SOUTH LAKE TAHOE  (AP) — A mandatory boat inspection program continues to pay dividends for Lake Tahoe, a conservation agency said, as 36 boats harboring invasive plants, mussels and snails were prevented from entering the lake in 2013.

In all, 4,221 boats were decontaminated with hot water at roadside inspection stations last year to prevent invasive species from entering Tahoe’s waters, according to the California Resource Conservation District

The results are a credit to the inspection program, said Patrick Stone, senior wildlife and fisheries biologist for the Tahoe Regional Planning Agency.

“We’re very happy with the watercraft inspectors’ diligence and accuracy,” he said in a statement.

In 2013, inspectors performed more than 7,000 new inspections as over 14,000 boats launched at Tahoe, including both newly inspected vessels and those with intact Tahoe-issued inspection seals.

Boat inspections began at Tahoe in 2008 to prevent the spread of invasive species such as quagga and zebra mussels into its pure, blue waters.

The two dreaded mussels native to Eastern Europe can overwhelm a lake’s natural ecosystem and cause blooms of noxious algae that reduce a lake’s clarity. Scientists have documented a steady and disturbing decline in Tahoe’s famed clarity for more than 40 years.

The Tahoe Regional Planning Agency has been concerned about the threat since the quaggas first turned up in Lake Mead in 2007. They have since spread to other water bodies in southern Nevada and California.

Zebra mussels were found in 2008 in California’s San Justo Reservoir, about 250 miles from Lake Tahoe, but so far neither invader has been detected in the lake.

Local officials have expressed growing concern as more bodies of water in the West have shown evidence of invasive mussels and other nonnative invaders.

Last year, the Nevada Department of Wildlife confirmed the establishment of New Zealand mud snails in sections of the Truckee River around Reno. The river flows more than 100 miles from Tahoe to Pyramid Lake, about 30 miles northeast of Reno.

Watercraft inspectors also staffed Boca, Prosser and Stampede reservoirs near Truckee, Calif., and assisted at Donner Lake — all in the Truckee River’s watershed.