UCD readies effort to repel Tahoe’s clam invasion

Call them unwelcome mats.

UC Davis staff and students on Tuesday worked at Bainer Hall to prepare 5 acres of rubber barriers meant to suffocate thumbnail-sized Asian clams invading Lake Tahoe.

Starting next week, excavators on a barge will lower 200 of the mats into the water near the mouth of Emerald Bay.

UCD’s Tahoe Environmental Research Center scientists first pioneered the use of the mats in 2010, killing all of the clams over 1 acre.

This new project will cost about $810,000. It is believed to be the largest of its type to have been attempted.

Other areas of the lake are now home to more clams: 6,000 to 7,000 per square yard in some places, compared to about 50 per square yard where the mats are being deployed.

Emerald Bay is viewed as a special case and a unique opportunity, however.

“This is Emerald Bay — it’s probably one of the most photographed spots in the country. It’s a state park,” said Geoff Schladow, the research center’s director. “And it’s an isolated population, so in a way it’s an experiment to see, ‘Can we hit these satellite populations and control them there?’ ”

Asian clams — which may have arrived on a boat or even a dumped aquarium — first turned up in large numbers about four years ago.

Though it’s too soon to know what their effect on other species will be, the clams have the potential to produce large algae blooms along the shore — clouds of green that can not only make Tahoe’s crystal waters less pleasing to the eye but in need of more filtering for drinking.

Plus, sharp, broken clam shells can make for one unhappy day at the beach.

“(The clams) filter out what’s in the water, algae, detritus and things like that. They concentrate it in their bodies, they consume a small amount of it, and then they excrete highly concentrated nutrients,” Schladow said.

“So what you have in the vicinity of these clam beds are locations of very high nutrients. Algae, which typically in Tahoe exist in small numbers, because the nutrient level is low, suddenly can flourish because of these high concentrations. It’s like an all-you-can-eat restaurant for them.”

The decision to try mats came in response to the question of how to control the invaders in a place where chemicals aren’t allowed and the water level can’t be lowered like a manmade reservoir. The idea has since been copied at Lake George in New York, which is battling an Asian clam problem of its own.

The mats are made of the same 1/16-inch-thick material used to line backyard ponds. They cut off oxygen to the clams, which burrow into the top few inches of silt.

Once the location of the furthest reaches of the glacier that carved out Emerald Bay, the uneven, sometimes rocky lakebed at the bay’s mouth presents a greater challenge than the flat areas where the researchers first tried out the mats.

Because oxygenated water flows through the silt to the clams, UCD is adding a second prong to its attack.

Before spreading out the rubber mats, divers unroll sheets made up of strands from aspen trees. The researchers believe the decomposing material will use up more oxygen, perhaps killing off clams more quickly.

Instruments in six locations will allow researchers to monitor nutrient levels under the mats. In another 48 spots, divers will be able to take samples from under the mat with a syringe.

The puzzle of how to modify the mats so they could be deployed and held in place fell to UCD research engineers Bill Sluis and Daret Kehlet.

Their lab is home to everything from the student concrete canoe building team to research looking at how rainfall moves pesticide down a sloped concrete surface. They’ve also built buoys that record meteorological data on the lake.

With their student workers, the engineers are unrolling the 100-foot-long, 10-foot-wide mats, then attaching grommets along each side. Divers later will use those when they zip-tie the mats to 10 miles of steel rebar, the weight of which will prevent the current from flipping the mats.

Each mat is riveted to two-inch plumbing pipe, which in turn holds a steel pipe strong enough not to buckle when the 300-pound rubber mats are hoisted over the water.

Sluis and Kehlet devised a system in which the ropes are attached by caps on the pipes. Once the rolled mat settles on the lake bottom, air will be shot through a hose into the pipe, popping the caps off. Then, divers can move in safely.

Readying the mats will take four to five weeks, their installation at the lake three or four. The work will be done on weekdays to interfere less with boating traffic heading into the bay, which can reach 1,000 boats a day during summer.

Schladow said the clams have invaded only “a couple of hundred acres” out of 191,588 square miles of lake.

Still, wiping them out would be all but impossible.

The researchers hope they’ll be able to kill off 99 percent of the clams under the mats, and eventually be able to hand off a refined process to about 40 public and private organizations who safeguard the lake. The Lahontan Regional Water Quality Control Board, U.S. Fish and Wildlife Service and U.S. Forest Service Pacific Southwest Research Station are funding the project.

Asked by a television reporter on Tuesday whether eating Asian clams might be one way to get rid of them, Schladow said they’re reputed to be less than tasty.

Plus, he added, “We need an invasion of linguine first.”

— Online: http://terc.ucdavis.edu

— Reach Cory Golden at cgolden@davisenterprise.net or 530-747-8046.