Biologists Finally Know What's Hunting Bull Sharks In American Rivers — And No One Can Explain Why

Bull sharks are one of the most feared predators in American waters, uniquely built to swim from the ocean deep into rivers where no other big shark can follow. For decades, biologists assumed that once a bull shark entered a river, it had nothing left to fear and nothing left to compete with. But a growing trail of evidence, starting with a shocking 2015 video from Florida, shows these sharks are turning up dead in the exact same rivers they were supposed to rule. Now researchers are trying to figure out why one of the ocean's toughest predators keeps losing fights it should never lose, and why it's happening more and more.