Biologists at the Aquarium of the Pacific in Long Beach are working to bring back the mountain yellow-legged frog, a species that has nearly vanished from Southern California. In a quiet room with three water tanks, scientists are raising these frogs in hopes of restoring their population, which once thrived in the region’s mountain streams. The program, now in its fifth year, has achieved a major milestone: more than 1,000 frogs have been raised and released into the wild. This number surpasses five times the estimated wild population in 2019, when only a few hundred frogs were believed to exist in scattered pockets of the San Gabriel, San Bernardino, and San Jacinto Mountains. The mountain yellow-legged frog was listed as endangered in 2002, with fewer than 100 adults remaining in Southern California at the time. Though a 2020 survey found around 188 frogs in the wild, biologists believe that number has likely changed due to ongoing threats. These include wildfires, mudslides, pesticides, fungal diseases, habitat loss, and competition from non-native species like trout, bullfrogs, and crayfish. These combined pressures have made the mountain yellow-legged frog one of the rarest vertebrates on Earth, prompting conservation efforts to reverse its decline. In the most recent release, about 653 frogs were returned to their native habitat by teams from the Aquarium of the Pacific, Los Angeles Zoo, and the U.S. Geological Survey. Of those, 394 had been raised from tadpoles at the aquarium, marking the largest release of the species to date. The frogs were originally bred at the Los Angeles Zoo, then transferred to Long Beach, where they were raised through their development from tadpoles to adult frogs. This process can take one to two years, though some frogs may remain in their tadpole stage for up to five years, making their exact age difficult to determine. The aquarium’s efforts began in 2021, following the 2020 Bobcat fire, which devastated nearly all of the frog’s remaining habitat in the San Gabriel Mountains. Biologists at the aquarium helped raise some tadpoles that had survived the fire, as well as captive-bred frogs from the Los Angeles Zoo. Since then, the project has expanded with collaboration from the U.S. Fish and Wildlife Service, the U.S. Forest Service, and the California Department of Fish and Wildlife. Some of the frogs raised at the aquarium are now sent to the Santa Ana Zoo each October to continue growing before being reintroduced to the wild. Careful genetic tracking ensures that each frog is released into its native range, preserving the health of the population. A major challenge for the biologists is the chytrid fungus, a microscopic pathogen that attacks amphibians by interfering with their ability to absorb salt through their skin, often leading to death. This fungus has caused amphibian declines worldwide, and mountain yellow-legged frogs appear especially vulnerable. Researchers are exploring ways to help frogs resist the fungus, including testing whether inoculating them before release or introducing beneficial microbes to their skin might offer protection. For Erin Lundy, a conservation biologist at the aquarium, the releases are both meaningful and bittersweet. She has participated in three release trips, watching frogs she has cared for over years being set free into the streams they were bred to repopulate. “It feels a little bit like Christmas Day,” she said. Some frogs quickly dart into the water, while others linger at the edge for a moment, almost as if saying goodbye.