Aquatic Restoration

High elevation aquatic resources in Yosemite National Park include the lakes, ponds, wet meadows, and streams located in the high country above Yosemite Valley and the diverse and vibrant plant and animal communities—including amphibians--that they support.

Historically, lakes and streams above 4,000 feet in Yosemite National Park were naturally free of fish due to the series of glaciations that scoured the Sierra Nevada creating steep waterfalls in Yosemite Valley and Hetch Hetchy Valley that kept fish from colonizing upstream habitats. As a result, aquatic organisms such as frogs in high mountain lakes and streams evolved without the presence of fish, and therefore, some of these organisms, like the Sierra Nevada yellow-legged frog, have not evolved defenses against these predators.

From 1897 to 1990 fish were stocked in lakes, ponds, streams, and rivers throughout Yosemite [1.66 MB PDF]. In the early days of fish stocking, fish were transported to waterbodies by hand, later they were transported by mule, and eventually fish were stocked by airplanes. In 1972, in recognition of the deleterious effects fish were having on native species, the National Park Service began phasing out its fish stocking practices. By 1975 most lakes were no longer stocked, and by 1990, Yosemite ended all fish stocking within the park. Although stocking no longer occurs, there are many self-sustaining non-native fish populations in the park. It is estimated that approximately 250 waterbodies and 800 miles of rivers and streams contain populations of nonnative fish.

 
Crews using gillnets in high elevation lake
Crew using a gillnet to remove fish from a high alpine lake in Yosemite.
 
The impacts of non-native fish on high elevation aquatic and adjacent terrestrial ecosystems are well documented and occur at all levels of the food web. Non-native fish feed on aquatic insects as well as frogs. Fish also compete for food with frogs, snakes, and bats and some birds, several of which feed on the adult stage of the aquatic insects. Predation and competition by nonnative fish decreases populations of many of these animals, most notably the Sierra Nevada yellow-legged frog, a federally endangered species that has declined from over 95% of their historic range.
 

The Sierra Nevada yellow-legged frog was once the most abundant amphibian found in these high-elevation aquatic systems. Visitors would often see hundreds of frogs during walks along mountain lake shores and thousands of tadpoles could also be seen piled on top of each other. Frogs were a vital link in the food chain—they were predators feeding primarily on insects and they were important prey for native birds, snakes, and mammals when still numerous.

The emergence of insects from the aquatic environment is another vital link in the food chain providing an irreplaceable food source for birds and bats and their young.

As early as the 1920s, negative impacts of non-native fish on frogs were documented in prominent natural history texts. One naturalist stated:

…frogs, in tadpole form at least, do not occur in lakes which are stocked with trout… the advent of fish in a lake sooner or later nearly or quite eliminates the frogs. It seems probable that the fish prey upon the tadpoles, so that few or none of the latter are able to reach the stage at which they transform.” - Animal Life in the Yosemite by J. Grinnell and T. I. Storer, 1924
 
Sierra Nevada Yellow Legged Frog in water
Sierra Nevada yellow-legged frog
More recent work from Dr. Roland Knapp and the Sierra Nevada Aquatic Research Lab found that fish negatively impacted the distribution of yellow-legged frogs across the landscape [179 kb PDF]. This research prompted an experimental fish removal restoration project in Sequoia and Kings Canyon National Park to help increase habitat for the mountain yellow-legged frog (the Sierra Nevada yellow-legged frogs’ sister species). Researchers in Sequoia and Kings Canyon found that once fish were removed, the yellow-legged frog would not only recolonize the lake, but they would thrive in the newly fish-free habitats. Yosemite National Park followed suit and began experimentally removing fish from select lakes starting in 2007.

The Sierra Nevada yellow-legged frog needs fish-free habitat now more than ever. Their depleted and isolated populations have been devastated by a newly discovered fungal disease, chytridiomycosis or “Chytrid” caused by the fungus Batrachochytrium dendrobatidis. Chytrid has been spreading across the Sierra Nevada since the 1970s causing extinctions of hundreds of frog populations. The fungus has infected most Sierra Nevada yellow-legged frog populations in Yosemite National Park, however there are a few populations that appear to be adapting to the fatal effects of the fungus and persisting even when infected. Creating habitat for the persisting populations of Sierra Nevada yellow-legged frog by removing fish is critical for the recovery of this endangered amphibian [1 MB PDF].
 
Yosemite has an ongoing lake restoration project to restore fishless ecological conditions to about 10% of the remote lakes in Yosemite (approximately 25 out of an estimated 250 lakes that contain fish in the park). Restoration includes the removal of fish to create habitat for the critically endangered yellow-legged frog as well as other aquatic species.

By restoring fishless conditions to these lakes, the park has increased the population of the Sierra Nevada yellow-legged frog while still providing high-quality fishing experience to visitors who value this activity in the numerous sites throughout the park where fish are not being removed.
 

More Information

References

  • Hoffman, R.L., Larson, G.L., Samora, B., 2004. Responses of Ambystoma gracile to the Removal of Introduced Nonnative Fish from a Mountain Lake. J. Herpetol. 38, 578–585.doi:10.2307/1565758
  • Knapp, R. A., Matthews, K.R., 2000. Non-native fish introductions and the decline of the mountain yellow-legged frog from within protected areas. Conserv. Biol. 14, 428–438.doi:10.1046/j.1523-1739.2000.99099.x
  • Knapp, R.A.,Fellers, G. M., Kleeman, P. M., Miller D. A. W., Vredenburg, V. T., Rsenblum, E. B., Briggs, C. J. 2016. Large-Scale recovery of an endangered amphibian despite ongoing exposure to multiple stressors. PNAS. 113-42. Doi: 10.1073/pnas.1600983113
  • Pilliod, D.S., Hossack, B.R., Bahls, P.F., Bull, E.L., Corn, P.S., Hokit, G., Maxell, B. a., Munger, J.C., Wyrick, A., 2010. Non-native salmonids affect amphibian occupancy at multiple spatial scales. Divers. Distrib. 16, 959–974. doi:10.1111/j.1472-4642.2010.00699.x
  • Pope, K.L., 2008. Assessing changes in amphibian population dynamics following experimental manipulations of introduced fish. Conserv. Biol. 22, 1572–1581.doi:10.1111/j.1523-1739.2008.00998.x
 

Last updated: September 11, 2017

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