May 16, 2008

Hope for Frogs in Southern California?

The southern mountain yellow-legged frog (Rana muscosa) has declined precipitously during the past century (check out for details), and nowhere has this decline been more severe than in the Transverse Ranges in southern California. Mountain yellow-legged frogs are now extinct from 98% of the sites they formerly inhabited and were listed as Endangered in 2002.

For the past decade, Robert Fisher and colleagues (U.S. Geological Survey) have been studying the eight populations of mountain yellow-legged frogs remaining in southern California in a last-ditch effort to prevent their extinction. Although the causes of the frogs' decline remain relatively poorly understood, trout introduced into the streams inhabited by the frog have been suspected as playing a role for many years. As such, the California Department of Fish and Game (CDFG) recently stopped stocking trout in those locations where mountain yellow-legged frogs were present. In addition, the CDFG began actively removing nonnative rainbow trout from one location (Little Rock Creek) to determine the effect of this removal on a remnant mountain yellow-legged frog population.

Surveys conducted in 2007 provide the first indication that these fish removals are beginning to have their desired effect (summary report available here). In 2000, a small population of mountain yellow-legged frogs was present above a natural barrier at the headwaters of Little Rock Creek, but were absent below the barrier (where trout were present). In 2001, the U.S. Forest Service built a fish barrier 1.5 km downstream of the natural barrier, and since 2002 the CDFG has been removing trout from the reach between the two barriers. Trout removals are conducted once per year via electrofishing.

In 2006, USGS scientists found a juvenile mountain yellow-legged frog in the trout-removal reach. In 2007, as many as seven mountain yellow-legged frogs were detected in the same area. The appearance of frogs coincided with the apparent elimination of trout from the upper portion of the trout-removal section.

Given these hopeful (although preliminary) results, the CDFG needs to immediately step up their fish removal efforts in Little Rock Creek. In recent years, fish removals have been conducted by CDFG volunteers and removal efforts have been limited to one two-day session per year. The electrofishing effort should be increased to include at least a few weeks of fish removals per year, and electrofishing could potentially be supplemented by efforts using gill nets placed into stream pools.

The CDFG deserves a lot of credit for the current Little Rock Creek frog recovery project, but has to redouble their efforts to carry the project to a successful conclusion.

Back to The Mountain Yellow-legged Frog Site.

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