Amargosa Speckled Dace (Rhinichthys osculus nevadensis)

General Habitat

The Amargosa Canyon form prefers pool-like habitat with deep (0.45-0.75 m), slow (less than 0.01 m3 sec-1) water. They are rare in the Amargosa River itself (Williams et al. 1982), but have probably never been very abundant there (Soltz and Naiman 1978). Dace are, however; abundant in Willow Creek and Willow Creek Reservoir (Williams et al. 1982). Willow Creek is a small, clear stream with low flow (1 cfs) and fine sand/silt substrates. It is characterized by a pH of 7.7, dissolved oxygen of 5-6 mg l-1, total dissolved solids of 700 ppm, and water temperatures of 21-28° C. The reservoir, however, is turbid, with a substrate of easily roiled fines. The periphery of the reservoir has dense stands of salt-cedar and cattails (Williams et al. 1982).

This population is confined to the Amargosa River in Amargosa Canyon and tributaries to it, especially Willow Creek and Willow Creek Reservoir (Williams et al. 1982). It was found in 1937 in a warm spring just north of Tecopa (Miller 1938), but that population is no longer present. Overall, its range has probably been reduced somewhat, but the exact extent is not known.


Speckled dace are small cyprinids, usually less than 90 mm total length, but Amargosa dace are small even for this species, rarely reaching 60 mm total length. They have a small, subterminal mouth, a pointed snout, thick caudal peduncle and slender body. The dorsal fin is set posterior to the origin of the pelvic fins. There are 6-9 dorsal fin rays (usually 8) and 6-7 anal fin rays (occasionally 8). Scales are small and there are 47-89 along the lateral line. The pharyngeal teeth are hooked with slight grinding surfaces. The dental formula is 1,4-4,1 or 2,4-4,2. They possess small barbels and a frenum that may or may not be attached to the premaxilla. Coloration is highly variable, but consists of a series of dark blotches on a lighter background. In reproductive individuals of both sexes, the bases of the fins become orange to red and males may develop tubercles on the pectoral tins.


During a 1981 survey of the Amargosa Canyon that included the river and Willow Creek, speckled dace comprised 1% of the fishes collected (Williams et al. 1982). Introduced mosquitofish comprised 40% of the fish fauna. This indicates that the dace are much less abundant than they used to be and are probably declining. There are, however, no historic estimates of abundance.

The major threat to the Amargosa Canyon speckled dace is the potential dewatering of its unique habitats, the Amargosa River and tributaries, by water withdrawals from both distant and near points on the aquifer that feeds the system and by local stream diversions. The Amargosa River apparently receives much of its permanent flow from springs fed by a large, ancient aquifer that extends into western Utah and central Nevada. The Las Vegas Valley Water District has proposed mining this water in large quantities to supply its ever-growing human population (E. L. Rothfuss, Superintendent of Death Valley National Park, letter to B. Bolster of CDFG, May 27, 1992). At the present time, farming operations and human settlements in the Amargosa region are withdrawing increasing amounts of water from the aquifer, which has already caused the water level of Devil's Hole in nearby Nevada (habitat of the endangered Devil's Hole pupfish, Cyprinodon diabolis) to drop (L. L. Lehman and R. G. Atkins, 1991, unpubl. report). If the Amargosa region withdrawals continue to increase and if Las Vegas proceeds with its planned withdrawals, it is highly likely that the Amargosa River could have its flows greatly reduced or even dry up completely during dry years. Already, diversions of springs and outflows on private land in the Tecopa area have probably reduced local flows in the river and local pupfish populations as well. With an increasing human population in Tecopa and the upper Amargosa Valley, demand for water and flood protection is increasing.

Although most of the land in the Amargosa Canyon is owned by The Nature Conservancy or the BLM, critical habitat for the dace includes a large tract of privately owned land, China Ranch. This ranch contains the headwater area of Willow Creek. Diversion of water from the creek or other alterations affecting water quality could cause dace populations to decline further.

A more immediate threat to the dace seems to be the introduced species present in the quiet-water habitat it prefers. In particular, mosquitofish may be reducing its numbers through competition andpredation. Because much of its habitat is on public land, additional introductions of undesirable species that may affect dace populations are also possible.

Conservation / Management

Populations should be monitored annually. Efforts should be made to ensure a natural flow of water in Willow Creek and the Amargosa River, including flood flows that reduce populations of introduced fishes. Fortunately, most of the canyon area is now owned by The Nature Conservancy and the BLM. Amargosa Canyon is part of an Area of Critical Environmental Concern and is closed to offroad vehicle use. Fences and barriers need to be properly maintained, however, because vehicle trespass is a common problem.

In Willow Creek, an evaluation should be conducted to see if permanent eradication of exotic species from speckled dace habitat is possible. If it is not, invasion-proof refuges for the dace (and Amargosa pupfish) should be created in the drainage.

The small population of dace in the Amargosa River may be dependent upon recruitment of dace from Willow Creek. If this is so, maintenance of adequate flows from China Ranch are critical to the survival of this fish. Efforts also should be made to locate the spring occupied by dace in 1937 (Miller 1938) to determine if this spring, or another nearby spring, could again support a dace population. As discussed for the Amargosa pupfish, frequent surveys of the Amargosa Canyon are necessary to monitor habitat conditions and the presence of introduced fishes.

The most difficult problem is dealing with the results of water removal from the aquifer that apparently feeds the river. The U.S. Supreme Court decision that protected the Devil's Hole pupfish from water withdrawals may be some help here, but its application on a larger, regional basis is uncertain. Listing of the dace and other regional fishes as threatened might forestall massive groundwater pumping by Las Vegas Valley Water District until it can be determined for certain whether or not the Amargosa River depends on the aquifer and would be threatened by the pumping. Protection of the fishes thus could protect an entire unique desert ecosystem.

Information in this account is excerpted from a CDFG publication link opens in new tab or windowFish Species of Special Concern in California - Second Edition (PDF) written by Peter B. Moyle, Ronald M. Yoshiyama, Jack E. Williams, and Eric D. Wikramanayake through the University of California, June 1995. This document will provide a more detailed account of this and other species.