Owens Speckled Dace (Rhinichthys osculus robustus)
Lead CDFW biologist: Steve Parmenter
Speckled dace from the Owens Basin are known to occupy a variety of habitats ranging from small coldwater streams and hot-spring systems, although they are rarely found in water exceeding 29° C. They also have been found in irrigation ditches near Bishop. Despite the large variety of habitats apparently suitable to speckled dace of the Owens Basin, their disappearance from numerous localities since the 1930s and 1940s suggests their vulnerability to habitat modifications or to invasion by exotic fishes.
Speckled dace occupied most small streams and springs in the Owens Valley. The Mojave River is the only river system in the Owens Basin that has not been occupied at some time by speckled dace. Sada (1989) reported that 17 different sites are represented in collections at the University of Michigan Museum of Zoology, California Academy of Sciences and files of CDFW. Speckled dace no longer occur at many of the sites they were collected from during the 1930s-1960s such as the Hot Creek system, or springs near Benton Crossing in Long Valley (Mono Co.) or Little Lake (Inyo Co.). Only one small population remains in one of three springs near Benton that were historically occupied. A survey of 166 sites in 1988-1990 found speckled dace extirpated from most historic localities). They currently persist at two Long Valley sites (Whitmore Hot Springs and Little Alkali Lake), one East Fork Owens River site near Benton (a spring on Mathieu Ranch/Lower Marble Creek), and live sites in the northern Owens Valley (North McNally Ditch, North Fork Bishop Creek, irrigation ditch in north Bishop, Lower Horton Creek, and Lower Pine and Rock creeks). In the northern Owens Valley, speckled dace no longer are found in Fish Slough, in irrigation canals between Bishop and Big Pine, or in the Owens River. They also were not found at Warm Springs, where CDFW biologists had planted 75 speckled dace in 1983. In the southern Owens Valley, speckled dace have been collected only from Little Lake but no longer occur there. Speckled dace now occur primarily in streams and irrigation ditches around Bishop, but the populations are scattered, mostly small and fluctuate widely in size. Some of these populations are ephemeral.
Speckled dace are highly variable in morphology but are generally distinguished by: small, subterminal mouths; pointed snout; small, irregularly placed scales; and torpedo-shaped body. Total body length is usually less than 90 mm. Typically, dorsal fin rays number 8 (range 6-9) and anal fin rays number 7 (range 6-8). As their common name indicates, numerous black speckles cover the body, except in fish from turbid waters which may lack them. Gilbert's (1893) description of Rhinichthys nevadensis generally fits the above details, with these additional characteristics: (1) lateral line incomplete and with 65 scales, (2) mouth terminal (rather than subterminal), and (3) well-developed maxillary barbel.
Owens speckled dace are highly variable; populations differ significantly for many morphological characteristics, and they also are distinct from R. o. robustus of the Lahontan basin (Sada 1989). The high variability in characteristics, however, results in high morphological overlap between populations. The following characteristics are based on Sada (1989): the pharyngeal teeth, development of the supratemporal canal, and presence and location of tubercles are similar among Owens drainage populations and are characteristic of R. osculus. The frenum is well developed only in the now extirpated Little Lake population; barbels occur in most populations but are poorly developed in Long Valley populations and absent from Walker River fish. The following ranges in mean counts are for four populations in the Owens River drainage and one in the Walker River: lateral line scales 59.3-70.7; lateral line pores 11.6- 61.7; dorsal rays 7.8-8.0; anal rays 7.0-7.1; pectoral rays 12.0-13.9; pelvic rays 7.0-7.6; total vertebrae 36.9-38.1.
Particular life-history adaptations of speckled dace from the Owens Basin have yet to be determined. In general, speckled dace feed on small aquatic insects and algae (Moyle 1976). They typically live three years and attain a maximum size of 80 mm SL in inland basins (Moyle 1976). Owens speckled dace, however, rarely exceed 50 mm SL in length.
There are little data available on the historic abundance of this dace. Given its greatly diminished range due to extirpation of many populations, it is undoubtedly much less numerous than it once was and is continuing to decline. Even in the streams and irrigation ditches around Bishop, where they are widespread, speckled dace now occur at low densities (Sada 1989).
A detailed account of the extant populations of Owens speckled dace, including human impacts and current threats, is given by Sada (1989). Sada (1989) includes among the causes of the decline of Owens speckled dace the diversion of waters from the streams (which may dry up on occasion), destruction of stream and riparian habitat by livestock, and predation by introduced fishes such as brown trout and green sunfish. The most significant threats are the increasing diversion of streams and associated habitat alterations, especially grazing and trampling of streambanks by cattle. Populations in springs and pools are under continued threat from illegal introductions of predatory fishes, such as largemouth bass. Other introduced fishes that may be competitors or predators of speckled dace are mosquitofish (Gambusia affinis) throughout Long Valley, and channel cattish (Ictalurus punctatus) and Sacramento perch (Archoplites interruptus) in Benton Valley springs. The two remnant populations in Long Valley and the one in the East Fork Owens River are small, occur in extremely small springs and are therefore vulnerable to extirpation. The single East Fork Owens River population is restricted to poorquality habitat that is 'frequently altered and occupied by introduced predators' (Sada 1989).
The most critical needs for Owens speckled dace are formal protection of existing habitat and creation of artificial refugia for populations in immediate danger of extinction. Sada (1989) recommends formal listing of the populations in Long Valley and near Benton as endangered, and populations in the northern Owens Valley as threatened. The Long Valley populations are especially in need of attention because of the high probability that they represent a distinct subspecies by themselves (Sada et al. 1993). However, all isolated populations of dace are susceptible to habitat changes and to the establishment of exotic fishes (Williams and Sada 1985). Establishment of speckled dace at additional sites in the Owens River drainage, as recommended by Sada (1989), would reduce the chance that they would be completely extirpated from this area. Remnant populations of Owens speckled dace should be monitored annually, particularly those in springs.
Information in this account is excerpted from a DFG publication Fish Species of Special Concern in California - Second Edition 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.