Watch Natural World Season 31 Episode 5 Online

The killer whale is one of the most feared predators in the ocean and most would consider it madness to enter the water with one. But New Zealander Dr Ingrid Visser thinks differently - and by swimming with her beloved whales she has come to know almost all of them by sight. But there's been an unusual number of deaths recently and Ingrid is on a mission to find out what is going on. Her findings reveal disturbing new information about the health of our oceans.