Brad's right, Gary. Assuming it's not a self contained lake, but one that has a river/stream leading into it at some point. Bull sharks are notorious for swimming up creaks, streams, rivers, etc. and are somehow adaptable from salt water to fresh, which is pretty much what makes them the #1 most feared shark, and the one that's made the most attacks on humans every year (above Whites and Tigers even).
In fact, the movie Jaws was loosely based on the notorious rampage of a bull shark back along the New Jersey shores in the early 1900's, with several of it's attacks coming in small creaks and rivers. If those lead to a lake, then it's certainly possible for the shark to get in there. Unless, by definition, lakes are entirely contained on all sides, with no outlets. I dunno...
Interesting speculation nonetheless.
