It started with the trout.

Until the late 19th century, the Sierra Nevada was mostly fishless above the waterfalls. But state policy of fish stocking eventually climbed to the high Sierra to transform those "barren" lakes into a fisherman's paradise. The California Department of Fish and Game began sending trout up the cliffs, first in barrels on muleback, and by the 1950s in the bellies of airplanes. (The planes would fly over the water and let drop their living cargo, much of which missed its mark and was left flopping on dry land.) All told, more than 17,000 mountain lakes were stocked.

As it turns out, trout eat tadpoles and young frogs. As trout multiplied, frogs disappeared.

Vredenburg's work in Sixty Lake Basin became an attempt to restore the lakes to their pre-1900s fishless status in order to bring back the frogs. He unfurled wide nets bank to bank, reeled them in, and disposed of the catch (often on the grill with a little salt and pepper). Eventually the National Park Service took over the project, and now 14 lakes are fish-free or virtually so. As more fish were netted out, Vredenburg says, the "frogs started to recolonize; the lakes were coming back to life."

But then came another blow. Chytrid, which had already invaded Yosemite National Park, arrived in Sixty Lake Basin and swept from lake to lake, around a hundred of them, in a predictable and deadly line. After removing fish and restoring habitat, "to have this disease wipe the frogs out again—it breaks my heart," he says.

Oddly, the fungus infects but doesn't kill tadpoles, which is why wriggling schools remain in otherwise lifeless ponds. Mountain yellow-legged frogs take some six years to mature. "Those tadpoles are from years ago—there's been no breeding in this pond since chytrid arrived," Vredenburg explains. "As soon as they transform into frogs, they'll die."

Yet Vredenburg remains doggedly optimistic. He calls pond number 8 his victory pond. When he saw the frogs start to die, he removed some of the adults and treated them with an anti­fungal medication, then put them back. The population—though tiny—has now been stable for three years running. Vredenburg plans to apply his painstaking capture-treat-release method to animals in other ponds in Sixty Lake Basin. (Recently announced, a similar treatment project by a U.K. team aims to mitigate disease in the Mallorcan midwife toad of Spain.) If enough fungal spores can be cleared from frogs' bodies, he says, the disease may lose its hold.