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Saturday, October 23, 2021

No happy clams in Lake Tahoe

When the non-native Asian clam species was spotted in Lake Tahoe back in 2002, it didn’t seem like a big deal. Boaters reported seeing strange, golden-colored shells, but it wasn’t an Asian clam invasion – not yet.

Then in 2008, researchers from the UC Davis Tahoe Environmental Research Council (TERC) discovered a booming Asian clam population under the placid surface of Lake Tahoe. For clams the size of a nickel, Asian clams can wreak havoc on an ecosystem. John Reuter, associate director of TERC, said Asian clams disrupt the natural food web.

“Typically, there are no native species that can out-compete these things,” Reuter said.

The clam’s method of eating is also a problem for the ecosystem. Clams filter lake water in order to consume microscopic algae. When they excrete waste, they also excrete nutrients like phosphorous. The result: huge “carpet-like” algae blooms that live off the clam waste and pollute the lake.

Luckily, UC Davis researchers have found a way to kill the invading clams. Scientists from TERC, in collaboration with scientists from the University of Nevada, Reno, discovered that rolling out rubber sheeting on the lake bottom helps control the clam population.

“If you put this very thin rubber on the bottom, it will block oxygen and kill the clams,” Reuter said.

The rubber-sheeting method kills the clams, but it also kills everything else living on the lake bottom. Reuter said native organisms in areas with a high density of clams are doomed anyway because the clams out-compete the native species.

Asian clams have taken over about 20 percent of the near-shore areas of Lake Tahoe. With the help of rubber sheets – each about one-half acre in size – the researchers hope to stop the spread.

The rubber-sheet method is successful, but now researchers must study how long it will take for Asian clams to repopulate the clam-free zones. Reuter doesn’t think the rubber sheeting will ever completely get rid of Tahoe’s Asian clam population – it is just a way to control the spread. Even if the clams disappeared from the lake, it would take time for the ecosystem to recover.

“Once an invader comes, the effects are long-lasting,” said Marion Wittmann, a postdoctoral researcher at TERC.

Wittmann said non-native aquatic invaders are a new phenomenon in the Western U.S. Scientists here must try out new methods – like rubber sheeting – to learn how to prevent the spread of organisms in the environment.

“We’re really just beginning to learn about the long-term impact of species like these,” Wittmann said.

The Asian clam population could also usher in other non-native shellfish species. Clams use calcium from the lake’s water column to construct their shells. Disintegrating shells from dead clams are a useful source of calcium for invasive species like quagga mussels and zebra mussels.

Educating boaters is a priority for conservationists. For the last three summers, Tahoe officials have inspected incoming watercraft for stow-away clams. Kim Boyd, manager of the invasive species program for the Tahoe Resource Conservation District, said boaters need to remember to clean out boats’ ballast tanks and bilge water areas.

“Our message is clean, drain and dry,” Boyd said.

Boat inspections keep the public aware of invasive species, but non-natives like quagga mussels and zebra mussels could still mount an attack.

“There’s a whole group of invasive species that may be on the way,” Reuter said.

When the fight is clams versus scientists, it’s good to have allies. The research by TERC and the University of Nevada, Reno has been supported by Tahoe Basin agencies like the Tahoe Resource Conservation District and the U.S. Fish and Wildlife Service.

MADELINE MCCURRY-SCHMIDT can be reached at science@theaggie.org.

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