Ten polar bears were recently spotted swimming in open water off of the northwest coast of Alaska, federal officials confirmed on Friday. Polar bears were not often spotted in open water until about 2004, said Susanne Miller, who heads up the U.S. Fish and Wildlife Service’s polar bear project. She and other biologists worry that the bears could exhaust themselves with long swims, which take more energy than moving on ice or land. Green groups point to the unusually high number of swimming bears as yet another sign of global warming, with melting ice forcing bears to swim farther than usual to hunt seals or reach stable territory. A higher-than-usual number of polar bears have also been seen on land this summer, perhaps because sea ice is retreating. The Bush administration in May declared that polar bears are a threatened but not endangered species, making sure that oil drilling could continue in their habitat.