Habitat Restoration as a Means of Controlling Non-Native Fish in a Mojave Desert Oasis

Transcription

1 Habitat Restoration as a Means of Controlling Non-Native Fish in a Mojave Desert Oasis G. Gary Scoppettone, 1,2 Peter H. Rissler, 1 Chad Gourley, 3 and Cynthia Martinez 4 Abstract Non-native fish generally cause native fish decline, and once non-natives are established, control or elimination is usually problematic. Because non-native fish colonization has been greatest in anthropogenically altered habitats, restoring habitat similar to predisturbance conditions may offer a viable means of non-native fish control. In this investigation we identified habitats favoring native over non-native fish in a Mojave Desert oasis (Ash Meadows) and used this information to restore one of its major warm water spring systems (Kings Pool Spring). Prior to restoration, native fishes predominated in warm water (25 32 C) stream and spring-pool habitat, whereas non-natives predominated in cool water ( 23 C) spring-pool and marsh/ slack water habitat. Native Amargosa pupfish (Cyprinodon nevadensis) and Ash Meadows speckled dace (Rhinichthys osculus nevadensis) inhabited significantly faster mean water column velocities (MWCV) and greater total depth (TD) than non-native Sailfin molly (Poecilia latipinna) and Mosquitofish (Gambusia affinis) in warm water stream habitat, and Ash Meadows speckled dace inhabited significantly faster water than non-natives in cool water stream habitat. Modification of the outflow of Kings Pool Spring from marsh to warm water stream, with MWCV, TD, and temperature favoring native fish, changed the fish composition from predominantly nonnative Sailfin molly and Mosquitofish to predominantly Ash Meadows pupfish. This result supports the hypothesis that restoring spring systems to a semblance of predisturbance conditions would promote recolonization of native fishes and deter non-native fish invasion and proliferation. Key words: Ash Meadows, Cyprinodon nevadensis, habitat manipulation, Mojave Desert, non-native fish control, Rhinichthys osculus nevadensis, thermo springs. Introduction Non-native fish alter native aquatic communities, are an agent of native fish decline and extirpation (Taylor et al. 1984; Moyle et al. 1986; Miller et al. 1989; Minckley & Deacon 1991), and are difficult to control or eliminate. Chemical treatment is often unsuccessful (Meffe 1983; Rinne & Turner 1991; Meronek et al. 1996) and is lethal to aquatic invertebrates (Morrison 1987; Mangum & Madrigal 1999), and physical removal has usually met with limited success (Meronek et al. 1996; Knapp & Mathews 1998). Thus, alternate and innovative strategies need to be developed to enhance population expansion of natives at the expense of non-natives. Physical changes in aquatic ecosystems can alter fish community structure, population demographics, and relative abundance of species (Bain et al. 1988; Rabeni & Jacobson 1993; Gido & Propst 1999). Therefore, nonnative fish control may be possible through the creation or re-creation of habitat that promotes native fishes over 1 Biological Resources Division, U.S. Geological Survey, 134 Financial Boulevard, Suite 161, Reno, NV 892, U.S.A. 2 Address correspondence to G. G. Scoppettone, gary_scoppettone@ usgs.gov 3 Otis Bay, Incorporated, 11 Mule Deer Drive, Reno, NV 87523, U.S.A. 4 U.S. Fish and Wildlife Service, 471 N. Torrey Pines Drive, Las Vegas, NV 8913, U.S.A. Ó 25 Society for Ecological Restoration International non-natives. Because non-native fish invasions are often associated with anthropogenically disturbed environments (Moyle & Nichols 1974), restoration of the aquatic system may counteract the effects of human disturbance. In undisturbed or more natural habitat, native fishes may have a better chance of tolerating a non-native fish invasion because such conditions contributed to evolution of the native species (Southwood 1988; Ricklefs 1991). Thus, restoring aquatic habitat to predisturbance conditions may serve to promote native fishes (Baltz & Moyle 1993; Moyle & Light 1996) if environmental conditions that favor natives over non-native species are emphasized in the restoration. Ash Meadows National Wildlife Refuge (AMNWR) in Northern Mojave Desert offers an opportunity to study the feasibility of non-native fish control through habitat manipulation associated with spring and stream restoration. Ash Meadows water resources are a series of thermal springs with discharge sufficiently low that flows are manipulable. The spring habitats have been significantly altered and invaded by non-native fishes. Typical of spring systems of the southwestern United States, there are few native species present (Miller 1961). In addition, Ash Meadows has few non-native fish species, thus simplifying the task of identifying habitat conditions favoring the native fishes over non-natives. The natives in spring systems of most of AMNWR had Amargosa pupfish (Cyprinodon nevadensis), Ash Meadows speckled dace JUNE 25 Restoration Ecology Vol. 13, No. 2, pp

2 (Rhinichthys osculus nevadensis), or both species. The primary non-native fishes are Mosquitofish (Gambusia affinis) and Sailfin molly (Poecilia latipinna). Like most aquarium fish invaders of Southwestern thermal springs, Sailfin molly evolved in lentic or slack water habitat (Harrington & Harrington 1961). This is also true of the Mosquitofish, which has successfully invaded slack and lentic temperate to warm water throughout the western United States (Swanson et al. 1996). Our impression from initial observations in Ash Meadows was that pupfish predominated over non-native fishes in warmer water, especially lotic warm water and that non-native fishes predominated in cool water, especially lentic habitat. Furthermore, Ash Meadows speckled dace do occur in warm water but flourish in cool fast water. We hypothesized that non-native lentic and slack water species can be controlled through spring system restoration so outflow channels retain their warmer temperature with velocities conducive to pupfish and speckled dace but detrimental to nonnative lentic forms. In this study we investigated habitat favoring native Ash Meadows pupfish and Ash Meadows speckled dace over non-native Sailfin molly and Mosquitofish. We also tracked species composition of a spring outflow before and after it was restored to promote native fishes over non-natives. Study Site The Mojave Desert is the driest region in North America, with Ash Meadows its largest oasis, and it harbors one of the greatest numbers of endemic species, for its area, in North America (Sada 199). Ash Meadows is situated within the Amargosa River Drainage, subdrainage of the Death Valley System at the southwestern edge of Nevada, just east of Death Valley, California (Fig. 1) (Hubbs & Miller 1948; Miller 1948). Ash Meadows primary water sources are approximately 24 thermal springs within a 7-km radius and with cumulative discharge of m 3 / second (Dudley & Larson 1976). Garside and Schilling (1979) reported near-source water temperatures from 18. to 33. C, with highly mineralized water and dissolved oxygen well below saturation. Historically, Ash Meadows spring water emerged from carbonated aquifers into small spring-pools and then through narrow outflow channels to discharge into Carson Slough. Using 1:24, scale 1-m digital elevation models and hydrologic modeling in ArcView GIS (ESRI, Redlands, CA, U.S.A.), we modeled the historic direction of the larger spring outflows and determined historic locations of over 45 km of perennial stream channel (Fig. 1). The simulated stream courses do not incorporate stream sinuosity, which would increase absolute channel length. Most Ash Meadows springs lie within an elevation of m above mean sea level (msl) and were historically connected via Carson Slough. Until years ago, three endemic fishes were known from these springs: Ash Meadows pupfish (Cyprinodon nevadensis mionectes), Ash Meadows speckled dace, and the now extinct Ash Meadows poolfish (Empetrichthys merriami) (Miller 1948, 1961). Centrally located on the eastern edge of AMNWR is a complex of higher-elevation (71 m msl) springs that are sufficiently isolated physiographically to harbor another endemic fish, Warm Springs pupfish (C. n. pectoralis) (Miller 1948). Each spring harboring Warm Springs pupfish is characterized by low water discharge (<.1 m 3 /second) and warm water (32 33 C). The most physiographically isolated of Ash Meadows fishes is the Devils Hole pupfish (C. diabolis), which occurs at 73 m msl in a 15-m depression on a hillside. At the northeast edge of AMNWR, Devils Hole has been part of the Death Valley National Park system since 1952 (Deacon & Williams 1991). Prior to its acquisition by the U.S. Fish and Wildlife Service for the preservation of its endemic species, Ash Meadows landscape had been greatly altered. Carson Slough was mined for peat and surrounding areas cleared and leveled for agricultural use. Several springheads were fitted with pumps, eliminating surface flow (Deacon & Bunnell 197; Pister 1974; Deacon & Williams 1991), and spring-pools were enlarged. Water was diverted from natural stream courses to a few earthen and concrete ditches and either stored in reservoirs or used directly for crop or pasture irrigation. Along with loss of natural channel, there was loss of native riparian corridors, and non-native vegetation became established along several of the new or altered stream courses. Massive levees constructed to protect agricultural fields and irrigation ditches from flash floods eliminated these intermittent events in most spring outflows. Mosquitofish became established in the lowerelevation springs by the 193s (Miller 1961), followed by Sailfin molly in most of the lower-elevation springs in the 196s (Deacon & Bunnell 197). Also in the 196s Largemouth bass (Micropterus salmoides) were stocked in Ash Meadows largest reservoir, Crystal Reservoir, and Brown bullhead (Ameiurus nebulosus) inhabited Davis Spring until removal through chemical treatment in Other introduced aquatic species include Bullfrog (Rana catesbeiana), Crayfish (Procambarus clarki), and Oriental snail (Melonoides tuberculata). Since the summer of 1993, AMNWR staff has annually removed Sailfin molly, Mosquitofish, and Crayfish from the larger spring-pools (St. George, unpublished data). Ash Meadows native fishes are all federally listed as endangered (U.S. Fish and Wildlife Service 1983), and their recovery is predicated on habitat restoration and elimination of non-natives (Sada 199). The once widespread Ash Meadows speckled dace was relegated to three spring complexes, a small fraction of the range described by Miller (1948). Warm Springs pupfish has been extirpated from one of the six springs and its range reduced in the others (Williams et al. 1985). Ash Meadows pupfish has remained widespread even though much of its former habitat had been eliminated (Pister 1974; Sada 199). When we began monitoring Ash Meadows fish populations in 1989, there were less than 1 km of spring lotic 248 Restoration Ecology JUNE 25

3 Figure 1. Map of AMNWR showing major spring systems as they existed in 1989 along with modeled historic connections of lower-elevation spring outflow channels preanthropogenic disturbance. Inset is AMNWR in relation to the state of Nevada. habitat, and this included concrete and earthen-lined irrigation ditches. Of this habitat, less than 5 km flowed in a well-incised channel with sufficient slope that water temperature was near constant and offered a variety of water velocities. In 1989 the source of one of these thermal streams (Kings Pool Spring) partially broke from its concrete channel approximately m from the source pool to form a headwater marsh, and by 1994 almost the entire.7 m 3 /second flow contributed to this proximal marsh (Fig. 2). In 1996 a 1-km meander channel was excavated along what was determined to be a historic outflow course (C. Gourley & E. Ammon 1997, Otis Bay, Incorporated, personal communication). It traveled southwest from the spring-pool, draining Kings Pool Spring Marsh before heading south (Fig. 2). In this article we refer to the concrete outflow channel as Kings Pool Spring Concrete Outflow and the excavated meander channel as Kings Pool Spring Restored Outflow. Discharge of individual Ash Meadows springs ranged from.4 to.18 m 3 /second. Emerging water typically issued into spring-pools ranging from 7 to 3 m in diameter and.5 to 1 m in depth and fed into stream channels JUNE 25 Restoration Ecology 249

4 Figure 2. Map showing Kings Pool Spring Concrete Outflow system as it was in 1989 and Kings Pool Spring Restored Outflow as it was in 23. ranging from.1 to 2.5 km in length before discharging into a marsh, pasture, or reservoir. Channels sampled ranged from.7 to 1.5 m in width and.2 to.8 m in depth. Materials and Methods Macrohabitat and Relative Species Composition To determine habitat that favored natives over nonnatives, we monitored fish species composition in five macrohabitat categories: warm water spring-pool, warm water stream, cool water spring-pool, cool water stream, and marsh-like habitat. Warm water spring-pool and stream habitats had near-constant year-round temperature from 25 to 33 C; this included six warm spring-pools and seven near-source outflows. Six cool water springpools also had near-constant temperature but the temperature ranged from 18 to 21 C. Cool water stream habitat had daily or seasonal water temperature falling below 22 C, but because of alteration of Ash Meadows spring systems, there was only one spring outflow (Jackrabbit) of this habitat type. Thermal springs cool in a downstream direction, and only Jackrabbit had a sufficiently long outflow to have cool water stream habitat. Marshy habitats, of which seven were sampled, exhibited a wide range in water temperature and included old irrigation channels clogged with Cattails (Typha) and Bulrush (Scripus), water seeping from man-made channels, and water that had originally been spread to irrigate pasture. Monitoring was by direct fish count with mask and snorkel or using standard Gee minnow traps lined with 1-mm mesh and baited with dog food. It was assumed that both methods would yield relative species abundance, and we did not mix methods within our five macrohabitats. Warm water spring-pools, warm water stream, and cool water stream were sufficiently open and clear that the snorkel method was used. Representative reaches along the outflow length (of warm water and cool stream) constituting 1 4% of the available habitat were snorkeled, whereas in spring-pools all fish were counted. In cool water pool and cool water marsh, water was often shallow, turbid, or heavily vegetated and could not be effectively snorkeled so we used Gee traps to obtain representative samples. We used a paired t test to determine significant predominance of native fishes in four of our five habitat categories (Sokal & Rohlf 1995). Cool water stream habitat was not tested because there was only one. For each of the four habitat types percentage of natives was paired against percentage of non-natives. Mean Water Column Velocity and Total Depth Used and Available We quantified mean water column velocity (MWCV) and total depth (TD) used by native and non-native fishes in warm and cool water stream habitat. MWCV and TD were studied because they can be integrated into stream restoration. Using mask and snorkel, fish were sighted and location was marked with a numbered washer to be revisited after completing a 1-m reach. Before placing numbered washers, fish species, size, and washer number were recorded. Warm water stream habitat use was studied in 2 Restoration Ecology JUNE 25

5 the Crystal Spring outflow where Ash Meadows pupfish occurred with Sailfin molly and Mosquitofish and in Jackrabbit Spring outflow, which supported Ash Meadows speckled dace along with the three species found in Crystal Spring outflow. Measurements in Crystal Spring were taken at 1 stations 1 m in length and spaced 2 m apart, extending 2,4 m downstream, and including concrete side channels. Sampling took place in winter and summer of 199, 1991, and 1992; a change in refuge water management curtailed sampling after summer In Jackrabbit Spring outflow, warm water stream conditions extended from the spring-pool downstream 7 m and cool water stream conditions from 1,1 m from the spring-pool to 3,7 m downstream. There were three 1-m-long stations spaced 2 m apart in the warm water lotic reach and four 1-m-long stations in the cool water stream reach spaced 1, m apart. Snorkeling was conducted seasonally from fall 199 to summer 1992 and then in winter and summer of Available MWCV and TD were quantified by conducting a cross-sectional profile at each station s downstream end, middle, and upstream end. Measurements were taken at nine evenly spaced points along the cross section. Outflow from Crystal and Jackrabbit springs varied little during the study, and measurements were taken in winter and summer 1991 and Velocity and depth measurements were made with a Marsh McBirney model 21D digital flow meter (Marsh McBirney, Incorporated, Frederick, MD, U.S.A.) mounted on a calibrated rod. One-factor analysis of variance (ANOVA) was used to test whether pupfish, speckled dace, molly, and Mosquitofish used significantly different velocity and depth in warm water stream habitat. Also tested were species MWCV and TD use in relation to available MWCV and TD among stations. Significant differences in MWCV and TD use were compared using a modified Bonferroni (a ¼.5) (Keppel 1982). Test Shift in Habitat Use Crystal Spring outflow (with Sailfin molly) and Fairbanks Spring (without Sailfin molly) were used to test the hypothesis that presence of Sailfin molly did not cause a shift in Ash Meadows pupfish MWCV and TD use. This was the only native non-native combination that presented the opportunity for testing. We used an ANOVA to test the null hypothesis that Ash Meadows pupfish use different MWCV and TD in the presence of Sailfin molly. Although Crystal Spring has substantially greater discharge than Fairbanks Spring, stream flow was split among its channels and we used monitoring stations that had flow comparable to Fairbanks and had the same fish complement (except for Sailfin molly). We took Ash Meadows pupfish habitat use information for Fairbanks outflow in winter 1999 and used winter habitat use information for Ash Meadows pupfish in Crystal Spring warm water stream habitat taken in 199, 1991, and Test between systems was not for the same years because flows of both had been manipulated during the course of the study and the two systems were only similar during these two time periods. Kings Pool Spring Marsh versus Kings Pool Spring Restored Outflow In 1997 Kings Pool Spring Marsh was drained and the water routed into an excavated channel configured to simulate the historic outflow stream (C. Gourley & E. Ammon 1997, Otis Bay, Incorporated, personal communication). The channel design incorporated MWCV and TD used by Ash Meadows pupfish and Ash Meadows speckled dace but used only infrequently by Sailfin molly and Mosquitofish. We compared fish species composition of Kings Pool Spring Marsh with Kings Pool Spring Restored Outflow to gauge the efficacy of habitat manipulation for the control of Sailfin molly and Mosquitofish. Kings Pool Spring Marsh was sampled in winter 1989 by placing 12 minnow traps in representative areas. In Kings Pool Spring Restored Outflow, three -m-long stations were snorkeled in winter 22, 5 years postrestoration. Stations were at the downstream end, middle, and upper end of the restored 1-km-long channel. Chi-square test was used to determine if there was a significant change in proportion of native to non-native fish after the change from marsh to stream habitat. In April 22 we quantified MWCV and TD used by Ash Meadows pupfish, Sailfin molly, and Mosquitofish at three -m-long stations along the restored stream reach to determine if they showed the same habitat use as in Crystal Spring and Jackrabbit Spring systems. Available MWCV and TD were quantified by conducting crosssectional profiles at three equally spaced transects at the upper, middle, and lower 1 m of each -m-long station. MWCV and TD were measured at nine points equally dividing each cross-sectional transect. One-factor ANOVA was used to test whether pupfish used significantly different velocity and depth than Sailfin molly and Mosquitofish in Kings Pool Restored Outflow. We also tested species MWCV and TD use in relation to available MWCV and TD. Significant differences in MWCV and TD use were compared using a modified Bonferroni (a ¼.5) (Keppel 1982). Results Habitat and Relative Species Composition Native fishes predominated in warm water spring-pools and streams, whereas non-natives predominated in cool water spring-pools and marshy habitat. An average of 8% of the fish in warm water streams and 65% in warm water spring-pools were native (Fig. 3). The difference was significant in stream habitat (t 6 ¼ 26.44, p ¼.1), but variance was very high among spring-pools and the JUNE 25 Restoration Ecology 251

6 Frequency (%) Native Non-Native In the cool water segment of this system pupfish and Sailfin molly did not use significantly (F [1] ¼ 2.72, p ¼.99) different MWCV (Fig. 4); both used significantly (F [1] ¼ 28.93, p <.1) faster water than the available MWCV and than that used by Mosquitofish, but the absolute differences were relatively small. Speckled dace used significantly (F [1] ¼ 15.27, p <.1) faster MWCV than Sailfin molly and pupfish. Pupfish and speckled dace occupied the deepest water, but the depth was not significantly (F [1] ¼.16, p ¼.692) greater than that used by Sailfin molly. Mosquitofish occupied significantly (F [1] ¼ 38.85, p <.1) shallower water than the other species. Stream Warm Stream Cool Spring-pool Spring-pool Warm Cool Habitat Marsh Cool Figure 3. Mean percent and standard deviation of native and non-native fishes in five aquatic habitat categories in Ash Meadows, Nevada. Numbers denote each habitat type sample size. difference was not significant (t 5 ¼ 21.16, p ¼.299). There was a significantly greater percentage (92%) of non-natives in cool water spring-pools (t 5 ¼ 1.44, p ¼.1) and cool water marshes (75%; t 6 ¼ 6.7, p ¼.1) than natives. Native and non-native composition was similar for the single cool water stream sampled. MWCV and TD Used and Available In warm water, native fishes occupied faster water than non-native fishes (Fig. 4). In Crystal Spring stream, warm water MWCV used by pupfish (21.7 cm/second) was not significantly different (F [1] ¼ 2.62, p ¼.16) from the stream s available MWCV (23.1 cm/second) but was over twice as fast as that used by Sailfin molly and Mosquitofish (<9. cm/second), and this difference was significant (F [1] ¼ 182.8, p <.1). Pupfish occupied the deepest water but not significantly (F [1] ¼ 1.97, p ¼.16) deeper than that occupied by Sailfin molly. Pupfish and Sailfin molly occupied mean TD significantly (F [1] ¼ 13.77, p <.1) greater than the mean available TD, whereas Mosquitofish occupied mean TD significantly (F [1] ¼ 51.4, p <.1) less than mean available depth. In Jackrabbit Spring stream, pupfish and speckled dace tended to use significantly (F [1] ¼ 24.91, p <.1) greater MWCV in warm water than the stream s MWCV, whereas Sailfin molly and Mosquitofish used MWCV significantly (F [1] ¼ 4.43, p ¼.36) less than the available MWCV. Speckled dace used the fastest MWCV at 3 cm/second, and this was significantly (F [1] ¼ 24.91, p ¼.1) faster than the 2 cm/ second used by pupfish. Jackrabbit Spring pupfish and speckled dace used significantly (F [1] ¼ 11.18, p ¼.1) deeper water than Sailfin molly and Mosquitofish, but the difference was small. Jackrabbit Spring outflow had the only substantial cool water stream habitat but with a limited range in MWCV. Test of Microhabitat Shift Sailfin molly did not cause pupfish to shift MWCV or TD use. In Crystal Spring outflow pupfish used an MWCV of 21.5 cm/second and mean TD of 35.2 cm compared to an MWCV of 21.6 cm/second and mean TD of 32.1 cm for Fairbanks Spring outflow. These differences were not significantly different for either MWCV (F [1] ¼.1, p ¼.976) or mean TD (F [1] ¼ 1.2, p ¼.275). Kings Pool Spring Marsh versus Kings Pool Spring Restored Outflow After the conversion of Kings Pool Spring outflow from marsh to stream warm water habitat, there was a significant shift (v 2 1 ¼ 68.63, p <.1) in species composition, from 23 to 91% native fish (Table 1). In addition, most of the Sailfin molly and Mosquitofish counted were in the downstream station in the restored channel, which was only 3 m upstream of marsh-like habitat. The fish found in the two upstream stations were 99.5% pupfish. In the restored channel, pupfish occupied faster and deeper water than did non-natives (Table 2). They occurred in MWCV of 27 cm/second compared to 18 and 15 cm/second for Sailfin molly and Mosquitofish, respectively. MWCV used by pupfish was not significantly (F [1] ¼ 2.61, p ¼.17) different than the 3 cm/second available but was significantly (F [1] ¼ 6.53, p ¼.11) greater than that used by Sailfin molly and Mosquitofish. There was no significant difference (F [1] ¼.13, p ¼.721) in stream mean TD and the mean depth used by Sailfin molly, but pupfish used significantly (F [1] ¼ 37.95, p <.1) greater TD than the mean stream depth and that used by Sailfin molly. Mosquitofish used significantly (F [1] ¼ 1.95, p ¼.1) shallower water than the mean TD used by pupfish and Sailfin molly. Discussion Water flow has been employed to promote native over non-native fishes in water systems that are both altered and highly regulated (Tyus 1992; Marchetti & Moyle 21; Propst & Gido 24). The spring systems of Ash Meadows are not highly regulated but are very much altered, and 252 Restoration Ecology JUNE 25

7 Warm Stream Mean Water Column Velocity (cm/sec) Crystal Spring 412 Jackrabbit Spring Total Depth (cm) Crystal Spring 412 Jackrabbit Spring Stream Pupfish Molly Mosquito fish Dace Stream Pupfish Molly Mosquito fish Dace Cool Stream Mean Water Column Velocity (cm/sec) Jackrabbit Spring Stream Pupfish Molly Mosquito fish 81 Dace Total Depth (cm) Stream Pupfish Jackrabbit Spring Molly Mosquito fish Dace Figure 4. MWCV and TD used by Ash Meadows fishes in warm and cool water lotic habitats in Crystal Spring and Jackrabbit Spring outflows. Numbers denote sample size, bars the mean use, and lines the standard deviation. there is thus ample opportunity for near-complete restoration of physical habitat with its ensuing benefits to native fishes. Our study suggests that a channel configuration that retains the spring s high stable temperature at a mean flow velocity of about 3 cm/second favors native Ash Meadows fishes, especially Ash Meadows pupfish. Like Sailfin molly, many of the successful invaders of thermal springs of the American Southwest are tropical aquarium fish that prefer lentic conditions (Courtenay et al. 1984). Although they can endure the chronically warm, high-salinity water, they tend to avoid the fast water used by Ash Meadows pupfish. Table 1. Fish species by percent composition in Kings Pool outflow before and after conversion from marsh to stream habitat. Native Non-Native Pupfish Molly Mosquitofish Marsh before restoration 23% (54) 61% (144) 16% (37) Stream after restoration 91% (814) 8% (71) 1% (9) Numbers in parentheses represent the number observed. That native fishes did not predominate in cool water lotic habitat does not imply that they are not suited to, or are competitively excluded from, relatively cool flowing water. Speckled dace flourish in the cool water stream conditions of the upper Amargosa River (Soltz & Naiman 1978). Ash Meadows speckled dace appear to require cooler water. They have been found to reproduce in water temperatures ranging from 17.5 to 24. C (Scoppettone, Table 2. MWCV and TD of Kings Pool Restored Outflow (KPRO), and MWCV and TD used by Ash Meadows pupfish, Sailfin molly, and Mosquitofish. Mean ± SD MWCV (cm/sec) TD (cm) n KPRO 3.3 ± 17.6 a 21.9 ± 6.9 a 216 Pupfish 26.9 ± 8.3 a 28.1 ± 5.3 b 63 Molly 18.5 ± 4.9 b 21.4 ± 8.8 a 29 Mosquitofish 14.7 ± 4.5 b 17.2 ± 2.8 c 31 Species and Kings Pool Restored Outflow followed by the same letter do not differ significantly using one-factor ANOVA and modified Bonferroni (a ¼.5). JUNE 25 Restoration Ecology 253

8 unpublished data), and two of the springs (Bradford 1 and Bradford 2) in which they occur are cool water. Speckled dace are known to persist in a broad array of habitats and has been described as a bottom browser on invertebrates (Moyle 22). In lotic habitat in Ash Meadows, they have been associated with faster water feeding on invertebrate drift (Scoppettone, unpublished data). The loss of cool water lotic habitat associated with habitat alteration may have contributed to the loss of speckled dace in spring systems throughout Ash Meadows. We focused on MWCV in stream restoration for controlling non-native fishes. However, enhancement of natives in streams influences adjacent spring-pools by supplying a source of native fishes rather than non-natives; pupfish especially are known to expand their range (Baugh et al. 1986). Although pupfish tended to be the predominant species in warm water spring-pools, their abundance was extremely low in cool water spring-pools and marshes, even though Amargosa pupfish are known to be tolerant of temperature extremes (Gerking et al. 1979; Gerking & Lee 1983). In fact, pupfish occurred in only one of six cool water spring-pools and composed only 11% of the population in the remaining spring (Forest Spring). All six systems had been disconnected from surrounding habitats during the period of agricultural development, and this isolation probably led to pupfish extirpation. Although Amargosa pupfish can feed and survive in these cool springs, they have only been found to reproduce in water temperatures from 25 to 31 C (Gerkin & Lee 1983). The chronically cool spring water (18 21 C) may not allow reproduction, leading to pupfish extirpation. Forest Spring now has pupfish, but they colonized after the partial restoration of Kings Pool Spring outflow reconnected to Forest Spring. There were no pupfish in this system in 1995 when it was isolated from warm water spring systems. Similarly, lack of connectivity caused the extirpation of Preston White River springfish (Crenichthys baileyi albivallis) from Lund Town Spring, a spring cooler than four others harboring the species (Scoppettone & Rissler 22). Thus, connectivity is another important element to evaluate for habitat restoration to promote native species. Non-native fishes may cause a shift in native fish habitat use (Brown & Moyle 1991; Douglas et al. 1994), thus obscuring the actual preference of natives. Sailfin molly did not cause a habitat shift in Ash Meadows pupfish in warm water lotic habitat. Mosquitofish is a sufficiently specialized surface dweller that it overlaps little with benthic natives and, consequently, is unlikely to cause a habitat shift. Therefore, the habitat in Kings Pool Spring Restored Outflow controls non-native Sailfin molly and Mosquitofish without compromising the quality of adult pupfish habitat. The influence of Sailfin molly and Mosquitofish on Ash Meadows speckled dace habitat shift requires further study. The transition of Kings Pool outflow from a marsh to warm water stream demonstrated the potential importance of lotic habitat for control of some non-native fish. In the design and construction of the restored outflow channel, emphasis was placed on creating salutary TD and MWCV condition for natives; Kings Pool Spring Restored Outflow had one of the highest ratios of native to nonnative fishes. We anticipate that this ratio will increase further once the channel is completed by eliminating the source of non-native fishes from the marshy habitat downstream. When the outflow channel of Kings Pool Spring Restored Outflow is completed, its length will extend to about 11 km and its lower reaches will be a cool water stream. Speckled dace will then be reintroduced into the system, and we can better define their habitat preference. Without a proactive restoration program, return of an aquatic ecosystem to its historic equilibrium may take centuries (Poff et al. 1997) if it returns at all. In the interim, there could be shifts in habitat features such as the formation of headwater marshes as occurred in Kings Pool Spring Concrete Outflow with habitat favoring non-native (poecilids) over native fishes. Headwater marshes pose an additional threat by providing excellent habitat for Largemouth bass and other predators. Restored lotic habitat increases the likelihood of native fishes predominating in Ash Meadows. The lotic habitat can be designed to favor the natives, which should further reduce the relative number of non-natives. Furthermore, connecting the stream channel with historic washes and their associated flash flood events may further serve to promote native fishes over nonnatives (Meffe 1984; Minckley & Meffe 1987). Restoration of preagricultural aquatic habitat is a management objective of AMNWR, and this study has shown that restoration can be used to reduce populations of non-native fishes. Acknowledgments Numerous individuals contributed to this study. Sean Shea, James Harvey, Stephanie Byers Mark Buettner, Peter Tuttle, James Call, Linda Hallock, and James Heinrich contributed in determining fish population density and habitat use. Sean Shea and Scott Cecchi assisted with graphics. The study was funded by Nevada Division of Wildlife, U.S. Fish and Wildlife Service, and the Biological Resources Division of the U.S. Geological Survey. We also thank Tom Strekal, Jim Deacon, Jerry Smith, Kristin Swaim, and Mark Fabes for reviewing this manuscript and for making useful suggestions. 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