High up in the Arctic Circle, Olivier Chastel begins his working day by scanning the horizon for polar bears, rifle at the ready. "In 25 years I've never had to use it, but you can't be too careful," he explains. There can't be many conservationists who go birdwatching while armed, but the danger to life from bears in Svalbard - the largest island of the Norwegian polar archipelago - is so high that it's a legal requirement.