"This is being investigated by Washington, Oregon and our fish cops," said National Marine Fisheries Service spokesman Brian Gorman. Authorities would not say if they had any suspects.

The deaths temporarily halted a new trapping program in Oregon and Washington, which won permission from the federal government in March to trap and kill the protected pinnipeds to save endangered salmon.

The Humane Society sued to block the killing and while the suit is pending, the states are limited to trapping the mammals and sending them to zoos and aquariums. Seven were trapped and transported to an aquarium in late April, where one died.

"We are are calling on the government to stop the trapping program," Sharon Young, marine issues field director of the Humane Society of the United States said.

For the past five years, the sea lions have been making the 150-mile (241-km) trip up the Columbia River, a favorite of sport and commercial fishermen. The sea lions eat salmon as they channel into the Bonneville Dam's fish ladders on their way upriver to spawn.

Authorities have tried to deter the sea lions with physical barriers, fire crackers and rubber bullets, without much success.

Although the salmon run on the Columbia River was strong this year, shrinking numbers have prompted critical restrictions on West Coast salmon fishing.

The Fisheries Service banned virtually all commercial and recreational salmon fishing off the West Coast for one year beginning May 1.