Bearded seals deserve to be listed as a threatened species because of the continuing loss of their sea ice habitat, a US court has been told.

The hearing in Anchorage, Alaska, was told that bearded seals, the largest of the Arctic seals, had been listed by the National Marine Fisheries Service (NMFS) in December 2012.

A court said the agency was wrong do so, and the case is now being heard by a panel of the 9th US Circuit Court of Appeals.

Robert Stockman, a lawyer for the NMFS, a government agency, urged judges to reverse the lower court ruling.

The agency's best projection is that ice critical to bearded seals will disappear or be greatly reduced by the end of the century in the Bering Sea, where 70% of bearded seals thrive. Ice loss is already outpacing models, he said.

"The minimum ice years will start being harmful soon," he said.

Lawyers for the North Slope Borough, the Alaska Oil and Gas Association and others who sued to reverse the listing said the NMFS not demonstrated that Bering Sea ice losses will harm bearded seals, which have thrived for centuries and survived other warming periods.

"Here all we have is information on sea ice loss," said lawyer Tyson Kade.

Bearded seals get their name from short snouts covered with thick, long, white whiskers. They grow as large as 8ft, weigh between 575 and 800lbs and can live to 25 years or more.

They eat Arctic cod and shrimp but also dive for crab and clams, usually in depths less than 325ft, according to the agency.

They give birth and rear pups on drifting pack ice. When females give birth, they need ice to last long enough in spring and early summer to successfully reproduce.

US District Court Judge Ralph Beistline ruled in 2014 that the threatened species listing was improper.

It did not appear, he wrote, that any serious threat of a population reduction, let alone extinction, existed before 2100.

The listing itself conceded that through the middle of the century, there would be enough sea ice to sustain Bering Sea bearded seals at or near current levels, the judge said.

Judges on Thursday questioned how far out an agency could project harm before it declared a species threatened or endangered.

Kristen Monsell of the Center for Biological Diversity, which sought the listing, said agencies must act to protect future generations of a species, not just the current population, and take measures with enough time to do so.

But Jeffrey Leppo, representing the Alaska Oil and Gas Association, said NMFS has not documented harm to bearded seals despite well-documented sea ice loss that already has occurred.

He dismissed the idea that bearded seals will face harmful competition if they migrate north of the Bering Strait into other areas with ice.

"If this was a small area, that would be a fair point," he said. "The area is a very, very large area."