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A sea lion feasts on a salmon along the Columbia River.

Two Pacific Northwest sea lions found shot dead this week, along with at least 18 others in the last two months, make up what one expert calls an "unprecedented" attack on marine mammals protected by federal law.

For years, sea lions along the Columbia River between Oregon and Washington have been gorging on salmon, and experts suspect angry fishermen have retaliated by killing some.

"Unfortunately and not unexpectedly, these deaths coincide with the spring run of salmon," Robin Lindsey, a member of the Seal Sitters Marine Mammal Stranding Network, told msnbc.com.

A few killings are typically seen at the start of each season along the Columbia River, but this year is running about four times the norm, Dalin D'Allesandro, a Portland State University research assistant, told The Oregonian. 

"Since the beginning of April, we've seen a surge in both Steller and California sea lions being shot," said D'Allesandro, who helps the local marine mammal stranding network.

The increase also comes after federal officials in March authorized local authorities to cull up to 92 California sea lions a year in waters just below the Columbia River's Bonneville Dam. Biologists estimate sea lions have eaten between 1.5 and 4 percent of returning salmon.

Related: 8 sea lions found shot to death near Seattle

Jim Rice, coordinator for the Oregon Marine Mammal Stranding Network, is among those who suspect that was also a factor in the dramatic increase.

It "may encourage some people to shoot sea lions" thinking they've got a green light as well, he told msnbc.com.

"Marine mammals have been protected since the early '70s," Rice earlier told The Oregonian. "Prior to that there were actually bounties on the animals and they were hunted quite actively. It is certainly unprecedented within that time frame."

As for the number of sea lions killed in recent months, Rice says the number is probably more than the 20 reported. "There are likely others that never came ashore or have not yet been reported to stranding network responders," he noted.

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Richard Clement / Reuters, file

Sea lions rest inside an open cage on the Columbia River at the Bonneville Dam in North Bonneville, Wash., in April 2008.

Oregon state authorities can resume killing California sea lions that feast on endangered salmon bottled up at a dam on the Columbia River, but fewer than one-third as many as federal biologists previously had authorized, a judge has ruled.

U.S. District Judge James E. Boasberg in Washington, D.C., on Thursday denied the Humane Society of the United States' request to stop the killing at the Bonneville Dam while a lawsuit challenging the program goes forward. But he limited the killing to 30 animals a year instead of the 92 authorized by federal authorities, and ordered that none of them may be shot. 

"Obviously we are very disappointed that this program was not halted," said Sharon Young, marine issues field director of the Humane Society. "But, we are grateful that the court put some restraints on it."

It was the group's third attempt to permanently halt the killings since they started in 2008.

The floating traps are out and if any of the 92 California sea lions branded as regular salmon eaters are seen inside them, the gates will be sprung, and the animals killed by lethal injection, said Oregon Department of Fish and Wildlife spokeswoman Jessica Sall. She said they have no plans to shoot any animals. California sea lions that hang around the dams eating salmon, and refuse to leave despite hazing by rubber bullets and firecrackers, go on a kill list.

Adult salmon and steelhead returning to spawn get bottled up at the fish ladders over Bonneville, located east of Portland, Ore. California sea lions, which are federally protected as marine mammals, but not as threatened or endangered species, swim about 145 miles upriver to the dam to feed on the fish in the spring.

Since 2008, 28 sea lions have been killed and 10 placed in institutions under similar salmon-protection programs overseen by the Fisheries Service.

The limits imposed by the judge should not pose a problem, Sall said. The department did not anticipate killing more than 30 animals in any one year. Over the past four years, only 41 have been trapped and killed or sent to a zoo or aquarium. The current authorization from the National Oceanic and Atmospheric Administration's Fisheries Service is good for four years.

The Humane Society lawsuit contends that the Fisheries Service erred when it decided that sea lions eating up to 4.2 percent of the fish passing over the dam amounted to a significant obstacle to the restoration of endangered salmon, when fishermen are allowed to take up to 17 percent. It adds that killing sea lions will have no effect on restoring salmon, which face a greater threat from fishermen and predation by walleye and bass introduced into the river for sport fishermen to catch.

The department, a co-defendant in the case, counters that while sea lions kill some protected salmon, fishermen are only allowed to kill hatchery-bred fish. The department says it is able to estimate how many wild fish die after being released, and to shut down the season if necessary.

Salmon returns to the Columbia Basin in Oregon, Washington, Idaho and Montana have declined steadily from harm caused by dams, logging, agriculture and urban development since settlement of the region began in the 1840s. Only a small percentage of the fish are wild, with the great majority produced in hatcheries. There are 14 different types of wild salmon and steelhead in the Columbia Basin protected by the Endangered Species Act.

The Associated Press and Reuters contributed to this report.

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