Walrus cows and yearlings rest on ice in Alaska in 2004. The U.S. Fish and Wildlife Service announced Wednesday that it will not add the Pacific walrus to the list of threatened species. (Joel Garlich-Miller/U.S. Fish and Wildlife Service via AP)

The Trump administration announced Wednesday it will not list the Pacific walrus as a threatened species based on diminished Arctic Ocean sea ice, concluding that the marine mammals have adapted to the loss.

U.S. Fish and Wildlife Service officials said they cannot determine with certainty that walruses are likely to become endangered “in the foreseeable future,” which the agency defines as the year 2060.

The decision could be challenged in court by environmental groups, which say a decline in Arctic Ocean sea ice due to climate change is a threat to the walruses’ future.

The agency said in 2011 that walruses deserve the additional protection of being declared threatened, but delayed a listing because other species were a higher priority. The agency revised the decision based on new information, said Patrick Lemons, the agency’s marine mammals management chief.

“Walrus demonstrated much more ability to change their behaviors than previously thought,” Lemons said. Their ability to rest on shorelines before swimming to foraging areas makes the threat of less sea ice uncertain, he added.

Older male walruses spend summers in the Bering Sea, which is west and south of Alaska. Females with calves, however, ride sea ice north as it melts in spring and summer all the way through the Bering Strait into the Chukchi Sea. The ice provides a moving platform, giving walruses a place to rest and nurse, and protection from predators.

In the last decade, however, ice in the Arctic Ocean has melted far beyond the shallow continental shelf over water too deep for walruses to reach the ocean floor. Walruses instead have gathered by the thousands on beaches in northwestern Alaska and Russia, where smaller animals are vulnerable to being trampled in stampedes if the herd is spooked by a polar bear, hunter or airplane.

In the past six years, Lemons said, protections put in place in Alaska and Russia have greatly reduced trampling deaths. Walruses also have shown a willingness to swim great distances — 130 miles or more — from coastal haul-outs to prime foraging areas.

The Center for Biological Diversity petitioned to list walruses in 2008 because of diminished sea ice tied to global warming. Arctic sea ice this summer dropped to 1.79 million square miles, about 610,000 square miles below the 30-year average.

Lemons said the Fish and Wildlife Service used climate models showing the Chukchi Sea between northwest Alaska and Russia could be ice-free in the summer by 2060. But he said information collected in the past six years makes predicting the walruses’ fate uncertain beyond then, so the decision was made not to list the species.

Shaye Wolfe, climate science director for the Center for Biological Diversity, wrote the listing petition filed in February 2008. She said last week that the group would probably sue if walruses were not listed as a threatened species.