This story originally appeared on Mother Nature Network.In the hierarchy of the ocean, sharks are the apex predator, not the prey. But the fearsome fish face a powerful enemy in humans, who've decimated their populations by commercial fishing. Enter Madison Stewart. The 20-year-old Australian who's been swimming with sharks since she was 12 has made it her life's mission to protect them, raise awareness about their importance to the ocean's ecosystem and stop the fishing and illegal finning that endangers them.