U.S. Fish & Wildlife Service

Sacramento Fish & Wildlife OfficeServing the people, conserving the fish, wildlife, and plants of California

A Unit Of The Pacific Southwest Region

Rescue for Recovery

Endangered mountain yellow-legged frogs get an emergency evacuation for treatment. Photo by Chris Templeton / NPS

Perched at the edge of a tiny mountain lake in the high Sierra on August 27th, the bright yellow helicopter sat ready to assist a dramatic rescue operation. Two biology technicians carefully carried buckets and a large cooler across the rocky landscape towards the waiting pilot and wildland firefighting crew. The helicopter team loaded the containers, waved goodbye and lifted off. They skimmed down from the remote high country of Sequoia and Kings Canyon National Parks, on a mission to deliver the precious cargo of critically endangered juvenile mountain yellow-legged frogs and tadpoles to waiting staff from the San Francisco and Oakland Zoos.

Species in Peril

Dr. Roland Knapp of the University of California, Santa Barbara was hiking in the Sierra Nevada twenty five years ago when he first saw mountain yellow-legged frogs, jumping before him on the trail and peeking up from the shallow lake. Dedicating his career to studying the amphibians, he struggles to describe the changes he has witnessed. "Places that we've gone to for years and years and seen thousands of frogs are now completely without frogs…and it's an incredibly dramatic and challenging transformation of that landscape."

Photo by Isaac Chellman / NPS

The rapid and severe decline of the Northern California Distinct Population Segment of mountain yellow-legged frogs led the US Fish and Wildlife Service (Service) to list them for protection under the Endangered Species Act in 2014. The Service is currently working with a number of partners to fight the species’ further deterioration. “Beginning in 2011, the US Fish and Wildlife Service took a leadership role in the Interagency Conservation Strategy to help this species,” says Dr. Steven Detwiler, Senior Scientist with the Endangered Species Program at the Sacramento U.S. Fish and Wildlife Office. The Service is collaborating with several partners including the National Park Service, U.S. Forest Service, California Department of Fish and Wildlife, affiliated experts from the University of California, and groups such as the San Francisco and Oakland Zoos.

Although the strategy has yet to be finalized, the team has been hard at work trying to save the frogs. "We have been actively coordinating on priority recovery actions, among them captive rearing and disease intervention," says Dr. Detwiler.

A Deadly Disease

Introducing predatory fish to mountain lakes has had a devastating impact on the frogs, but their latest challenge comes from an emerging, highly infectious fungal disease of the skin called chytridiomycosis (chytrid). "Every continent that has amphibians has the disease. It has spread around the world very quickly," says Dr. Knapp, a research biologist with the Sierra Nevada Aquatic Research Lab at UC Santa Barbara.

Within 15 years, chytrid has led to the extinction or decline of over 200 species of frogs and salamanders. While researchers cannot eradicate the disease, they can investigate ways to help amphibians develop resistance and support declining populations until they can recover.


This summer's rescue targeted two populations of mountain yellow-legged frogs in Sequoia and Kings Canyon National Parks. "Both sites are in danger of being extirpated," explains Danny Boiano, Aquatic Ecologist with the National Park Service (NPS). "If this emergency salvage did not occur, considerably more than 90% of these metamorphosing individuals might not survive, and we could lose both of these critical populations," agrees Dr. Detwiler.

Photo by Larissa Perez / NPS

Chytrid begins to affect frogs as they mature into adults, which may take up to three years in the wild. Even without the threat of disease, many tadpoles may not survive the long journey to adulthood. By collecting early-stage froglets and tadpoles and rearing them in captivity, biologists can intervene and give the frogs a "head start." Juveniles that are vulnerable in the wild develop in the safety of a zoo's lab, where they are inoculated against the disease, mature in just six months, and can then be reintroduced back into their native habitat.

If enough frogs develop a resistance to the disease, they may be able to rebuild their decimated numbers over several years. "But time isn't on our side," notes Boiano, as he describes the challenge of supporting infected frog populations until they can recover. "We might wait two decades for a population of 200 frogs to naturally recover in a diseased population. Head-starting allows us to introduce many more adult frogs that are disease-resistant. Instead of 200 in twenty years, we may give those populations 200 frogs in one year."

Boiano and Detwiler collaborated to arrange and coordinate the rescue in partnership with staff from the San Francisco and Oakland Zoos. After the Strategy team identified the two locations as a high priority in the spring of 2015, they had to wait until just the right time in the frogs' development and then implement the high-mountain evacuation rapidly. "It was through having the working elements of the Conservation Strategy in place that we were able to communicate so quickly, work as a team, and respond in a timeframe that made this rescue feasible," Dr. Detwiler describes.

Special Delivery

Collecting and delivering the tadpoles and froglets to their new home at the zoo involved intricate logistics. Both locations where the frogs were in trouble are incredibly remote and only accessible by hiking. Two teams of NPS biology technicians camped on site and began collecting tadpoles and juvenile frogs early in the morning, keeping them moist and protecting them from the sun. They were able to capture 80 animals from one location that has suffered a slow decline over the past 10 years, and 190 animals from another location that recently became infected.

Photos by Chris Templeton and Larissa Perez / NPS

Down at the Ash Mountain helibase, an NPS helicopter-based team that spends most of their summer fighting wildland fires prepared instead to transport a delicate, endangered species. Nearby, Victor Alm, Zoological Manager of Oakland Zoo, and Jessie Bushell, Director of Conservation at San Francisco Zoo, staged their vehicles and waited.

From their alpine home, 270 young frogs successfully traveled towards treatment. "They were essentially given a life-flight out to us. These frogs most likely would have perished without intervention," according to Alm. The Zoo staff met the returning helicopter, divided the populations and drove them back to the city: 79 tadpoles and froglets went to Oakland, and the remaining animals went to San Francisco.

Building Resistance

Marianne Hale / SF Zoo

"These animals came into the zoos highly infected," says Jessie Bushell. "The minute they entered, we started treating them with anti-fungal medication." When infected frogs metamorphose from tadpoles to adults, they can slowly suffocate as the chytrid disease causes their fragile skin to thicken. Caretakers can stop the infection over a two week process by bathing the young frogs for 10 minutes a day in a solution of Itraconazole, a common drug that kills fungal pathogens. If the frogs are still infected after the two week process, they will undergo treatment until they are clear.

The zoos will continue to care for the frogs over the next year, raising them to adulthood and keeping them healthy. Bushell describes the ultimate goal to release the frogs next summer with added protection: "we have developed, in collaboration with UC Santa Barbara and researchers working on this project, an experiment to try and immunize these frogs." The zoos re-infect the frogs short-term with chytrid, let them build up a minor infection, monitor them, and then treat them aggressively. Over a course of several months of this treatment, the frogs appear to develop a resistance to chytrid. "And that is the key part to making sure that these populations that we’re putting back out in the wild have a chance at survival."

Helping a Backyard Species

Both Oakland and San Francisco Zoos have enthusiastically volunteered their facilities and staff, excited by the opportunity to help a local species recover. "The [Oakland] Zoo gets the benefit of helping a California heritage species, a local animal that lives just a few hours away in the Sierra Nevada," says Victor Alm. "Saving animals in our backyard is a really unique opportunity to make a difference in our community," agrees Jessie Bushell, "we have these cool frogs, these special animals that only live here."

Photo by Isaac Chellman / NPS

Dr. Detwiler visited both zoos this winter and got a first-hand look at the treated frogs. "Seeing the actual animals we were able to save brought it home for me how the partnerships we have developed facilitate these important conservation actions," he says. Detwiler gratefully acknowledges the volunteer efforts of the zoos, and the collaboration among all of the partners who made this rescue a success.

Ensuring that the frogs recover and can be enjoyed by future generations is a shared goal among the rescue team. A healthy population of mountain yellow-legged frogs will also help rebuild the high mountain aquatic ecosystems, where the frogs are a keystone species and their decline has had a major impact.

For the Sequoia and Kings Canyon National Parks animals, the future looks hopeful: the majority have already metamorphosed into active frogs and are gradually building their immunity for a return to the wild in the summer of 2016.

Story by Michelle Donlan and Lisa Hupp, U.S. Fish and Wildlife Service

Last updated: December 6, 2017