After more than two decades of research that has included faunal surveys of thousands of lakes and ponds and fish removal experiments, there is now no doubt that introduced fish are a major cause of the mountain yellow-legged frog's decline.
Prior to the mid-1800s, virtually all Sierran lakes, ponds, and streams above 7,000 feet (2,100 m) were fishless (Knapp 1996). These habitats were created by glaciers as recently as 10,000 years ago, and when the glaciers receded they left behind waterfalls that prevented fish that were native to lower-elevation streams from moving to higher elevations. Although naturally lacking fish, these montane habitats were home to a unique community of invertebrates and amphibians, including the mountain yellow-legged frog.
Beginning in the 1850s, Euro-American settlers began moving trout into these naturally-fishless habitats to provide recreational fisheries. In the 1920s, the state of California assumed control over the fish stocking program. Early fish stocking efforts were conducted using horses and mules, but in the 1950s the California Department of Fish and Game began using airplanes to stock even the most remote backcountry lakes. Although fish stocking was halted in Sierra Nevada national parks (Sequoia, Kings Canyon, and Yosemite) by the early 1990s because of concerns over the impacts of nonnative trout to native aquatic fauna, fish stocking continues today in hundreds of lakes within Sierra Nevada national forests, including in federally-designated wilderness areas. As a result of past and current fish stocking efforts, the majority of these naturally-fishless lakes now contain nonnative trout. The most widely introduced species include rainbow trout (Oncorhynchus mykiss), golden trout (Oncorhynchus mykiss aguabonita), and brook trout (Salvelinus fontinalis) (Knapp 1996).
Predation by nonnative trout on mountain yellow-legged frog tadpoles and adults has long been recognized as a major factor in the decline of this amphibian. As early as 1924, Joseph Grinnell, a professor at the University of California, Berkeley, stated that
"It is a commonly repeated observation that [mountain yellow-legged] frogs, in tadpole form at least, do not occur in lakes which are stocked with trout. Adult frogs are sometimes found around the margins of such lakes and they occur in numbers along the shores of streams inhabited by trout, but the advent of fish in a lake sooner or later nearly or quite eliminates the frogs." (Grinnell and Storer 1924, pg. 664).
A study conducted in the 1930s on the growth of golden trout following their introduction into two fishless Sierra Nevada lakes stated that
"The stomach of one trout, 9.8 inches long, from Bright Dot Lake contained an adult Sierra Nevada yellow-legged frog (Rana boylii sierrae). On Septeber 10, 1937, an occasional trout was seen pursued by others, swimming toward deeper water with one of these frogs held crosswise in its mouth, attempting to swallow it." (Needham and Vestal 1938).
Not surprisingly, Sierra Nevada yellow-legged frogs are no longer present in fish-filled Bright Dot Lake.
More recent studies based on surveys of thousands of lakes and ponds have confirmed that lakes containing nonnative trout are much less likely to contain mountain yellow-legged frogs than fishless lakes (Bradford 1989; Bradford et al. 1998; Knapp et al. 2003; Knapp 2005). As a result, areas heavily stocked with trout (e.g., national forest wilderness areas) have a much lower proportion of water bodies inhabited by mountain yellow-legged frogs than do areas where stocking intensity has been lower (national parks). For example, 90% of lakes in the John Muir Wilderness contain nonnative trout versus only 50% of lakes in adjacent Kings Canyon National Park. As a result, frogs are currently found in only 5% of John Muir Wilderness lakes versus 35% of Kings Canyon National Park lakes (Knapp and Matthews 2000a).
Fish Removal Experiments
Two recent fish removal experiments have added another powerful line of evidence that introduced trout are a major cause of frog decline. Collectively, nonnative trout were eliminated from a total of eight lakes using gill nets (Knapp and Matthews 1998). All eight lakes had Rana sierrae or Rana muscosa populations nearby. In all cases, frog and tadpole numbers increased rapidly following fish removal and within 3-4 years had reached densities typical of fishless lakes (Vredenburg 2004; Knapp et al. 2007). Frogs subsequently spread to neighboring lakes and streams and established additional populations. Subsequent fish removal projects led by the California Department of Fish and Game and National Park Service have produced similar results (see Conservation).
Why are frogs so vulnerable to fish?
The extreme vulnerability of mountain yellow-legged frogs to introduced trout is largely the result of very similar habitat requirements of these two groups. Mountain yellow-legged frogs and trout both require deep-water habitats that don't dry up during the summer or freeze solid in the winter (usually deeper than 6 feet (2 m)). In R. muscosa and R. sierrae, this requirement is a consequence of tadpoles taking multiple years to reach metamophosis and adults overwintering underwater. Because trout have now been introduced into most of these deeper water bodies, mountain yellow-legged frogs no longer have access to the majority of deep-water habitats that are essential for their survival (Knapp and Matthews 2000a). This impact of fish will likely have increasingly severe effects on frogs in future decades due to global warming. The reduced snowpacks predicted under realistic climate change scenarios will result in more frequent drying of the smaller ponds to which frogs are now often restricted (Lacan et al. 2008). In addition, because mountain yellow-legged frogs evolved in the absence of trout, they lack any effective defenses such as distastefulness or behavioral changes that could provide protection from predatory fish (Vredenburg 2002).
In addition to introduced trout excluding mountain yellow-legged frogs from their essential deep-water habitat, trout introductions have also isolated remaining frog populations from each other (Bradford et al. 1993; Knapp and Matthews 2000a). Prior to fish introductions, frogs were abundant in the majority of Sierran lakes and ponds. Given their historic ubiquity, if any particular frog population went extinct as a result of natural events such as droughts, avalanches, or disease, the extinction would soon be reversed as frogs moved into this empty habitat from nearby populations. Today, the scarcity of frogs and the abundance of predatory trout virtually assures that water bodies from which mountain yellow-legged frogs go extinct will never be recolonized.