet a shark could though…no bones to break in the fall.
So could, say, a Great White live in fresh water? I’ve been told (long, long ago) that saltwater fish can’t live in fresh water. Perhaps it was the other way around, or perhaps my info is completely wrong. Anyway, to sum up, in general, would fresh water kill a saltwater fish, or vice-versa?
Not Great Whites, but bull sharks can, this article explains why, says 2 people attacked in freshwater last year
http://news.nationalgeographic.com/news/2005/07/0719_050719_bullsharks.html
No, Great Whites are saltwater aniamls.
It depends entirely on the fish species. There are a great many fish of diverse species that will happily live in either fresh or salt water as need and fancy dictates. Everything from trout to catfish to eels to perch. And of course sharks and rays.
And of course there are many fish species that are restricted to either freshwater or saltwater and will die if taken out of that environment.
Sharks have bones, don’t they?
Nope only cartiliage
According to this site, no. Their skeletons are made of cartilage.
Nope, just cartilage. In most sharks most of it has a texture similar to the cartilage in your nose or ear, IOW it’s reasonably flexible. However the jaws are more rigid and in some species much of the skeleton has a similar rigidity producing a sort of psuedo bone. But it’s still not bone.
What fall? Niagara Falls fall the other way. Otherwise, it would be salt water and sharks flowing into Lake Erie, instead of the much-preferred system of fresh water and morons in barrels flowing from Lake Erie to Lake Ontario and the St. Lawrence Seaway.
The Falls don’t keep bull sharks out of Lake Ontario, though. I assume the temperature does. Bull Sharks do wander into freshwater lakes in Africa and South America, but not Canada. Yet.
I just read a book on shark attacks called Close To Shore, by Michael Capuzzo, which mentioned attacks by a bull shark in the Mississippi River in Illinois. Apparently, they have been caught there more than once.
Sea lampreys (a coastal fish that can live in salt and fresh water) invaded the Great Lakes around 80 years ago. The upper lakes are available to coastal fish by way of the Welland Canal that bypasses the Niagara River. Canadian freighters use it to take iron from Ontario or Wheat from Saskatchewan and Manitoba down to Toronto. The 1950s Saint Lawrence Seaway extended the shipping toward the ocean by providing locks to avoid rapids in the upper St. Lawrence River, but the Welland Canal is quite a bit older.
I know of nothing in the Saint Lawrence Seaway (aside from adverse temperatures) that would prevent a (coastal) shark from entering the Great Lakes, but I have never heard of it happening. (The bull shark would probably avoid the colder waters of the Northern rivers and lakes.)
The implication of the above statement seems to suggest that the Welland canal is reserved for Canadian commerce. The Welland Canal is part of the St Lawrence Seaway which is a joint operation for betwwen Americans and Canadians and services the American Great Lakes cities shipping as well.
In spite of the fact that Bull Sharks can live in waters with reduced salinity (they are able to retain salt in their systems unlike other saltwater marine life) I believe the Great Lakes are far too cold.
I am going to keep saying that it is impossible, over and over, as my mantra.
I have an irrational fear of sharks and I kayak, swim, and fish in Lake Erie.
Dammit, AnabolicDoberman, for throwing me into a panic attack. 
But I seriously did have a panic attack. 
South of the border, bull sharks are referred to as “Lake Nicaragua Sharks”.
I would just like to point out that watching giant sharks go flying over Niagara Falls would be simply…awesome.