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<br />cur in the Warm Springs area but are represented
<br />by relatively few individuals. Moapa speckled dace
<br />Rhinichthys osculus moapae occur at the lower
<br />boundary of the Warm Springs site, and roundtail
<br />chub Gila robusta extend further' into the area
<br />(Deacon and Bradley 1972; Cross 1976). Both are
<br />most abundant in the cooler, downstream reaches.
<br />Two nonnative fishes, western mosquito fish
<br />Gambusia affinis and shortfin molly Poecilia mex-
<br />icana, are established within the range of the Mo-
<br />apa dace and Moapa White River springfish. The
<br />western mosquitofish was already present when
<br />the Moapa dace was discovered in 1938 (Hubbs
<br />and Miller 1948), and it was the predominant spe-
<br />cies in the upper Muddy River in 1963 (Deacon
<br />and Bradley 1972). Western mosquitofish are om-
<br />nivorous and formidable predators on fish larvae,
<br />sometimes replacing native fish populations
<br />(Minckley 1973; Meffe 1985). Early effects of
<br />western mosquito fish on Moapa dace are undoc-
<br />umented, but the dace population appeared to be
<br />stable in the early 1960s (Cross 1976). The short-
<br />fin molly, introduced in 1963 (Hubbs and Deacon
<br />1964), was the most abundant species in the upper
<br />Muddy River in 1968 (Deacon and Bradley 1972).
<br />The Moapa dace population declined after short-
<br />fin mollies were introduced (Cross 1976). It is un-
<br />likely that the decline was caused by habitat deg-
<br />radation, because there had been no readily
<br />apparent physical changes in Moapa dace habitat
<br />(J. Deacon, University Nevada, personal com-
<br />munication). Short fin molly and mosquito fish oc-
<br />cur throughout the range of Moapa dace and Mo-
<br />apa White River springfish except for a 170-m
<br />stream reach at the south edge ofthe Warm Springs
<br />area and within the bounds of the Moapa National
<br />Wildlife Refuge. Little published life history in-
<br />formation is available on the shortfin molly except
<br />that, like the western mosquitofish, it bears live
<br />young (Meffe and Snelson 1989).
<br />In this paper I examine present niche overlap
<br />(habitat use and diet) of Moapa dace with both
<br />nonnative (shortfin molly and western mosqui-
<br />tofish) and native (Moapa White River springfish)
<br />species as a possible explanation for the observed
<br />decline of Moapa dace. Although overlap may
<br />suggest the potential for interspecific competition,
<br />lack of overla,p may imply that it has not been an
<br />important factor-as long as that lack is not the
<br />result of intense competition that caused niche
<br />shifts (Werner and Hall 1979). I then explore the
<br />value of an overlap analysis under present con-
<br />ditions for identifying causes of species decline. I
<br />also test the hypothesis that shortfin mollies and
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<br />western mosquito fish are more aggressive preda-
<br />tors on fish larvae than are Moapa White River
<br />springfish and that such predation may have caused
<br />the decline of the Moapa dace population.
<br />
<br />Methods
<br />
<br />I define the origin of Muddy River as the con-
<br />fluence of South Fork Stream and Upper Muddy
<br />Stream, both of which are spring fed (Figure 1).
<br />Two other spring-fed tributaries (Muddy Spring
<br />and Refuge Stream) enter the Muddy River in the
<br />Warm Springs area, which at the time of study
<br />was largely agricultural. Several spring-fed tribu-
<br />taries served as irrigation ditches, some lined with
<br />concrete; up to 25% of the river flow was used for
<br />irrigation.
<br />Established in 1979, the Moapa National Wild-
<br />life Refuge was a former swimming resort. Its
<br />spring discharge had been intermittently manip-
<br />ulated and chlorinated. Cumulative refuge spring
<br />flow was 0.09 m3/s. Its primary stream channel
<br />was treated with a piscicide in 1984 to eliminate
<br />shortfin molly and western mosquitofish. Moapa
<br />dace and Moapa White River springfish were re-
<br />introduced in 1984, and they were isolated from
<br />nonnative fishes by a 75-cm-high waterfall.
<br />Relative abundance. -I determined relative
<br />abundances of the various fishes in tributary
<br />streams where all life stages of Moapa dace and
<br />Moapa White River springfish were known to oc-
<br />cur. Numbers of fish in these streams, used by
<br />Moapa dace for reproduction and rearing, had not
<br />previously been documented. Numbers of adult
<br />Moapa White River springfish, shortfin mollies,
<br />and western mosquitofish were calculated by
<br />mark-recapture methods, and adult Moapa dace
<br />were counted with the aid of mask and snorkel.
<br />In summer 1984, I estimated relative abun-
<br />dance of adult Moapa White River springfish (> 27
<br />mm total length, TL), western mosquito fish (> 20
<br />mm TL), and shortfin mollies (> 20 mm TL) in
<br />four .tributaries: Refuge, Upper Apcar, Muddy
<br />Spring, and South Fork (Figure 1). Sampling oc-
<br />curred during 16-21 July in Upper Apcar, 18-22
<br />July in South Fork, 23-25 July in Refuge, and 16-
<br />23 August in Muddy Spring. Estimates were made
<br />by isolating representative lO-m reaches with a
<br />3-mm-mesh block net at each end. At least 10%
<br />of each stream's length was sampled. Fish within
<br />the reach were captured with minnow traps lined
<br />with I-mm-mesh fiberglass screen; traps were
<br />fished overnight for 14-16 h. The upper tip of the
<br />caudal fin was clipped from each captured fish and
<br />the fish was released. An adjusted Petersen esti-
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