Escondido, California - Just before dawn on Thursday, a team of staff and volunteers from the San Diego Zoo Institute for Conservation Research was preparing to load and transport young frogs and tadpoles from the Arnold and Mabel Beckman Center for Conservation Research in Escondido, to release them into the wild at Fuller Mill Creek in the San Bernardino National Forest.

The process is part of the recovery program for the Southern California mountain yellow-legged frog (Rana muscosa)—one of the most endangered frogs in North America. The San Diego Zoo Institute for Conservation Research, along with its partners, has been working to save this species from extinction since 2006.

This year, the program, which includes captive breeding, resulted in more than 5,600 eggs laid and the most viable embryos in a single season: nearly 1,800. On Aug. 6, 200 tadpoles that hatched this year, along with 27 metamorphs from last year’s breeding season, were released into Fuller Mill Creek.

The animals were moved into buckets with a few inches of chilled water, and kept cool with ice packs underneath the buckets. Water temperatures in the buckets were kept close to 50 degrees Fahrenheit, to match the facility where the frogs were hatched and the wild streams where they would be released later that morning. Buckets with frogs and tadpoles were transported to the release site in a temperature-controlled vehicle.

The release site was surveyed by staff from the Zoo’s Institute for Conservation Research, the U.S. Fish and Wildlife Service and the U.S. Geological Survey, who looked for the deepest pools with some overhangs where the frogs naturally like to live. The deep-water locations with banks are also the least likely to dry out before fall, when rain is expected. The team carried the buckets to three pools that had been selected as the release site, and measured the temperature of each pool.

Water from the pools was added to each bucket—about one-half inch to one full inch—to slowly bring the temperature and pH level of the bucket water into alignment with the creek water. Staff gave the frogs five minutes to acclimate before adding more creek water. This process continued until the temperature and pH levels of the water in the transport buckets matched the creek. Then, staff slowly poured the frogs into the pools.

“We vaccinate the frogs before release, to provide some resistance to the Chytrid (Chytridiomycota) fungus once they are released into the wild,” said Debra Shier, associate director of Applied Animal Ecology at the San Diego Zoo Institute for Conservation Research. “Our goal with captive breeding and reintroduction is to facilitate species recovery by increasing the numbers of mountain yellow-legged frogs in the wild, as well as buying the species time to evolve resistance to the fungus.”

In addition to breeding the endangered frogs, the San Diego Zoo Institute for Conservation Research participates in the field monitoring of the species, led by the U.S. Geological Survey. The monitoring is critical to determining if the releases are successful, and documenting population declines and increases in this native Southern California species.

The mountain yellow-legged frog is a species watched over by a team of scientists, land managers and regulators, while it maintains a perilous toehold in the mountains of Southern California. Mountain yellow-legged frogs in Southern California live in perennial streams in portions of the San Gabriel, San Bernardino, and San Jacinto Mountains. The upper-elevation stream segments inhabited by the frogs are generally 1,214 to 7,546 feet above sea level.

The U.S. Fish and Wildlife Service listed the mountain yellow-legged frog in Southern California as endangered in 2002, and fewer than 200 of the frogs remained by 2003. Efforts to boost the species’ population have included captive breeding, reintroducing captive offspring to historic habitat, regular surveys to assess the species status and conducting scientific research into the causes of the species’ decline.

Myriad factors have influenced the species’ decline, including introduced trout and bullfrogs, pesticides, large wildfires that bury the species’ stream habitats in ash and debris, and recreational activities that can impact frog recovery by damaging egg sacs when people swim or cross streams. And, while land managers are making headway on removing some of these threats, there is an even more serious one that cannot be removed: a fungus called Chytridiomycota. Chytridiomycosis, a disease that occurs when an amphibian is infected with a large amount of the fungus, is a serious threat to thousands of frog species in the U.S. and around the world, and will never be removed from the wild.

Bringing species back from the brink of extinction is the goal of San Diego Zoo Global. As a leader in conservation, the work of San Diego Zoo Global includes on-site wildlife conservation efforts (representing both plants and animals) at the San Diego Zoo, San Diego Zoo Safari Park, and San Diego Zoo Institute for Conservation Research, as well as international field programs on six continents. The work of these entities is made accessible to children through the San Diego Zoo Kids network, reaching out through the Internet and in children’s hospitals nationwide. The work of San Diego Zoo Global is made possible by the San Diego Zoo Global Wildlife Conservancy and is supported in part by the Foundation of San Diego Zoo Global.