The U.S. Court of Appeals for the D.C. Circuit agreed with a lower court in siding with the FWS, prohibiting those imports.
The hunting advocates had argued that the FWS had not properly triggered additional protections for polar bears under the Marine Mammal Protection Act by appropriately designating that their supply was sufficiently depleted. The court, however, dismissed that idea and found the agency had declared polar bears depleted in determining that they were threatened.
It is "quite clear," Judge David Tatel wrote in the court's decision that Congress intended to grant the law's protections "to all depleted species, regardless of how they achieve their depleted status."
The court also disagreed with the hunters' argument that the law does not prevent imports of species that were not protected at the time they were killed.
"The provision refers not to mammals taken from species the Secretary had designated as depleted but instead mammals taken from species the Secretary has so designated," Tatel wrote. That difference of wording, the court determined, protects animals killed before additional protections were granted.
The distinction should discourage hunters who anticipate that a species is going to be protected and rush to kill it, according to Ralph Henry, deputy director of litigation with the Humane Society of the United States.
"In the view of the law and in the view of good conservation measures, we should never allow someone who knows that an animal is in trouble to rush to kill a member of that species just in time to beat out the final sort of legal ruling deadline," he said.
In addition, the court disagreed with claims that the lack of a specific prohibition on trophies killed in sport hunting allowed for their importation and charges that the FWS did not go through the proper procedure to protect polar bears.
The appeals court's ruling should refute hunters' arguments that trophy hunting helps preserve threatened species by raising the animals' value and paying fees that fund conservation efforts, Henry added.
"We feel that there's significant writing on the wall to the trophy hunting lobby that taking the most robust and reproductively viable animals out of the species stock because it is a sport and getting a hide to hang on your wall or use as a rug is not something that would benefit that species in the long term," he said.
-- This story was updated at 5:06 p.m.