Can a salt water shark survive in fresh water?

Well, back to the thread subject. For years there have been rumors that a bull shark has been occasionally spotted in southern Lake Michigan and that it or its ancestors made their way up the Mississippi River and eventually into the Chicago River. From there, it escaped into Lake Michigan. It is more likely that the "shark" that some people have spotted was either a large sheephead or a lake sturgeon (the largest of this species ever caught in the Great Lakes was over 150 years old and weighed in at 310 lbs.!). Spooky thought, though!