Authorities in the western US state of Oregon have euthanized four sea lions in the last month as part of a program to protect salmon runs and steelhead trout that are at risk of going extinct.

A spokesman for the state's Department of Fish and Wildlife told AFP the lethal removals of the California sea lions began in mid-December and would continue through May.

Rick Swart said the killings were necessary to save migrating steelhead that have been ravaged by the sea lions in recent years as they swim upstream from the ocean to spawn.

"Our scientists believe that if these sea lions aren't removed, that run of steelhead could go extinct anytime," Swart said.

He said salmon were also threatened by the sea mammals, whose population has exploded since the Marine Mammal Protection Act was passed in 1972.

"What's going on here is we have a run of wild native Oregon steelhead and they are moving right now from the ocean upstream to spawn," Rick Swart said. "And when they get to Willamette Falls in Oregon, in downtown Portland, they come up against the waterfall and a dam and it takes them a while to get across that."

He said that as the fish gather in a big school while preparing to go over the waterfall and dam, the sea lions swim in and feast on the trout.

"Two years ago, the sea lions destroyed 25 percent of that wild steelhead run," Swart added. "They killed about 512 fish and that represented about one fourth of all the fish that we had left."

He said the state was authorized to kill up to 93 sea lions a year but he expected that no more than 40 would be removed this year.

Authorities in the past have used various non-lethal techniques, including loud noise or rubber buckshot, but the animals have adapted and learned how to avoid the threat.

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