Birch Aquarium sent 223 endangered mountain yellow-legged frogs into creeks in the San Jacinto Mountains in August, the La Jolla facility announced Sept. 8.

It is the aquarium's second successful reintroduction of the species and brings its total to more than 440 frogs returned to the wild since joining the regional recovery program in 2024.

Staff at Birch Aquarium at Scripps Institution of Oceanography at UC San Diego received the frogs as tadpoles from the San Diego Zoo Wildlife Alliance (SDZWA) in summer 2025. They raised the animals behind the scenes at the La Jolla campus until the frogs were ready for release.

The first group of about 74 frogs went into a creek near Idyllwild in early August. A few weeks later, a second batch of 149 frogs was released along another creek a few miles away. SDZWA added two frogs and 170 tadpoles of its own at the second site.

The results were encouraging.

Sean Bruce, assistant curator of fishes and invertebrates at Birch Aquarium, said the team found adult frogs at the first release site that SDZWA had placed there as tadpoles in earlier years. Researchers tagged one of those adults to track its progress.

"Spotting wild frogs at one of the release sites was especially meaningful, as it was an encouraging sign of a healthy habitat and gave me hope that our newly released frogs can thrive there, too," Bruce said.

The U.S. Fish and Wildlife Service listed the frog's Southern California population as endangered in 2002. Wild adult numbers have dropped roughly 98 percent. Surveys in 2023 counted fewer than 200 adults remaining, Mongabay reported in October 2025.

In an interview with Mongabay at the time, Bruce described the scale of the decline: "There were hundreds of locations historically where these frogs were present, and now we're down to only a few left in Southern California."

Researchers from SDZWA and the U.S. Forest Service joined Birch Aquarium staff at both release sites. Partners in the long-running recovery program also include the U.S. Fish and Wildlife Service, U.S. Geological Survey and California Department of Fish and Wildlife.

Researchers plan to monitor the release sites several times a year.

Visitors can learn more about the frog's story at Birch Aquarium's "Imagine Wild: Mountain Yellow-legged Frog" exhibit, an immersive installation recreating the species' mountain habitat. The exhibit is presented by The Lodge at Torrey Pines.