Invasive smallmouth bass have become established in Lake Tahoe, posing what researchers call a "much more ferocious threat" to native fish than anything that has come before.

The reason is that smallmouth are more adaptable to cold temperatures and varied spawning conditions than other invaders already present in the high-Sierra lake, including largemouth bass, crappie and bluegill.

"We've had reports of smallmouth bass before, but now we've 100 percent confirmed its presence in Lake Tahoe," said Kevin Thomas, environmental scientist at the California Department of Fish and Game. "The population could explode and put more stress on the native fish."

If that were to happen, smallmouth bass could deplete native minnows in the lake, such as Lahontan redsides. These small fish are important food for Tahoe's prized sportfish, including Mackinaw trout and Kokanee salmon. While these species are also not native, they underpin a summer sportfishing economy worth at least $1 million a year, said Ed Sheetz, a longtime fishing guide at the lake.

"It would be disastrous if we had the bass in large numbers," said Sheetz, whose business is based at the Tahoe Keys Marina in South Lake Tahoe. "I think the bass need to go."

The finding was announced Monday by the University of Nevada, Reno, which is engaged in a partnership with Fish and Game and the U.S. Fish and Wildlife Service to eradicate warm-water invasive fish at the lake, including largemouth bass, crappie and bluegill. The project uses electroshocking equipment and gill nets.

Anglers have reported catching smallmouth on occasion over the past year, Thomas said. But one was caught for the first time during the eradication program, in early June.

That smallmouth was caught in the Tahoe Keys, a 25-mile network of man-made canals that winds through a housing subdivision. The warm, shallow water of the Keys has become a haven for invasive fish.

The smallmouth is a greater threat because it can more readily spread beyond the Keys, Thomas said. It tolerates colder water, and it can breed in sand, gravel beds and rocky outcroppings, which are numerous at Tahoe.

Sudeep Chandra, a UNR limnologist, found that the problem may be linked to water pollution, a decades-long problem that is reducing Tahoe's famous clarity.

Nonnative fish, including smallmouth bass, have less tolerance to ultraviolet radiation from the sun. As Tahoe waters become cloudier from algae and pollution, UV rays don't penetrate as deeply into near-shore waters.

The news came on the eve of an annual Tahoe Summit held Tuesday at Homewood, where politicians and policymakers discussed projects to reverse environmental decline at the lake.

In a statement, Chandra urged planning officials to begin monitoring aquatic ultraviolet levels as a measure of water quality. This, he said, would be more useful and precise than the Secchi disk employed since the 1800s. The disk is basically a round, white plate lowered into the water. Depth is measured until the plate can no longer be seen from the surface.

"Given the potentially high cost of a full-scale warmwater fish invasion, any expense to implement these largely preventative measures is money well spent," Chandra said.

No one knows for sure how the smallmouth bass arrived at Tahoe. Thomas believes that – like the largemouth, bluegill and others – it was probably intentionally introduced by a misguided angler hoping to start a new fishery, or by someone dumping out an aquarium.

One advantage, he said, is that smallmouth bass generally take a long time to proliferate, so early control efforts can be successful.

The current eradication program costs about $104,000 a year, Thomas said, and is funded through 2012. More funding is needed to continue the program beyond that.

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