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On the Rebound
The removal of nonnative fish from several parks is helping the mountain
yellow-legged frog jump back from an 80-percent decline.
BY ELIZABETH DAERR
It's a painful trade-off—killing fish to save frogs. But
years of research among the high elevation lakes of the Sierra Nevada have
proven that allowing stocked, nonnative fish to remain in once-fishless
lakes means certain extinction for a unique frog that is an important link
in the web of life above 8,000 feet.
In the last century, the mountain yellow-legged frog, Rana
muscosa, has led the declining populations of amphibians throughout the
Sierra Nevada, thinning to 20 percent of its historic territory. [Muscos
is Latin for "smelly" and describes the strange garlic-like odor
given off by male frogs during the breeding season.] Scientists believe
that the species has evolved over 1 million years with the advance and
retreat of glaciers in the area. As the ice sheets receded, glacial lakes
formed, and the accompanying steep waterfalls prevented fish from
colonizing any of the lakes.
Beginning in the 1850s, however, settlers began stocking the
lakes with trout to fish for food and recreation. The practice continues
even today in all national forest wilderness areas, and nearly 80 percent
of the Sierra Nevada lakes now contain nonnative fish.
Trout have been particularly problematic to this frog because
of its extended metamorphosis. Unlike frogs that mature from a tadpole to
an adult in one year, the mountain yellow-legged frog takes three years.
This requires them to remain in water deep enough to avoid summer drying
and winter freezing; here, they become prey for the fish.
Much of the frog's remaining habitat lies within Yosemite and
Sequoia/Kings Canyon national parks where the Park Service has stopped all
stocking and is removing established fish populations. In three years,
aquatic ecologist Roland Knapp and a crew of 30 volunteers have surveyed
nearly all of the parks' water bodies for fish—an impressive feat
considering that there are more than 3,000 lakes in Yosemite alone. They
use gill netting to kill the fish. "I abhor it," says Knapp.
"Killing species is the antithesis of why we got into this work. But
you swallow your discomfort and go do it," he added. The method is
safer, however, than the previous alternative of poisoning the lake, which
not only killed the fish but many other species, said Annie Esperanza, an
air quality specialist who is working on the project at Sequoia/Kings
Canyon.
Although biologists say the evidence is overwhelming that
predation by fish is taking its toll on the species, other factors are
being researched. Pesticides and herbicides used for agriculture in the
Central Valley may be compromising the frogs' immune systems. Scientists
are also working to understand the possible impact of chytrid fungi, which
are commonly found in the soil but have not been known to attack
amphibians until recently. The fungi have been linked to die-offs and
extinctions in Australia and Central America. Red-leg disease, caused by
the bacterium Aeromonas hydrophile, has also increased, and some
scientists believe that the frogs are susceptible to it when stressed. As
its name suggests, the infection turns their legs red and is fatal.
The species has yet to be listed under the Endangered Species
Act even though a petition has been filed. According to U.S. Fish and
Wildlife Service spokeswoman Pat Foulk, a backlog of court-ordered
listings has prevented the agency from pursuing research on the frog. The
agency is working to push through the current petitions to get to the
frog.
Initial results from fish removal projects indicate some
success. A lake Knapp surveyed in 1996 had 20 frogs. All the fish were
removed by 2000, and by last year 750 frogs were found. "Now when you
walk the lake, they jump into it—sometimes five at a time."
ELIZABETH G. DAERR is news editor.
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