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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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