Above: a researcher releases mountain yellow-legged frogs in a 2013 release.
On June 19, 2018, 259 endangered mountain yellow-legged frogs (MYLF) were reintroduced in the San Bernardino Mountains. This was the largest reintroduction of froglets by San Diego Zoo Institute for Conservation Research in the program’s reintroduction history!
This release is part of a managed care breeding program started by San Diego Zoo Global in partnership with the U.S. Fish and Wildlife Service, U.S. Geological Survey (USGS), California Department of Fish and Wildlife, San Bernardino National Forest, and other zoological institutions, including the Oakland Zoo and the Los Angeles Zoo. The program began in 2006 when scientists from USGS rescued 80 tadpoles from a drying creek bed in the San Jacinto Mountains and brought them to the San Diego Zoo Institute for Conservation Research. Since then, thousands of frogs have been bred and released back into the region.
This release was also notably different from our past reintroductions, in that the frogs were divided into either a soft or a hard release group. Frogs in the hard release group were reintroduced into the stream directly from the transport bucket, with little to no acclimation period. Whereas the froglets in the soft-release group were housed for seven days in soft release enclosures, which were custom built to provide a predator-protected environment where the frogs could acclimate to the environmental conditions of the release site.
A hard release is how we typically release the mountain yellow-legged frogs. Data from frogs recaptured during last year’s post-release surveys showed that hard-released froglets moved an average of 397 feet (121 meters) in just 3 months following release. The purpose of this experiment was to examine how a soft vs. hard release may affect long distance dispersal movements, stress levels and establishment of long-term territories at the release site.
With the help of a few volunteers, we deployed six soft release cages into a stream in the San Bernardino Mountains. Each enclosure was approximately 4.9 x 2.3 x 2.3 feet (1.5 x .7 x .7 meters) and housed between 21 and 22 frogs. The enclosures were constructed with a mesh fabric, a PVC pipe frame, and a zipper opening to allow researchers easy access to the frogs throughout the acclimation period.
Each enclosure was furnished with floating platforms, artificial leaf litter, and vegetation from the surrounding habitat, to encourage natural behaviors such as basking, foraging, and seeking shelter. Environmental data was collected inside the enclosures for the entire duration of the trial with the use of data loggers programmed to collect, air and water temperature, light level and humidity every hour. Trail cameras were deployed to capture any event or activity that might occur outside of the enclosures.
Throughout the acclimation period, researchers conducted daily health checks on all the frogs. During the daily checks, the frogs were offered a combination of insects from their captive diet (crickets, flies, beetles, and worms) and insects found throughout the habitat. Terrestrial insects were staff-collected by sweep-netting adjacent vegetation, while aquatic insects were collected with a small dip net.
Frogs from both treatment groups were released on the same day, at the same time. 129 frogs were released from their mesh enclosures and 130 froglets were released from transport buckets.
We will conduct post-release surveys at the release site over the next four weeks, then monthly through September or October. Stay tuned!
Author Michelle Curtis is a research associate in Recovery Ecology at the San Diego Zoo Institute for Conservation Research. Read Frogs in Winter and Spring and How We Like Our Eggs to learn more about the science behind this conservation project.