SOUTH LAKE TAHOE ” As measures to prevent the introduction of aquatic invasive species into Lake Tahoe increase, one invader ” which already is here ” has begun to draw greater attention from scientists and land managers.

Asian clams are a “larger problem than we thought,” John Singlaub, executive director for the Tahoe Regional Planning Agency, said at August’s Governing Board meeting.

An April survey found living clams or shells at seven out of 10 sites examined at Lake Tahoe’s South Shore, with some of the densest patches found north of Nevada Beach.

A more comprehensive survey is being developed with the Tahoe Resource Conservation District to get a better idea of where the clams exist around the lake.

Lake Tahoe has Asian clam beds with densities as high as 1,000 clams per square meter, said TRPA spokesman Dennis Oliver.

“The clam beds are a big concern,” he said.

One of the fears surrounding the clams is that they will help introduce quagga mussels into the lake.

The quagga mussel, and its close relative the zebra mussel, have caused billions of dollars of damage to Midwestern waterways and have been found in Western states since early 2007.

It’s unknown if the mussels could thrive in Lake Tahoe’s calcium-poor environment. The mussels need approximately 12 parts per million of calcium to survive, Oliver said.

While the lake’s average calcium levels are about 10 parts per million, the water near dense clam beds can have calcium levels as high as 26 parts per million, Oliver said.

UC Davis researchers also have pointed to the clams as a possible source of nutrients that fed an algae bloom in Marla Bay this summer.

TRPA staff had hoped to retrofit a boat used to remove invasive Eurasian watermilfoil from the lake with a suction device to remove Asian clams, but the extent of the problem may require a larger vessel, Oliver said.

TRPA staff are hoping to include removal of aquatic invasive species into an update to the Environmental Improvement Program so a “steady stream” of funding can be applied to the problem, Oliver said.

The clam is yellowish-brown to black with evenly spaced ridges on the shell’s surface, which is smaller than 55 millimeters across, according to information in the May survey.

Asian clams are native to parts of the Mediterranean, Australia and Asia, but have spread throughout much of North America after being discovered in 1938 along the Columbia River in Washington state, according to the survey.

Observations by University of Nevada, Reno, researcher Sudeep Chandra suggest the clams have been in Lake Tahoe since at least 2002. It’s unknown how the clams got into the lake, although they have been known to attach to boat hulls or be transported in bait buckets, according to the survey.