Army Corps of Engineers

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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Todd Haney / Field Museum

Some of the 38 deep-sea limpets found aboard the research sub ALVIN. Each is less than half an inch long.

Sure, it sounds like a sci-fi movie: Alien species deep below the ocean latch onto scientists' gear, surface and cause havoc. But it could happen, scientists reported Thursday in a study that concludes those free rides can ruin ecosystems.

"I don't worry too much about deep-sea aliens taking over," lead researcher Janet Voight told msnbc.com, "but the worse-case scenario would in fact be a fundamental change in the ecosystem" if the new species brought with it a disease or parasite.

What triggered the study was the discovery of 38 deep-sea limpets, a kind of saltwater snail, inside a suction system aboard the research submarine ALVIN. They were found just after a dive in 2004 to the deep-water hydrothermal vents along the Juan de Fuca Ridge off Washington state -- and didn't seem like limpets native to the zone.

Voight, who was the mission's chief science officer, said, "I examined the specimens, and contacted my co-authors to help me understand why these limpets were apparently collected on Juan de Fuca Ridge." 

Further research determined that the limpets were from Gorda Ridge, a deep-water site 400 miles to the south where ALVIN had been a few days before the Juan de Fuca dive.

Janet Voight / Field Museum

Clumps of limpets are seen among sea worms at the bottom of Gorda Ridge, from where 38 limpets were mistakenly taken for a ride to another ecosystem.

"We realized instead that they had been collected on Gorda Ridge and 'stowed away'," said Voight, a biologist with the Field Museum of Natural History in Chicago.

Most surprising was that the limpets had survived the pressure change that comes from rising 8,900 feet in a submersible. 

"Our small limpets and their associates accrued somewhere in the suction sampler, perhaps in the corrugated hose, where enough water pooled to keep them alive," the researchers wrote in their study.

So while ALVIN was busy collecting new samples at Juan de Fuca, the Gorda Ridge limpets were hiding out in the hose.

Ships and even air travelers, via shoes or clothes, are also known to redistribute species to new areas, potentially altering local ecosystems.

"In retrospect, we should have cleaned the sampling gear more thoroughly, but we honestly believed that no animals could survive on ALVIN at sea level pressure for more than a day it took to get to the next dive site," Voight said. "We were naive. This is why we are admitting to our mistake, which could have, but we don't think did, introduce this species to Juan de Fuca Ridge. We want to warn other scientists that it is possible."

Raymond Lee

Limpets easily attach themselves to other objects, as seen in this photo of research equipment.

The danger at Juan de Fuca, Voight said, is that the Gorda Ridge limpets could have established themselves, along with parasites or disease, and potentially wiped out the native limpets.

Voight doesn't think that happened because the Juan de Fuca dive was some 300 feet from any vents, which the limpets need to survive. Still, she adds, "it might be worthwhile to go back to that spot" to check things out.

The team's advice? "We urge our colleagues to assume that physiologically tough stowaways are present on deep-sea research tools and to guard against transport of non-native species by clearing hoses and rinsing containers with freshwater, or even a peroxide solution, and drying tools before transporting them to different sites," they wrote.

The peer-reviewed study was published in the journal Conservation Biology.

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Utah Division of Wildlife Resources via AP

Quagga mussels cover a boat's propeller at Lake Mead, Nev. The invasive freshwater mussel first entered the U.S. through the Great Lakes.

Nonnative species like zebra mussels, which clog pipes and have caused billions of dollars in damage along Great Lakes shores, are the target of the first-ever federal rule on how many living organisms ships can discharge with their ballast water. A decade in the making, the rule is being published Friday, but environmentalists say it doesn't do enough to protect U.S. waters from the invaders.

Calling it a "starting point," activists said the Coast Guard rule needs to be strengthened, either by states or the Environmental Protection Agency.

A major concern is that the rule gives shipowners until 2019 to implement controls that prevent invasive species from being discharged when a ship releases ballast water.

"Getting treatment technology on board ships is a welcome step, but we're disappointed the Coast Guard is manning a slow boat toward a truly protective standard," Joel Brammeier, president of the Alliance for the Great Lakes, said in a statement.

"The onus now rests with the EPA and states to shut the door on invasive species to protect our waters, jobs and way of life," added Marc Smith, a program manager with the National Wildlife Federation.

The rule sets limits for how many living organisms may be discharged.

Those limits, the Coast Guard said in a statement, "are supported by reports from the National Academy of Sciences and the U.S. Environmental Protection Agency Science Advisory Board in 2011 as the most stringent that vessels can practicably implement and that the Coast Guard can enforce at this time."

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