Frog Fable #3 - Fish Removals Cause Frog Disease Outbreaks

I recently read a letter that stated that fish removals, far from benefiting mountain yellow-legged frogs, actually cause outbreaks of the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd"). The purported basis for this is essentially that fish removal causes frog populations to grow and that the resulting increased frog density boosts the likelihood of Bd outbreaks. Furthermore, by removing fish populations from clusters of lakes, frog dispersal between adjacent habitats is increased and this enhances Bd spread.

Ecological theory suggests the plausibility of the above-outlined scenario. However, a decade of field research in the Sierra Nevada provides absolutely no support for the idea that fish eradications cause Bd outbreaks.

This idea seems to have its origin in two sources, the first an article written by Tom Stienstra - the outdoor columnist for the San Francisco Chronicle - published August 17, 2003, and the second a scientific paper written by me and two colleagues published in 2007 (PDF). Both sources mention cases in which initial frog population increases following fish removal were followed by frog population crashes. So, let's look at both of these in turn.

To quote the key section of Tom Stienstra's piece, "The father, Don Vachini of Novato, and sons Jason and Matt had hiked eight miles over 11,000-foot Piute Pass and into Humphreys Basin, where several gorgeous lakes are set in granite pockets. They set up a base camp at Lower Desolation Lake, set at the timber line at 11,375 feet, then trekked another two miles cross-country to their favorite lakes at the foot of towering Mount Humphrey (sic), looming overhead at 13,986 feet. These lakes are cobalt blue with pristine clarity. On windless mornings, there is a perfect mirror image of the mountain on the lakes' surfaces. The surrounding landscape is stark batholithic granite, with pockets of ice and snow and a few scraggly pines. But Vachini shook his head, a hollow look in his eyes. 'Can you believe all the trout are gone from this magnificent water?' he asked. 'After 20 years of hiking into Humphreys Basin, it breaks my heart.' In a program that could become the prototype for hundreds of high Sierra lakes, all the trout have been netted out and killed at Vichini's favorite lake.... But last month, Roland Knapp of the Sierra Nevada Aquatic Research Laboratory made his own shocking discovery in Humphreys Basin: at Vachini's favorite lake, where all the trout were cleaned out to protect the frogs, all the frogs were suddenly gone, too. They were wiped out last winter by chitrid (sic) fungus, Knapp said."

Quite a dramatic outcome, isn't it? Unfortunately, Tom Stienstra fabricated this story. Following the removal of fish from several small lakes in this part of Humphreys Basin, the frog population has thrived and in marked contrast to Tom Stienstra's vivid imagination, to date there has been no Bd-caused population crash. During my visit to this area in early September of this year, I counted more than 8000 tadpoles and 1500 adult and juvenile frogs, counts that continue the trend of a steep increase following fish removal. The above photo and the photo accompanying my October 10 post were taken immediately following this survey.

Which brings me to the article we published in 2007 (Knapp, R. A., D. M. Boiano, and V. T. Vredenburg. 2007. Removal of nonnative fish results in population expansion of a declining amphibian (mountain yellow-legged frog, Rana muscosa). Biological Conservation 135:11-20). This article focused primarily on the recovery of mountain yellow-legged frogs following fish removal in Humphreys Basin (John Muir Wilderness) and LeConte Canyon (Kings Canyon National Park). It also mentioned the important role played by Bd in limiting frog populations and in particular, the Bd outbreak in 60 Lake Basin (Kings Canyon National Park) a few years after fish removal and subsequent frog population increases in several lakes. This is decidedly not a case of fish removal causing the Bd outbreak, however. In fact, Bd is spreading across Sequoia-Kings Canyon National Park as a west-to-east moving front. Having arrived at the basin just west of 60 Lake Basin in the mid-1990s, I expected its arrival in the adjacent 60 Lake Basin by 2000. It took a bit longer than that, but in 2004 Bd outbreaks began in 60 Lake Basin and in the next four years spread across the entire watershed, reducing the mountain yellow-legged frog population to a fraction of its pre-Bd size. The first Bd outbreaks actually began in a portion of the basin that was most distant from the fish removal sites. Furthermore, Bd caused high mortality in all frog populations in this basin, regardless of frog numbers or density, providing another suggestion counter to the notion that larger frog populations are more susceptible to Bd outbreaks.

In summary, despite many years of data collection at numerous sites across the southern Sierra Nevada, there is no evidence that frog population recovery following fish removal causes Bd outbreaks. The long-term fate of the recovering frog populations in now-fishless lakes in 60 Lake Basin and elsewhere will provide the ultimate test of whether fish removal can benefit mountain yellow-legged frogs despite the presence of Bd. Until those results are in, let's stick to the facts as we currently know them. Wild extrapolation from these facts only results in the loss of credibility on the part of the person making the extrapolation.

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