Born to be rewilded

Authors: Aaron Ambos, Raymond Saumure, Thomas O’Toole, Zane Marshall

Year: 2019

Abstract

In May 2018, 290 adult Pahrump Poolfish, Empetrichthys latos, and 100 juvenile Relict Leopard Frogs, Rana onca, were released into two constructed ponds at the Las Vegas Springs Preserve in Las Vegas, Nevada. These introductions mark the first time in over sixty years that native fish and frogs have occupied the site of the historic Las Vegas springs. Once a lush oasis within an arid Mojave Desert landscape, the Las Vegas spring complex ceased flowing prior to 1962 due to increasing groundwater usage. The loss of this spring complex led to the extinction of the endemic Las Vegas Dace, Rhinichthys deaconi, around 1957. Reduced spring flows along with habitat alteration also led to the extirpation of the Vegas Valley Leopard Frog, Rana fisheri, within the Las Vegas Valley by the late 1940’s. In the following decades, the site of the Las Vegas spring complex existed solely as a well field, providing water for the growing Las Vegas community. In 1997, however, the Las Vegas Valley Water District Board of Directors approved a plan to develop a 180-acre preserve to protect and manage the natural, cultural, and water resources of the site. The Springs Preserve opened in June 2007, and by 2010 plans were made to restore some surface water habitat within the original spring complex channels. The imperiled Relict Leopard Frog and the federally endangered Pahrump Poolfish were identified as candidates to inhabit these ponds. Both species are native to Nevada and need additional refuge populations and would fill the niches of the extirpated species. The initial two ponds were completed in early 2018 and by late May, Pahrump Poolfish from an existing refuge population and captive reared Relict Leopard Frogs representing two natural populations were introduced. Surveys in October 2018 documented 4 large adult Relict Leopard Frogs (1 male and 3 females) and provided a population estimate of 386 (95% CI 278-605) Pahrump Poolfish. Several cases of Fall/Winter mortality of the Pahrump Poolfish were documented. In Spring 2019, 101 large Relict Leopard Frog tadpoles and 111 juvenile frogs were released in the ponds. A nocturnal survey in March 2019 documented 2 large adult frogs including a calling male. Numerous small Relict Leopard Frog tadpoles were observed in late April 2019, which demonstrated natural recruitment. Larval Pahrump Poolfish were observed in May 2019, and a mark-recapture survey in June provided an estimate of 173 (95% CI 131-232) adult Poolfish between the two ponds. A visual encounter survey of Relict Leopard Frogs in August 2019 documented 195 juveniles and 4 small adults. In September 2019, a survey of Pahrump Poolfish documented 130 individuals including fish under 35 mm in length, which represented new recruitment. An additional eight ponds at the Springs Preserve may eventually hold Pahrump Poolfish and Relict Leopard Frogs.