The Trump administration has refused to designate the Pacific walrus as an endangered or threatened species.
The move announced Wednesday reverses the Obama administration finding that the walrus deserves protection because of diminished Arctic Ocean sea ice.
The Fish and Wildlife Service has instead concluded the walrus population is healthy and "can adapt to the changing conditions in the Arctic," said Alaska Republican Senator Lisa Murkowski, chair of the Senate Energy and Natural Resources Committee and supporter of the initiative.
The decision could be challenged in court by environmental groups, who say a decline in Arctic Ocean sea ice due to climate change is a threat to the walruses' future.
"This is a truly dark day for America's imperiled wildlife," said Noah Greenwald of the Center for Biodiversity. "You couldn't ask for a clearer sign that the Trump administration puts corporate profits ahead of protecting endangered species."
While older male walruses spend summers in the Bering Sea, females with calves ride sea ice north as it melts in spring and summer. The ice provides a moving platform, giving walruses a place to rest and nurse, and protection from predators.
Arctic sea ice this summer dropped to 4.64 million square kilometers, about 1.58 million square kilometers below the 30-year average.