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. lqq3 SeDp~sl\oV\~ Transactions of the American Fisheries Society 122:599~08. 1993 .. tILl 7 Interactions between Native and Nonnative Fishes of the Upper Muddy River, Nevada G. GARY SCOPPETTONE U.S. Fish a.nd Wildlife Service, National Fisheries Research Center Reno Substatj.~n, 4600 Kietzke Lane. C-l20, Reno. Nevada 89502, USA Abstract.-I investigated interactions between native and nonnative fishes in the upper Muddy River system to add insight into (I) the mechanism causing the decline of the Moapa dace Moapa coriacea after the introduction of the shortfin mony Poecilia mexicana, (2) the reason Moapa White River springfish Crenichthys baileyi moapae were less affected by the introduction, and (3) the reason interactions between natives is relatively benign. I investigated the hypothesis that the shortfin mony caused the decline of the Moapa dace through competition or predation on larvae, pressures not experienced by the Moapa White River springfish. Relative interspecific competition was analyzed by contrasting the ranges of spatial and dietary overlap among larval, juvenile, and adult life stages. There appeared to be moderate to low spatial overlap between the various life stages of native and nonnative fishes. Overlap in diet was highest between adult Moapa White River springfish and shortfin mollies. Laboratory experiments suggested that shortfin monies prey vigorously upon fish larvae. In terms of spatial habitat use, Moapa White River springfish larvae were less available to adult shortfin mollies for consumption than were Moapa dace larvae. When predatio,n on larvae is the mechanism by which nonnative fish reduce native forms, aggressiveness of the predator and the degree to which the predator overlaps in habitat with the prey may influence the degree to which a native fish population is affected. The addition of nonnative fish species may greatly alter the structure of a native fish com- munity (Moyle et al. 1986; Brown and Moyle 1991). Frequently, nonnative fishes replace or greatly diminish natives, and several western spe- cies are extinct or have experienced dramatic de- clines following the invasion of nonnative fishes (Miller et al. 1989; Moyle and Williams 1990). The mechanisms of these adverse interspecific in- teractions have received little attention (Moyle et al. 1986; Ross 1991). After the upper Muddy River, Nevada, was in- vaded by nonnative fishes, the two native fishes, Moapa dace Moapa coriacea and Moapa White River springfish Crenichthys baileyi moapae, de- clined (Cross 1976). The Moapa dace declined suf- ficiently that it was federally listed as endangered (U.S. Department of the Interior 1973). The Mo- apa White River springfish also .declined but less dramatically, and it is a candidate 2 species (being considered for listing as endangered or threatened; U.S. Fish and Wildlife Service 1991). Both species are endemic to the headwaters of the Muddy River (also called the Moapa River), Clark County, Nevada, a tributary to the Colo- rado River system. The river originates from about 20 warm springs (31.0--32.0oC at their sources) having a combined discharge of 1.1 m3/s (Eakin 1964). Water cools in a downstream direction. Moapa dace and Moapa White River springfish are thermophilic, restricted to temperatures be- tween 26.0 and 32.0oC. They are hence relegated to the upper 2 km of the approximately 40-km river and to several small headwater tributaries (Deacon and Bradley 1972; Cross 1976; Scoppet- tone et al. 1992). The general area in which they occur is known as the Warm Springs area (Hubbs and Miller 1948; La Rivers 1962). Reproduction occurs year-round and is confined to the upper, spring-fed tributaries, typically within 150 m of the warmwater discharges (Scoppettone et al. 1992). In these areas, water temperatures vary be- tween 29.0 and 32.20C and dissolved oxygen con- centrations vary between 4.1 and 6.2 mglL, both seasonally and over 24-h periods; conductivity is about 964 /-Ls/cm and pH is 7.5 (my unpublished data). Juveniles are found almost exclusively in the spring-fed tributarie~, whereas adults are also found in the main stem (Scoppettone et al. 1992). There is little published information pertaining to the life history of Moapa White River spring- fish. Other subspecies of C. baileyi are omnivo- rous and opportunistic in diet (Williams and Wil- liams 1982) and withstand unusual extremes in temperature and dissolved oxygen (Sumner and Sargent 1940; Hubbs and Hettler 1964; Hubbs et al. 1967). White River springfish reproduce year- round, laying eggs 1.9 mm in diameter that are typically attached to aquatic vegetation (Kopec 1949; Espinosa 1968). Their daily activity pat- terns were studied by Deacon and Wilson (1967). Two other, nonthermophilic, native fishes oc- 599