Status of Two Endangered Fishes, Cyprinodon nevadensis mionectes and Rhinichthys osculus nevadensis, from Two Springs in Ash Meadows, Nevada

Abstract

The largest remaining populations of the endangered Ash Meadows speckled dace, Rhinichthys osculus nevadensis, and two of the largest remaining populations of the endangered Ash Meadows Amargosa pupfish, Cyprinodon nevadensis mionectes, occur in Jack Rabbit and Big Springs. Although both springs contain large populations of C. n. mionectes and small, but apparently viable, populations of R. o. nevadensis, they also contain large populations of exotic fishes, crayfish and bullfrogs. Natural barriers exclude exotic fishes from the spring pool and part of the outflow of Jack Rabbit Spring. Below the barriers, sailfin mollies, Poecilia latipinna, predominate, with mosquitofish, Gambusia affinis, occurring in relatively small numbers. Both native fishes and the exotics, P. latipinna and G. affinis, occur throughout the Big Spring system. In Big Spring, the large, relatively still-water habitats present in the spring pool and wider outflow channel allow G. affinis to predominate. The rarity of R. o. nevadensis appears attributable to the abundance of exotic species. Notes on life history, population sizes and management of the endangered fishes are also provided. Ash Meadows is an approximately 16,200 ha oasis surrounded by the Amargosa Desert in southwestern Nevada. More than 20 springs and innumerable seeps within Ash Meadows provide a myriad of aquatic habitats in the midst of this otherwise arid region. Endemism of aquatic organisms is high, particularly among the molluscan and fish faunas. A species flock of snails, most of which are undescribed, is restricted to various springs in Ash Meadows (D. W. Taylor, pers. comm.). All five of the native fishes are endemic. Unfortunately, physical and biological alteration of the habitats has been pervasive. Numerous exotic species, such as mosquitofish, Gambusia affinis, sailfin mollies, Poecilia latipinna, bullfrogs, Rana catesbeiana, and crayfish, Procambarus clarkii, have become established in the larger springs (Deacon and Williams, 1984; Hardy, 1980). Largemouth bass, Micropterus salmoides, were introduced into Crystal Pool, the largest spring in Ash Meadows, but have subsequently disappeared. During the early 1960's, an illegal tropical fish farm resulted in the temporary introduction of such exotic predators as the arawana, Osteoglossum bicirrhosum (Soltz and Naiman, 1978). Physical habitat alteration has included draining of marshes, removal of extensive mesquite (Prosopis pubescens, P. glandulosa) and ash (Fraxinus velutina var. coriacea) thickets, leveling of the land surface and excessive groundwater pumping for agriculture, channelization of spring outflows, and concrete lining of certain spring pools. The impact of these changes on the five native fishes has been severe. Historically, the Ash Meadows poolfish, Empetrichthys merriami, inhabited the larger, lower-elevation springs in the valley (Miller, 1948). The species was last collected in 1942 and is now considered extinct (Miller, 1961). The