A fishing boat captain and a member of his crew have been charged with using a shotgun to harass and kill Steller sea lions, federal prosecutors say.

The investigation began when the bodies of 15 sea lions were found during the opening of the 2015 Copper River salmon season. The U.S. attorney's office Thursday announced felony charges against Jon Nichols, 31, and Theodore "Teddy" Turgeon, 21.

Prosecutors say Nichols, a Cordova resident, was the captain of the 32-foot F/V Iron Hide with Turgeon, of Wasilla, working as a deckhand. Nichols is accused of telling his crew, including Turgeon, to use a shotgun to shoot Steller sea lions as the boat traveled the mouth of the Copper River fishing district between May and June 2015.

Steller sea lions are listed as threatened under the Endangered Species Act.

Nichols sometimes fired on the animals himself and would occasionally "drive the F/V Iron Hide in the direction of the Steller sea lions to allow Turgeon to get a better shot," the U.S. attorney's office said in a news release.

The men removed the shotgun from the fishing vessel after learning they were under investigation and coordinated false stories among crew members, according to prosecutors. Nichols tried to obstruct a grand jury investigation by attempting to influence crew members to provide false information, prosecutors say.