Fish Versus Frogs in the High Sierra

by Cliff Riebe

In a plight that scientists and policymakers are scrambling to understand and reverse, mountain yellow-legged frogs are vanishing at alarming rates from high-altitude Sierra Nevada lakes. The species has historically lived in an area ranging from northern Plumas County to southern Tulare County, with additional isolated populations in Southern California mountains. The frogs were once so abundant that, according to a 1915 report by biologist Joseph Grinnell, you could hardly avoid stepping on them around some of the more densely populated Sierra lakes. Scientists suspect that the frogs began disappearing about three decades ago. A recent, dramatic acceleration in their decline has put the frogs on the Global Amphibian Assessment's list of amphibians considered to be threatened. Sierra high country visitors can now expect to encounter mostly frogless lakes, or, worse, shores littered with frog carcasses. Scientists now fear that the mountain yellow-legged frog faces imminent extinction, despite recent efforts to preserve and restore its habitat.

What's killing the frogs? According to Dr. Roland Knapp of the Sierra Nevada Aquatic Research Lab (SNARL) in Mammoth Lakes, biologists have identified several factors, including introduced species, pesticide fallout and outbreaks of a devastating new fungal disease.

The introduction of non-native trout for sport fishing in the Sierra Nevada began in the late 1800s, and intensified dramatically in the 1950s and 1960s with the use of airplanes. But outside of a few anecdotal accounts of voracious trout wiping out native frogs, little was known about the distribution of fish or their effect on native Sierra species until about ten years ago, when Knapp and other biologists at SNARL began surveying fish and frog populations in more than 7,000 Sierra lakes. Initial results showed that frogs were nearly absent in lakes of the John Muir Wilderness, where fish were flourishing thanks to a decades-old policy of intensive stocking. By contrast, frogs appeared to be doing much better in the Yosemite, Sequoia and Kings Canyon National Parks, where stocking had generally been more restrained and was terminated altogether starting in the late 1970s. Ongoing surveys, including many from Tahoe area lakes, continue to show that non-native trout and the few remaining mountain yellow-legged frog populations don't often overlap, leaving little doubt that non-native fish introductions were at least partly responsible for the decline of the frogs.

According to the studies of Dr. Vance Vredenburg, of the University of California, Berkeley, frogs returned and began to flourish within just one to three years after fish were eliminated. Frog recovery was also observed in a similar, independent study by Knapp's group. Policymakers and scientists with the California Department of Fish and Game (CDFG) and the National Park Service have taken note of the frogs' decline and are now experimenting with fish removal as well.

According to biologist Curtis Milliron of the CDFG, the number of backcountry lakes that are stocked in the eastern Sierra Nevada has been reduced from about 225 to 75 as a result of new policies. In Desolation Wilderness and the northern Sierra Nevada, backcountry stocking was halted altogether as of 2001, according to Stafford Lehr of the CDFG. Stocking resumed in 2004 at select lakes, but only after the CDFG determined that no frogs would be endangered.

The fish removal experiments and changes in stocking policies have some backcountry anglers worried that sport fishing in the high Sierra will be lost. But eliminating an established fish population from even the smallest lake is no small chore. Manual removal, using gill-nets and electro-fishing, is labor-intensive. Moreover, the CDFG has no intention of removing fish at more than a handful of lakes, according to Milliron. "Anglers can definitely expect to see some changes," said Milliron, "but there will still be lots of places with good fishing." According to Lehr, the revised policies will actually improve some fisheries. Less stocking in lakes with self-sustaining fish populations will lead to less competition for food, and ultimately, to bigger fish. Recent findings suggest, however, that despite the creation of fish-free habitats the frogs are still disappearing rapidly, at a rate of about 10 percent per year.

According to Lara Rachowicz, of the University of California, Berkeley, much of the ongoing frog die-off can be explained by deadly outbreaks of chytridiomycosis, a disease caused by the chytrid fungus. Chytrid has been implicated in recent amphibian declines elsewhere in the world, but little is known about it. Researchers aren't even sure yet how it spreads from basin to basin. One thing they do know: it moves fast. In just one year, Knapp's group witnessed the near-complete chytrid-induced annihilation of frogs from all lakes in a remote southern Sierra basin. Frustratingly little can be done to stop the spread of chytrid, Rachowicz says.

Ongoing studies are testing the theory that chytrid has long existed in the ecosystem but is only now affecting frogs weakened by other factors, such as pollution and habitat loss. Non-fungal diseases may have also contributed to recent frog die-off in the Sierra. At least one lake's frogs were wiped out by red-legged disease, caused by a bacterial pathogen, according to a study by Dr. David Bradford of the U.S. Environmental Protection Agency. Wind-borne pesticide fallout from the Central Valley has recently been implicated as yet another significant factor in frogs' decline at many lakes. Scientists also list climate change and UV radiation as potential threats to the frogs.

It is estimated that the mountain yellow-legged frog population has declined by 90 percent overall. The U.S. Fish and Wildlife Service recently listed the Southern California populations as officially endangered. Northern populations are on the candidate list for endangered species status. However, even with the protection of the Endangered Species Act, the frog will not survive long if mortality from the rapidly spreading chytrid fungus remains high, according to Knapp. On the brighter side, say scientists, it looks as if other native species will benefit mightily from the changes in policy inspired by the mountain yellow-legged frog's plight.