The picturesque lakes of Yosemite National Park’s High Sierra are truly breathtaking. However, for over a century, these beautiful bodies of water have faced serious ecological threats, as they were stocked annually with non-native fish that decimated the local population of Sierra Nevada Yellow-legged frogs that once thrived in these areas.

This decline had major repercussions for the ecosystem. These frogs were a critical part of the diet for many animals during the summer, including bears, coyotes, snakes, and several bird species like the Clark’s Nutcracker and the Gray-crowned rosy finch.

Ultimately, the last remaining frogs faced a devastating blow from the arrival of the lethal chytrid fungus, which wiped them out in the few lakes that remained free of fish.

“It was a double whammy that nearly drove the species to extinction,” stated Roland Knapp, a research biologist at the University of California, Santa Barbara, who has been studying these frogs since 1995.

Then, something incredible happened.

For the past three decades, Knapp and a dedicated team of biologists have been searching for any remaining populations of these iconic frogs in the few lakes devoid of fish. To their surprise, they discovered that some frogs were developing a resistance to the chytrid fungus over time.

After extensive research and countless requests to state and federal agencies, they successfully reintroduced these resistant frogs into 12 different lakes, where they observed significant population growth.

“The lakes are bursting with life again, it’s unbelievable,” exclaimed Knapp.

This research involved collaboration with scientists from the University of Tennessee, Colorado University, and Yosemite National Park, with findings published this week in the journal Nature Communications.

“You can literally look along the shoreline and see 50 frogs on one side and another 50 on the other, with hundreds to thousands of tadpoles swimming around. It’s an entirely different environment now,” he stated.

The arrival of fish: a disaster for tadpoles

The decline of the Sierra Nevada Yellow-legged frog can be traced back to the California Gold Rush, which started in 1848. The influx of thousands of miners, many from Europe, into the western mountains led to significant changes in the local environment.

Amidst their quest for gold, they also came across the stunning landscapes that prompted the establishment of Yosemite National Park in 1890 for preservation.

They also discovered over 1,500 pristine alpine lakes in the high Sierra that were not only clear but teeming with life—except for fish.

“There were countless people arriving in search of sustenance, many having come from Europe, where their alpine lakes had been stocked with fish for centuries,” explained Knapp.

To stock the lakes, several species such as rainbow trout, golden trout, brown, brook, and cutthroat trout, along with Atlantic salmon and grayling, were introduced. For many years, live fish were brought in and released into the lakes using buckets.

“This had a significant impact. Many people who visited the high country witnessed these changes. The yellow-legged frog, once the most common amphibian in the region, disappeared just a few years after the fish were stocked,” said Knapp.

By the post-World War II era, the frogs were mostly limited to about 20% of the most secluded, unstocked lakes. However, this changed drastically when aerial fish stocking began.

“Many pilots returning from the war had exceptional targeting skills,” Knapp noted. They quickly utilized these abilities to drop fish into lakes from planes. “With aerial stocking, no lake was too remote anymore.”

Eventually, only a tiny fraction of lakes remained untouched by fish and rich with frogs.

The National Parks ceased fish stocking in the 1990s due to concerns for native species, but it was later discovered that the annual restocking had never been necessary. The fish populations thrived without intervention.

The fight against fungus

In 1992, Knapp set out to determine whether the frog population could be revived by removing the invasive fish species. He obtained permission to use gill nets to eliminate fish from a select few lakes, hopeful for a rebound of the frog population.

In general, when the fish were removed, it allowed the frogs to quickly return to the area.

Various organizations, including the California Department of Fish and Wildlife, the National Parks Service, and the U.S. Forest Service, initiated their own fish removal programs and noticed that frog populations began to recover.

Then, a serious setback occurred.

In the early 2000s, studies revealed that the deadly chytrid fungus, which affects amphibians worldwide, had made its way to the Sierra Nevada, causing significant declines in the already dwindling frog populations.

First identified in Australia and Central America during the 1990s, this fungus triggered mass die-offs and significant declines in frogs and other amphibians across the globe.

“It was an incredibly challenging time for efforts to save the species. For nearly a decade, we felt like we were just detailing their extinction,” noted Knapp.

The Sierra Nevada yellow-legged frog was officially listed as an endangered species in 2014.

However, scientists later discovered a glimmer of optimism.

As the lethal fungus spread, frog populations plummeted in various high Sierra lakes. Yet, some populations started showing signs of recovery.

“It appeared they had developed some level of resistance,” Knapp remarked.

This recovery was observed only in lakes devoid of fish, as these locations had sufficient frogs and genetic diversity to adapt and build resistance.

In 2006, Knapp and fellow researchers decided to explore whether they could preserve the frog populations in fish-free lakes. After extensive research and preparation, they initiated the reintroduction of frogs resistant to the fungus into these lakes. “They are actually transported in standard Ziploc bags that you can buy at a grocery store. We just made some holes for ventilation,” Knapp explained.

The effort proved successful.

Nowadays, Knapp can observe the remarkable transformation at numerous high-altitude lakes. Early explorers noted lakes surrounded by yellow-legged frogs basking in the sun. When they approached, frogs would jump into the water in a dazzling display,” Knapp recalled.

This “frog shower,” which he had previously only read about, is now something he can witness firsthand.

However, for those who make the trek to these lakes, which are situated 7,000 to 10,000 feet above sea level, don’t expect to hear a loud peeping sound from frogs.

Sierra Nevada yellow-legged frogs lack vocal sacs, so they do not call out during their breeding seasons. “If you hear frogs at one of these lakes, it’s actually the Pacific tree frog,” Knapp clarified.

Despite this, they do make sounds—you just have to dip your head underwater to catch it.

“They produce their mating calls by grinding their teeth together,” he described. Having submerged his head in the icy water to listen, he attempted to convey the sound. “The best comparison I can come up with is it resembles a wet hand rubbing against a balloon. It creates a squeaky noise that is quite loud underwater,” he shared.