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Return of the Frogs

It was weather only a frog could love on the day of the historic release of red-legged frog eggs in San Diego County. Between the rain and cloud cover brushing the tops of our heads, I was glad I had opted to observe the release on private property, with lovely landowners and their bottomless pot of coffee. 

I had arrived early at the release site in case the egg transport went faster than anyone anticipated, so we had plenty of quality time, sipping coffee and monitoring the minute-by-minute drama via email and texts. This quiet overcast day marked the culmination of many years of hard work to return California red-legged frogs to their historic range in southern California, where they have been absent for about 20 years.

When the eggs finally arrived in their humming temperature-controlled Yeti cooler, the reintroduction process was relatively quick and simple. Adam Backlin, the red-legged frog expert from U.S. Geological Survey, checked the water temperature to ensure it matched the temperature of their new pond.  He dipped the mesh bag of eggs in clean pond water to rinse them, and provide biocontrol against chytrid fungus, and carefully emptied the bag into a mesh cage floating near the edge of the pond.

The simplicity of this action belied the decades of research and planning, and fevered pitch of obtaining permits and arranging transport across an international border. More than 20 people worked for years to see this happen. Some of the people who had the greatest longevity on the project missed the actual reintroduction, staying in Baja California in case there was an opportunity for a second egg transport in this field season.

It required almost unheard of binational cooperation between the United States Fish and Wildlife Service (USFWS), the U.S. Geological Survey (USGS), the San Diego Natural History Museum (The Nat), The Nature Conservancy (TNC), and the Mexican nonprofit organization, Conservación de Fauna del Noroeste (FAUNO). It also required two years of work to rehabilitate the pond that received the eggs.

A species in decline

Red-legged frogs, Mark Twain’s celebrated jumping frogs of Calaveras County, once occurred along the coast of California from Point Reyes, inland to Redding, and south to northwestern Baja California, Mexico. When bullfrogs were introduced to California around 1896, they led to the decline of red-legged frogs, and populations were further decimated by other exotic species, fungal disease, drought, and urbanization. One hundred years after the bullfrogs arrived, red-legged frogs were listed as threatened under the Endangered Species Act. The last known red-legged frog in San Diego County was observed in 1974.

Scientists from USGS and The Nat launched a study in 2006 to look for the species south of the border, in Baja California. In 2016, FAUNO, the Mexican conservation organization working with the team, discovered remaining remnant populations of red-legged frogs in the mountains in northern Baja California. Staff from the Museum and FAUNO worked energetically on habitat conservation for the species and even dug additional breeding ponds, which successfully attracted frogs. It was one of the relatively new breeding ponds that provided the source of egg masses for this historic translocation event. 

Egg laying and permitting: a perfect storm

Red-legged frogs have been successfully transplanted between ponds in Northern California, and as far south as the Santa Monica Mountains. However, the populations that went locally extinct in Riverside and San Diego counties and points south are genetically different from those to the north. 

With the discovery and nurturing of a population genetically related to the extirpated southern California frogs, it seemed like a straightforward concept: collect eggs in Mexico and reintroduce them in southern California. Of course, binational work requires permits from multiple agencies in two nations, so it is nothing short of a miracle the timing worked out.

Frogs wait for the late winter rains to begin breeding in Baja California’s Sierra San Pedro Martir. Once the rains start, there is a period of weeks for egg laying. Frogs also need the correct amount of rain: too little rain and ponds can dry up, too much rain and they overflow, washing eggs away. 

This year brought a dry spell in January that continued through much of February, followed by an abundance of rain that washed away some of the egg masses. With a gestation period of about 14 days from egg to tadpole, the scientists had to monitor for perfect conditions. Meanwhile, there was a race to obtain permits—to collect and transport eggs, and export them from Mexico, and to import and release them in the U.S. and particularly in the state of California.

Race against Coronavirus

It was a nail-biter of a week waiting to hear about field conditions, permits, transportation options, and weather forecasts. It was also a race against the spread of the coronavirus. Reading the preserved email string makes my heart race. Weather was deteriorating, eggs were maturing, transportation was arranged, and then planes were unable to fly. We were only missing one permit, then a new permit need emerged, then tadpoles emerged.  New egg masses were laid, then some were lost to rain and pond overflow.  

Meanwhile the significance of COVID-19 was emerging. On March 12, when it was determined that the project was a “go,” there were 1,276 reported cases of the coronavirus in the United States. On March 13, the same day we made the difficult decision to close the Museum to the public, the frog team was doing last-minute prep for the translocation. Rumors of an imminent border closure and possible bans on international travel by government employees further complicated the situation. No one knew what the next few days held, and it felt like things were playing out on a split screen—business closures and minute-by-minute changes in San Diego, while an exciting environmental milestone played out in the wilderness of Baja California.

The weather took a turn for the worse, grounding the plane that had been standing by to transport eggs, so the only way the translocation could work was to hand off the eggs at the border. With a team of experienced problem-solvers, every potential roadblock was met with a calm move to Plan B, C, or D. In the end, the eggs were driven across the border at Tecate, Mexico in the lone car lane.  Although the line was short, each car was being checked for potential coronavirus, so the projected 15-minute wait took two hours.

By the time the cooler was transferred to the last vehicle that could handle the increasingly muddy roads at the release site, a few of the eggs had already hatched into tiny wriggling tadpoles. The installation of the eggs and tadpoles into their new ponds went smoothly. Years of planning, preparation, and clockwork timing floated into fruition.

Blinking out/blinking on

When the last of a species is extirpated—or goes locally extinct—scientists say it has “blinked out.” The seemingly simple act of gently emptying the mesh bag with about 500 eggs and tadpoles was a rare and amazingly moving sight, a “blinking on.” 

Recent follow-up reports indicate that the tadpoles are growing and thriving. My fervent hope is that by August when the froglets are hopping around the release site, we will also have been released from confinement, and the team can gather again to witness the remarkable vision of red-legged frogs restored to San Diego.


Adult male and female California red-legged frogs from the source population in Baja California, México. Breeding takes place in the spring, and a female can lay more than 1,000 eggs each year. (Photo by J.A. Soriano/Fauno)

Before the translocation, a field team led by Fauno and The Nat conducted weekly surveys to ensure that a sufficient number of eggs had successfully hatched in Mexico. (Photo by Dr. Bradford Hollingsworth/San Diego Natural History Museum)

The last leg of the journey required a vehicle that could handle the muddy road. Clark Winchell of USFWS met the egg cooler at the U.S. side of the border, bearing permits and wearing a brand-new uniform in honor of the occasion. (Photo by Judy Gradwohl/San Diego Natural History Museum)

The final step in the process, rinsing the eggs in water from their new home, helped ensure that the painstakingly rehabilitated pond would remain uncontaminated. (Photo by Judy Gradwohl/San Diego Natural History Museum)

Eggs and tadpoles were released into a tiny sanctuary, a mesh cage floating in the pond. (Photo by Judy Gradwohl/San Diego Natural History Museum)

Posted by President and CEO Judy Gradwohl on April 9, 2020

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