Rubber mats laid atop beds of Lake Tahoe's invading clams seem to have done their deadly job, according to reports from the Tahoe Resource Conservation District.In early November, divers removed mats from two half-acre-sized swaths of clam-infested lake bottom. While data still are being analyzed, the test indicates the mats appear to be the most effective weapon against Tahoe's Asian clam population, experts have said.
"There are definitely lots of visibly dead clams," said Kim Boyd, an invasive species specialist with the Tahoe Resource Conservation District. "It looks very promising."
The mats, measuring 10 feet wide and 100 feet long, kill clams by robbing them of oxygen and nutrients. The mats were laid on the lake bottom in July at Marla Bay and South Lake Tahoe's Lakeside Marina.
The $648,000 experiment was built on a previous test in 2009 suggesting "bottom barriers" were effective in killing the dime-sized clams. But this one targeted them on a larger scale. Scientists first noticed Asian clams in Tahoe's waters in 2002, but they have since exploded in number, infesting a large part of the southeast corner of the lake. They also have become established on a football field-sized piece of lake bottom near the mouth of Tahoe's landmark Emerald Bay.
Clams can adversely impact Tahoe's sensitive ecology and are linked to blooms of noxious algae. A bigger danger, experts say, is that the clams could elevate calcium levels in the lake's waters to the point they could support populations of quagga or zebra mussels, which would have devastating effects on Tahoe's ecology and economy.
"The fear is Asian clams could facilitate the introduction of the quagga," said Geoffrey Schladow, director of the University of California, Davis Tahoe Environmental Research Center.
Researchers now will study the areas the mats were laid to determine how quickly Asian clams recolonize on the bottom. They also want to study long-term impacts on other macroinvertebrates native to the lake, such as pea clams, Boyd said.
Other, smaller tests are planned in Marla Bay to gauge effectiveness of the mats in killing Asian clams during the winter, when cold water temperatures are thought to shut down their metabolism and make them invulnerable to this particular method of control, Boyd said. If bottom barriers still can be effectively used in winter, she said they would prove much more valuable over the long run.
By late February, researchers plan to install some barriers in Emerald Bay to determine their stability when encountering underwater turbulence produced by paddle-wheel tour boats, Boyd said. Scientists said they hope to begin using the mats to kill Emerald Bay's clams after next summer's boating season.
While the barriers appear effective, Asian clams probably will always have an unwelcome presence at Lake Tahoe, Schladow said.
"Eliminating them probably isn't feasible. Controlling them could be possible," Schladow said.
Photo: UC Davis
