PORTLAND — For years, the sea lions lounging at the Bonneville Dam have had easy pickings from salmon waiting to go up fish ladders to upriver spawning grounds.

Over the weekend, the federally protected sea creatures were themselves easy prey for a gunman who shot and killed six of the sea lions as they lay in traps meant to humanely catch them.

State and federal authorities were investigating the shootings, which came less than two weeks after an appeals court issued a temporary injunction against authorities killing the salmon-gobbling mammals. Agents have been trapping them instead, but trapping will be suspended during the investigation, said Rick Hargrave, a spokesman for the Oregon Department of Fish and Wildlife.

Fishermen and American Indian tribes have pushed to protect the salmon and remove the sea lions, by lethal force if necessary.

The carcasses of the four California sea lions and two Steller sea lions were found Sunday around noon below the Bonneville Dam on the Columbia River on the border of Oregon and Washington.

The six animals appear to have been shot by somebody on the Washington side during the night, said Brian Gorman, a spokesman for the National Marine Fisheries Service.

Two closed cages each contained the carcasses of two California sea lions and one Steller sea lion, he said.

Necropsies were planned for all the animals, and the area was being treated as a crime scene by state and federal agencies, Gorman said.

The discovery came one day after three elephant seals were found shot to death at a breeding ground near San Simeon in central California.

 Investigators will try to determine whether there is any link between the shootings, Gorman said.

Seven California sea lions were trapped on the Columbia starting April 24 after the 9th U.S. Circuit Court of Appeals approved their capture. One died during a medical inspection before transfer to a Sea World park.

Washington and Oregon have been granted federal authorization to capture or kill as many as 85 sea lions a year for five years at the base of the dam.

The Humane Society of the United States has gone to court to challenge the authorization, with another hearing set for May 8. Until a judge rules, no animals may be legally killed.

“We’re really shocked,” said Sharon Young, a Humane Society spokeswoman, who learned about the sea lion deaths from a reporter. “We’re a nation of laws, and we should expect people to abide by them.”