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The Science of Stowaways

A dock dislodged by 2011's Japanese tsunami washes ashore in Oregon, posing an invasive species threat, but also serving as an unprecedented natural experiment in open-ocean dispersal.

By | October 1, 2012

image: The Science of Stowaways EXOTIC MUSSELS: Mytilus edulis and M. galloprovincialis mussels cling to the derelict dock among unidentified barnacles.Oregon Parks and Recreation department

This June a dock weighing more than 180 tons washed up on the Oregon coast, about a year after being dislodged from Japan’s seashore during the devastating earthquake and tsunami that struck there in March 2011. The dock is one of the largest pieces of debris to wash up on the opposite side of the Pacific, and it arrived earlier than anyone expected. But a bigger surprise for researchers and natural-resource managers dealing with the debris was the profusion of marine organisms clinging to the dock, many of which have already been pegged as potentially aggressive invasive species. “We have already identified very bad things on this debris,” says John Chapman, an expert on marine invasive species at Oregon State University. “We can already see things that we definitely do not want.”

One species clinging to the dock that is causing concern to Chapman and his colleagues is wakame (Undaria pinnatifida), a brown alga that can suffocate entire ecosystems when its natural predators are not around to keep it in check. Chapman likens the alga’s rapacious effect to “cutting down the whole forest,” and warns that the consequences of an invasion by such a prolific species might not be known for years to come. And there were many more invasive organisms present, including a thriving population of Mytilus galloprovincialis, a blue mussel that was itself introduced to Asia from the Mediterranean.

Pre-tsunami, the mussels only covered the part of the dock that was below the water’s surface. But when the structure was ripped from its pilings, it sank a little and dipped a fresh surface into the ocean. This previously high-and-dry surface was also covered in mussels by the time the dock reached Oregon, making M. galloprovincialis the dominant species on the structure.

There were very few small mussels on the dock. Did the entire population grow rapidly in Japanese waters as the dock bobbed just offshore before setting off on its open ocean journey? Or did the mussels continue to grow, encountering nutrients along the way? To try to answer these questions, marine ecologist Jessica Miller, also at Oregon State University, is working to unravel growth patterns from the creatures’ shells.

 

DOCK WORKERS: Oregon Department of Fish and Wildlife staff and volunteers remove marine organisms from the wayward dock, which landed at Agate Beach.
DOCK WORKERS: Oregon Department of Fish and Wildlife staff and volunteers remove marine organisms from the wayward dock, which landed at Agate Beach.
Credit: Oregon Parks and Recreation department

 

Much as trees’ yearly rings record environmental changes, hard-shelled marine organisms lay down annual layers of shell—slower growth and unfavorable seasons corresponding to thinner lines. The age and nutritional history of any mussel can therefore be determined, but chemical analyses of changes in elements across growth lines can also tell Miller something about the proximity to shore and the temperature at which the organism grew, allowing her to reconstruct the colonization history of the dock. “This chance natural experiment gives a better opportunity to understand transoceanic dispersal,” she says.

Many of the dock organisms were collected and await analysis in freezers and labs across North America; much of the remaining biomass has been obliterated in an effort to restrict the spread of invasive species. Scientists recovered around 23–27 kg of organisms for research, but local responders scraped off and disposed of another 2 tons of material. Miller says that 92 species have already been identified from what was saved, and the team is confident they destroyed a large percentage of the total species present.

But when Chapman and his team looked for organisms that might have made landfall as the dock crashed into the Oregon coast, they found a large debris field of non-native mussels stretching down the beach. Chapman says they’ll try to determine what proportion of organisms was pulverized as the dock came to shore and what was washed up on the beach. “The remainder is what we’re afraid of—what’s offshore and didn’t get crushed, and could still be alive,” he says.

Invasive species expert Rick Boatner of the Oregon Department of Fish and Wildlife says it could be a while before we really know which, if any, species have managed to take hold on the Oregon coastline. He says that the clean-up operation on the dock was largely successful and the department is ready for another invasion, noting that at least one other Japanese dock is known to be adrift, along with ships and other large debris. However, he says, removing the first dock cost almost $100,000, and further funding is an issue, even in the face of the severe economic havoc invasive species can wreak. “If [an invasive species] got into the oyster aquaculture, it could devastate that,” Boatner says.

Among the fears, though, Chapman sees the silver lining: an experiment that could never have been simulated. Many invasive species travel in ship ballast water, but these are typically discovered only after they have become established, with researchers left trying to reconstruct when and how they arrived.

“This is the first time in history that a dispersal event could be really watched,” Chapman says. Boatner agrees: “As a biologist, it’s kind of a cool thing. As an invasive species person, I don’t care for it.”

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