Polar bears must have ice floes for hunting and fatty seals for food, and a new scientific study has concluded that the world’s largest bruins cannot survive on food scrounged when they come ashore. The study, published Wednesday in Frontiers of Ecology and the Environment, comes as the Arctic has just recorded its lowest-ever ice pack at the end of winter. In recent years, extent of the ice pack has also reached record lows in September, when summer ends.