UC Berkeley post-doctoral fellow Vance T. Vredenburg showed that introduced trout have devastated native frog populations over the past 50 years in formerly fish-free high-Sierra lakes, but that removing the fish can allow the frogs to flourish once more.
Vredenburg's study was published this week in the online edition of the Proceedings of the National Academy of Sciences.
"The mountain yellow-legged frog used to be the most common inhabitant of the high Sierra, but frog populations have declined dramatically enough to put it on the endangered species list," said Vredenburg, who works at the campus's Museum of Vertebrate Zoology. "I'm not saying that other things didn't cause a decline as well, but this report shows that most of the problem came from fish."
Though anglers might resist efforts to remove trout from Sierra lakes, Vredenburg's preliminary data and the results of an earlier survey of Sierra frog populations were critical pieces of evidence that led the park service and Fish and Game to consider that approach, at least on a limited basis.
"People want their trout, but they don't realize what they're getting along with their trout - changed ecosystems, diseases and all sorts of things," noted David Wake, a UC Berkeley professor of integrative biology who organized the first international conference on amphibian decline in 1990.
In a 1915 survey of wildlife in the Sierra Nevada, UC Berkeley biologist and museum director Joseph Grinnell complained that mountain yellow-legged frogs (Rana muscosa) were so abundant that his survey team was stepping on them. Today there are probably fewer than 200 populations of the frogs - perhaps 5,000 adults - in their range from north of Lake Tahoe to below Sequoia National Park, Vredenburg said. (A separate endangered population, and perhaps a distinct species, exists in the San Gabriel and San Bernardino mountains of southern California.)
Non-native trout dumped into the high-Sierra lakes have been suggested as one cause of the decline, but many biologists dismissed this as improbable, since the trout - rainbow, golden and brown - were introduced more than 100 years ago, while the decline has been noted only since the 1980s. Some biologists even claimed that trout don't eat tadpoles, Vredenburg said.
Vredenburg's study, conducted in a remote and rugged area of Kings Canyon National Park, shows both these assumptions to be false. He noted that, while trout were planted in the fishless high-altitude lakes beginning in the late 1800s, mules carrying fingerlings in milk cans could not reach many lakes because of their remoteness and the lack of trails. Thus, the main habitat of the mountain yellow-legged frog - clear, cold, rocky lakes up to 12,000 feet - remained free of fish.
All that changed when the California Department of Fish and Game began planting fish by airplane in the 1950s and '60s, flying low over thousands of lakes in the Sierra and dumping fingerlings everywhere. Today, some 17,000 lakes in the American west are stocked with fish, while 90 percent of the habitat of the mountain yellow-legged frog is now home to introduced trout.
Vredenburg demonstrated in his study that trout are voracious eaters, quickly decimating the tadpoles in a lake. The delay in the decline of adult frogs is most likely due to the fact that they probably live 10 years or more, so the loss of tadpoles would not produce a dramatic drop in adult frogs for a decade or longer.
As part of his study, he and UC Berkeley undergraduate volunteers removed trout from five lakes and documented a rebound in the frog population in all of them. Three years after trout removal, the frog populations in all five lakes were indistinguishable from populations at lakes that had never seen a trout.
"The response was incredibly dramatic and rapid," Vredenburg said. "Every time you plant hundreds of thousands of fish, you're hammering a nail in the frogs' coffins. But we can certainly do something about it, we can turn the decline around if the political will is there for people to do it. We know it's possible."
"The results are very convincing," Wake said. "The mountain yellow-legged frog has suffered a 90 percent decline in the Sierra, and here is remedial action that can restore the natural populations. There are other problems these populations face, but this immediate problem is solvable."
"Vance's study is fabulous and of tremendous importance," agreed David Graber, senior science advisor to Kings Canyon and Sequoia national parks. He said that the parks are now engaged in a 10-year "management experiment" to see if removing fish from 11 high-elevation lakes will bring back frog populations. So far, with six of the lakes expected to be declared fish free this summer, it seems to be working.
Nevertheless, he admits that doing this broadly in the high Sierra is neither physically nor politically feasible. Many lakes are too large and deep to allow removal of fish without the use of poisons, and fishermen would protest any loss of trout lakes, no matter what the impact on native amphibian populations.
"This is a stopgap. At this point, we are just simply trying to prevent extinction," Graber said. "These frogs have nearly disappeared from the high Sierra, so we're (conducting) a panicked rear-guard action."
There also are other possible causes of frog declines, he said, including a fungus now attacking frogs worldwide and pesticides blowing into high-altitude lakes from the Central Valley.
"We don't want to interfere with recreational fishing, but create reserves to help recover the species," added Harold Werner, wildlife ecologist for the two parks. "We want to give the public options."
Fish and Game also is testing fish removal as a way to help declining frog populations, Werner said, and the National Forest Service is interested in the approach.
Vredenburg began his study in 1996 in the Sixty Lake Basin, a bare, rocky landscape between 10,000 and 11,500 feet in elevation that is a two-day hike from the nearest roadhead. He spent several years, from June until the snows came in October, documenting the fish and frog populations at the more than 60 lakes in the basin, and noticed a trend first established by UC Santa Barbara researcher Roland Knapp in an earlier survey of Sierra lakes.
"I documented a pattern that showed there are lakes with frogs only, lakes with fish only and some overlap, but in the lakes with both fish and frogs, there were very few frogs or tadpoles," he said. "The question was, 'Why?'"
Using gill nets, he and his undergraduate assistants removed trout completely from three lakes and greatly reduced the population in two other lakes. In all lakes, both tadpoles and frogs rebounded within three years to levels found in naturally fish-free lakes.
In three other fish-free lakes, he fenced off a small portion and introduced a few trout along with masses of frog eggs. The trout ate every tadpole in all three enclosures.
"It's clear that trout are largely responsible for the decline of this frog, that they explain a vast amount of the decline," Vredenburg said. "It wasn't until the 1950s and '60s, when Fish and Game began dropping fish out of airplanes, that the landscape really changed. The reason there were no native fish in lakes above 6,000 feet is that they couldn't get there, so when you put them in at the top of the watershed, they swim downstream and really alter the ecosystem."
Vredenburg, who has conducted more than 900 censuses of the frog population at lakes throughout the basin, is continuing his studies of the mountain yellow-legged frog to find out how far the frogs roam, where they breed, and with whom. He has already implanted microchips in 1,200 frogs to enable easy identification, like running a bar code across a scanner, and has outfitted many frogs with radio transmitters.
He also has teamed up with other researchers at the Museum of Vertebrate Zoology and at UC Berkeley's departments of integrative biology and plant and microbial biology to determine the effect of a chytrid fungus, Batrachochytrium dendrobatidis, on the mountain yellow-legged frog. The fungus, which is threatening frog populations around the world, attacks the mouthparts of tadpoles as well as adults, and can kill adult frogs. It was discovered in the Sierra Nevada in 2001.
The research is supported by a grant from the Ecology of Infectious Disease Program that is run jointly by the National Institutes of Health and the National Science Foundation, and by the U.S. Geological Survey.