Thermal tolerances and preferences of fishes of the Virgin River system (Utah, Arizona, Nevada)

—Critical thermal maxima (CTM) and thermal preferenda of the common fishes of the Virgin River were examined. Differences in final temperature preferenda and CTM for species with low thermal lability (speckled dace, spinedace, roundtail chub) correspond well with differences in their distribution and abundance in the river. These species shifted their acute thermal preferences relatively little as acclimation temperature increased. For thermally labile species (woundfin, red shiner, desert sucker, and fiannehnouth sucker), the final preferendum is a less precise indicator of probable distribution. The woundfin, an endangered fish, has a high CTM (39.5 C at 25 C acclimation) and a labile acute preferendum (slope nearest 1) compared to other species in the system. The introduced red shiner likewise has a high CTM and a labile acute preferendum. In cooler temperatures, its acute preferendum shifts more rapidly than does that of the woundfin. At higher temperatures (above 15 C), the red shiner does not shift its acute preferendum as rapidly as does the woundfin. The red shiner, however, has a higher final preferendum. For thermally labile species, influence of acclimation temperature on mean preferendum, together with CTM, provides a better insight into distributional relationships within the system. In recent years, agricultural, municipal, and industrial water uses in arid regions of the southwestern United States have reduced both stream flows and water quality. The consequent alterations in thermal, chemical, and flow regimes, coupled with the discharge of various effluents and the introduction of nonnative fishes, have seriously reduced many native fish stocks (Deacon and Minckley 1974, Deacon 1979, Pister 1979, 1981). The Virgin River in Utah, Arizona, and Nevada is an example of such a system. Its shifting, sandy bottoms, steep gradients, high sediment loads, variable flows, large daily and seasonal fluctuations in temperature, and other physical and chemical characteristics are typical of desert streams (Cook 1960, Deacon and Minckley 1974, Naiman 1981). Below Zion National Park, Utah, several natural physicochemical and geographic barriers disrupt the continuity of the biotic communities. These include Pah Tempe Springs, a series of over 100 saline hot springs emerging along the Hurricane Fault in Utah; and the Virgin River Gorge in Arizona where, during much of the year, the entire flow seeps below ground and reemerges in springs above Littlefield, Arizona (Sandberg and Sultz 1982). The fish fauna of the Virgin River consists of only six native species: speckled dace (Rhinichthys osculus); flannelmouth sucker (Catostomus latipinnis); desert sucker (Catostomus [Pantosteus] clarki); Virgin spinedace (Lepidorneda mollispinis mollispinis) and Virgin roundtail chub (Gila robusta seminuda), the latter two of which are endemic subspecies; and woundfin (Plagopterus argentissimus), which is an endemic species. The woundfin is listed as endangered (U.S. Fish and Wildlife Service 1986), the Virgin roundtail has been recommended for endangered status, and the Virgin spinedace has been recommended for threatened status (Deacon 1979, Deacon et al. 1979). The red shiner, Notropis lutrensis, was introduced into the Colorado River system as a bait fish in the early 1950s (Hubbs 1954). Of the 13 introduced fish species recorded from the lower Virgin River (Cross 1985), only the red shiner has become well established (Williams 1977, Cross 1978a). Agricultural diversion and groundwater use since 1900 have reduced flows in the mainstream to the extent that long stretches may be dry during summer months. Following completion of the Quail Creek Reservoir project early in 1985, an unanticipated, dramatic increase in discharge of Pah Tempe Springs 'Department of Biological Sciences. University of Nevada at Las Vegas, Las Vegas. Nevada 89154. Environmental Science and Engineering Program, University of California School of Public Health, Los Angeles, Los Angeles, California 90024. Present address: Office of Waste Programs Enforcement, U.S. Environmental Protection Agency, Washington, D.C. 20460. Department of Physics, University of California, San Francisco, School of Medicine, San Francisco, California 94117.