The Frog and The Fish

Almost all of the thousands of lakes and ponds lying above 7,500 feet in the Sierra Nevada were fishless before settlers began stocking them with trout in the 1800s. After World War II, the California Department of Fish and Game began an intensive program of regularly stocking hatchery-raised trout in the waters, most of which today lie within the boundaries of U.S. Forest Service and National Park wilderness areas.

But a new environmental study of the lakes indicates that the introduced trout, which are still regularly stocked, have profoundly changed the basic ecology of the High Sierra lakes.

The trout, well-known predators of young amphibians, are the primary cause of the steep decline of at least one native amphibian, the mountain yellow-legged frog, the study concludes. The frog was commonly found at high-elevation Sierra lakes as recently as the 1970s, but it has now vanished from much of its native range. Its numbers have plummeted to the point that it is considered qualified for listing as an endangered species.

“The mountain yellow-legged frog has undergone precipitous declines in the past 50 to 100 years, and is in danger of being [exterminated] from the Sierra,” said University of California fish biologist Roland Knapp, who conducted the study with U.S. Forest Service biologist Kathleen Matthews. “Of 1,200 bodies of water we surveyed, we found the frogs in less than 3%. Historically, they were found in approximately 50%. There is no question that the decline is dramatic, and our data support the idea that introduced trout are the primary cause.”

Many Sierra amphibians that populate lower elevations of the range are also in trouble, according to the report of the Sierra Nevada Ecosystem project, a comprehensive environmental study of the entire Sierra commissioned by Congress and completed last year. Half of the 29 native Sierra species of frogs, toads and salamanders were reported in danger of disappearing from the Sierra, according to the study.

Among the causes of the declines are introduced fish such as trout, which are voracious predators that devour amphibians, particularly frog tadpoles and other young. Other factors in the declines include loss of habitat, pesticides, air pollution and other introduced species such as bullfrogs.

“The Sierra Nevada is viewed as this pristine, relatively untouched ecosystem,” said Knapp, who also authored a chapter in the ecosystem project report on the effects of introduced trout. “To find declines in the national parks and wilderness areas was a real shock to most people.”

However, other scientists are not convinced that introduced fish are the primary reason for the frogs’ disappearance.

Gary Fellers, a federal biologist from the Point Reyes National Seashore who has studied amphibians in the Sierra extensively, said, “Introduced trout are clearly having a very significant negative impact on mountain yellow-legged frogs. Whether trout are the primary cause of their disappearance isn’t clear yet. Airborne contaminants [like pesticides or herbicides] could be a bigger factor than introduced fish, but it hasn’t been demonstrated yet.”

Phil Pister, a retired state fisheries biologist who spent most of his career working in the Sierra region, and state fisheries biologist Alan Pickard also said they are not convinced that trout are the main cause of the decline. They noted that mountain yellow-legged frogs have disappeared from some bodies of water that have never contained fish.

Nevertheless, the disappearance of the mountain yellow-legged frog intensified concern within the Forest Service about the effects of fish stocking in wilderness areas, Knapp said.

Part of the mandate of the Forest Service is to preserve the native species of wilderness areas. If the mountain yellow-legged frog, which ranges across the Sierra, were listed under the Endangered Species Act, it could lead to a cessation of trout stocking in the range, Knapp said.

Non-native brook, brown and rainbow trout have already been blamed for the declines of Little Kern and California golden trout, which are native to a few high-elevation Sierra streams. The Little Kern subspecies has already been listed under the Endangered Species Act, and the California subspecies is currently under consideration for listing.

The ongoing study by Knapp and Matthews--which was funded by the Forest Service and the National Science Foundation--has also found that non-native trout have changed the balance of water insects and zooplankton, which are microscopic crustaceans, in the mountain lakes. The fish eliminated some species of the organisms, and ultimately altered the basic ecology of the high lakes.

The mountain yellow-legged frog is more vulnerable to trout because it spends most of its life in the water, unlike other Sierra amphibians that are more terrestrial. In addition, the frog remains in the tadpole state for two years or longer, while most other frogs metamorphose in just weeks or months. That confines the frog’s reproduction to deeper lakes that do not freeze in winter or dry up in summer, and virtually all of those Sierra lakes contain trout.

Knapp said that when the agency began using airplanes to stock the high-elevation lakes in the early 1950s, it expanded the range of the trout to many lakes that had been inaccessible before. It also enabled the agency to stock more regularly, which gave the frog less of an interval to recover.

Airplanes are still used by the agency to regularly stock High Sierra waters. Pilots fly 200 feet above the lakes and drop fingerling trout into them, which are small enough to float down and survive the fall. Knapp said that according to Fish and Game Department records, the agency particularly sought to stock lakes where mountain yellow-legged frogs were abundant during the ‘50s.

“They were considered fish food,” he said.

The study by Knapp and Matthews has become part of a scientific debate over how large a role introduced fish have played in the disappearance of the frog species.

UC Davis biologist Peter B. Moyle, an authority on Sierra fish and aquatic systems, backed the study’s findings, saying, “In the southern Sierra, fish are probably the single biggest factor causing problems with the [mountain yellow-legged] frog. . . . The impacts of fishes on amphibians were known, but the extent was not known and not appreciated.”

Pickard said the agency has already stopped stocking some mountain lakes, in part because of the effects on the frog.

Knapp also said that continuing to regularly stock lakes sometimes only serves to overpopulate the lakes with fish, resulting in trout of stunted size.

Knapp, who described himself as an avid fisherman, does not advocate ceasing all stocking of Sierra back-country lakes, suggesting it is not only unnecessary but likely to spur a backlash among some fishermen and businesses dependent on tourism. Instead, he suggested that a reserve system be set up, halting stocking or entirely eliminating fish from selected lakes, and reestablishing mountain yellow-legged frogs in about 10% of the high-elevation Sierra lakes.