Unusual dominance by Desert pupfish (Cyprinodon macularius) in experimental ponds within the Salton Sea Basin.
The desert pupfish (Cyprinodon macularius) is a small cyprinodontid with an unusual ability to tolerate water-quality so extreme (e.g., temperatures >42[degrees]C, salinities about twice that of seawater, dissolved oxygen ca. 0.1 mg/L) as to exclude nearly all other species of fish (Schoenherr, 1988, 1992; Martin and Saiki, 2005). however, possibly due to geographical isolation from natural predators and competitors under which many species have evolved, desert pupfish usually do not fare well when forced to interact with nonnative species (Kodric-Brown and Mazzolini, 1992; Gido et al., 1999; Martin and Saiki, 2005; Rogowski and Stockwell, 2006). Loss and modification of habitat and pollution also may represent threats to desert pupfish. Thus, although originally occurring throughout the lower colorado and Gila river drainages of Arizona, California, and northern Mexico, the desert pupfish is now present mostly as small isolated populations around the salton sea and in the colorado River delta (Moyle, 2002). Due to plummeting numbers throughout its natural range, this species was listed as endangered in 1986 (united states Fish and wildlife service, 1986). A recovery plan was completed in 1993 (United States Fish and Wildlife Service, 1993).
As recently as the 1950s, desert pupfish were common or abundant in shoreline pools of the Salton Sea (Barlow, 1961; Walker et al., 1961). According to Barlow (1961), density in a small (50 [m.sup.2]) pool was ca. 150 desert pupfish/[m.sup.2], and one school of juveniles contained [less than or equal to]10,000 individuals. Today, greatly reduced densities of desert pupfish occur in natural tributaries such as the San Felipe Creek system and Salt Creek, in a few shoreline pools of the Salton Sea and, occasionally, in the Salton Sea itself (Moyle, 2002; Sutton, 2002; Martin and Saiki, 2005; S. Keeney, pers. comm.). Portions of San Felipe Creek and two of its tributaries, Carrizo Wash and Fish Creek Wash, are designated as critical habitat for desert pupfish (United States Fish and Wildlife Service, 1986). In addition, desert pupfish have colonized numerous agricultural drains and artificial ponds, although it is unknown if such human-made habitats could exist for more than a few years without periodic maintenance (Moyle, 2002). In ponds that rely on pumped water, desert pupfish are likely to succumb to desiccation within a few days if pumping is discontinued.
Except for the population in San Felipe Creek, desert pupfish typically represent only a minor portion of fish in the Salton Sea Basin, which usually is dominated by nonnative species such as western mosquitofish (Gambusia affinis), sailfin molly (Poecilia latipinna), red shiner (Cyprinella lutrensis), hybrid Mozambique tilapia (Oreochromis mossambica x O. urolepis), and redbelly tilapia (Tilapia zillii; Schoenherr, 1979; Black, 1980; Martin and Saiki, 2005).
In April 2006, an experimental system of four serially connected ponds was constructed by personnel of the united States Geological Survey and united States Bureau of Reclamation on the southern shore of the Salton Sea. These ponds were part of an investigation to determine if seleniferous brackish inflows from rivers could be mixed with low-selenium saline water from the Salton Sea to create shallow wetland habitats for aquatic birds. Because ponds received water pumped from the Alamo River and Salton Sea, an attempt was made to avoid entraining fish, especially desert pupfish. Specifically, intakes of pumps were screened with 6.4-mm-mesh hardware cloth. In addition, a rock barrier was installed in the intake trench (water-supply ditch) leading from the Salton Sea to block entry by small fish (a similar barrier was not installed in the Alamo River because high velocity of current at the intake was believed to discourage occupancy by desert pupfish).
Pumping of water into the experimental ponds began immediately after construction was completed in April 2006. however, despite efforts to exclude fish, brown pelicans (Pelecanus occidentalis) and other birds were observed feeding on unidentified fish in October 2006 (D. A. Barnum, pers. comm.). By Spring 2007, numerous small fish resembling desert pupfish were observed in the ponds. On 20 June 2007, cursory dip-net samples of fish collected from two ponds were identified as desert pupfish. in August 2007, baited minnow traps and crayfish traps were placed into ponds for ca. 3 h and captured 1,843 desert pupfish, but no nonnative species (J. Crayon, pers. comm.).
The purpose of our study was to better understand use of the four experimental ponds by fish. Specific objectives were to document quality and depth of water in the ponds, and to determine species and relative abundance of fishes. Although desert pupfish initially were abundant and seemingly the only species present, we were interested in determining if nonnative species also had colonized ponds and, if so, whether the proportions of desert pupfish and nonnative species would begin to resemble ratios in agricultural drains; i.e., where desert pupfish constituted a minor portion of the overall community of fishes.
MATERIALS AND METHODS--The study area consisted of four, shallow, flat-bottomed, earthen ponds, each with a surface area of ca. 10 ha, in imperial County, California (33[degrees]12'33"N, 115[degrees]35'01"W). Soil excavated during construction of ponds was used to create levees, roads, and several small islands for nesting birds. Except for a small infestation of salt cedar (Tamarix chinensis or T. ramossisima), which was removed by hand-pulling, these levees and islands were free of riparian vegetation. Although not abundant, rooted macrophytes such as wigeon grass (Ruppia maritima) occurred in ponds 1 and 2, whereas filamentous algae occurred in ponds 1, 2, and 3 (algal mats were especially prominent in shallower portions of pond 2 from spring through autumn).
For the first one-half of our study, brackish water from the Alamo River and saline water from the Salton Sea were alternately pumped through a 2.5-km-long pipeline to pond 1, where waters were mixed by wind. Mixing in pond 1 was intended to achieve a target salinity of 20 (expressed as a dimensionless unit in accordance with the Practical Salinity Scale of 1978, which defines salinity as the conductivity ratio of a sample of sea water to a standard solution of potassium chloride; United Nations Educational, Scientific, and Cultural organization, 1981), with salinity increasing through evaporation as water flowed by gravity through culverts first into pond 2, then into pond 3, and finally, into pond 4. Although excess water in pond 4 was supposed to discharge through a gated culvert into the Salton Sea, water levels did not reach the elevation of this culvert during our study. Consequently, pond 4 functioned largely as a terminal sink for the pumped water.
In autumn 2007, and continuing through November 2009, receding levels of water in the Salton Sea resulted in reduced salinity of water pumped from the intake trench due to incursions of flow from adjacent agricultural drains and the Alamo River. To maintain the salinity regime of experimental ponds, beginning in January 2009, a substitute source of saline water from pond 4 was pumped into pond 1 for dilution with brackish water from the Alamo River.
[FIGURE 1 OMITTED]
Sampling began in October 2007 and continued at seasonal intervals through November 2009. During each sampling effort, minnow traps were deployed; then, quality (temperature, dissolved oxygen, ph, specific conductance, and turbidity) and depth of water were measured at each trap. Measurements of water-quality were taken ca. 15 cm below the surface with a hydrolab Datasonde 4a multiprobe (hach Environmental, Loveland, Colorado). Measurements of specific conductance (mS/cm @ 25[degrees]C) were converted by the electronic meter to salinity by using the formula, [salinity.sub.calculated] = 5.995 x [10.sup.-8] * specific conductance4 -2.312 x [10.sup.-5] * specific [conductance.sup.3] + 3.4346 x [10.sup.-3] * specific [conductance.sup.2] + 0.53532 * specific conductance -0.015494 (Hydrolab Corporation, 1997). When specific conductance in pond 4 exceeded measuring capacity of the sensor, we diluted samples of water to achieve measurable readings of 26-70 mS/cm @ 25[degrees]C, then adjusted final estimates of specific conductance for amount of dilution. in January 2008, a hydrometer was used to estimate salinity directly from diluted samples of water. Depth was measured with a calibrated wooden pole.
Fish were sampled with collapsible minnow traps (25.4-cm high by 25.4-cm wide by 43.2-cm long, with 3.2-mm-square mesh and ca. 5-cm-diameter, funnel-mouth opening; Stock AMT-F, Netco, LLC, Memphis, Tennessee). Each uniquely identified trap was baited with 57 g of canned, fish-flavored, cat food placed in a capped, plastic, film canister drilled with several evenly spaced holes. Although the goal was to deploy 10 traps for 1 h each along the four shorelines of each pond, low levels of water often resulted in deployment of fewer traps (a depth of ca. 17 cm was needed to cover the mouth of the funnel).
Immediately upon retrieval of individual minnow traps, captured fish were removed for identification to species and total length was measured. All desert pupfish and most individuals of nonnative species were then released alive at site of capture (a few samples of nonnative species were sacrificed and preserved).
Raw data were compiled and stored as spreadsheets in Microsoft office Excel 2007. Summaries of data were computed with SAS version 9.2 (SAS institute, inc., Cary, North Carolina) and graphs were created with office Excel 2007.
RESULTS AND DISCUSSION--With one exception, variables for water-quality did not exhibit consistent spatial patterns among ponds (Table 1, Fig. 1). Estimates of salinity were the exception, with lowest salinities occurring in pond 1 and progressively higher salinities occurring in ponds 2-4. This spatial pattern persisted throughout the study (Fig. 1d).
Temporal patterns were observed for temperature and pH of water, but not for other variables of water-quality (Fig. 1). Temperature exhibited a strong seasonal pattern, with highest values occurring during summer (July) and lowest values during late autumn and winter (November and January; Fig. 1a). Concentration of hydrogen-ions (pH) did not exhibit a seasonal pattern; instead, higher pH-values occurred in October 2007-January 2009, then seemingly declined through November 2009 in most ponds (Fig. 1c).
Depth of water in individual ponds averaged 18-46 cm, with a maximum of 120 cm occurring in pond 3 within an excavated area immediately downstream from the inflow culvert that drained pond 2 (Table 1). On most sampling dates, mean depth was deepest in pond 3 and shallowest in pond 4 (Fig. 1f).
A total of 3,620 fish representing five species was captured during our study (Table 2). The desert pupfish, the only native species encountered, was the most numerous and comprised ca. 93% of the total number of fish. Nonnative species included the western mosquitofish (4.1%), sailfin molly (2.8%), and tilapia (a mix of hybrid Mozambique tilapia and redbelly tilapia, 0.1%).
Although we did not determine if fish colonized ponds through entrainment in water that was pumped, several small (total length ca.10 mm) tilapia were caught with a dip net in June 2007 in the intake trench leading from the Salton Sea; the rock barrier should have blocked their entry. After traversing the rock barrier, small fish could have passed through the 6.4-mm-mesh hardware cloth covering intake of the pump. Fish also could have entered ponds through water pumped from the Alamo River if they were able to pass through the hardware cloth that was covering the intake of the pump. In addition to entrainment, fish may have been transplanted inadvertently into ponds by fis-heating birds. Adult black skimmers (Rynchops niger) with fish in their bills often were observed landing on small islands within ponds, where fish subsequently were presented to mates or fed to chicks. After landing, these birds would walk around for several minutes while carrying desert pupfish and tilapia in their bills. Digital photographs of birds included several images of dropped fish that, if still alive, could have escaped into the water and survived to reproduce. Finally, unknown persons could have surreptitiously planted fish in ponds.
Except for pond 4, which was devoid of live fish, desert pupfish dominated catches in minnow traps. Combined abundance of desert pupfish and nonnative species increased in a progressive fashion from pond 1 (19.0 fish/10 trapping hours) to pond 2 (28.3 fish/10 trapping hours) and to pond 3 (67.0 fish/10 trapping hour). Although proportions of desert pupfish and nonnative species differed significantly among the three ponds ([chi square] = 648.52, df = 2, P < 0.001), pond 2 seemed most unusual because the percentage of desert pupfish was much lower in this pond (73.9%) than in the other two ponds (pond 1, 97.4%; pond 3, 99.3%).
With one exception (October 2008), desert pupfish dominated catches in minnow traps on all dates that were sampled. The catch in October 2008 was exceptional because sailfin mollies (5.3 sailfin mollies/10 trapping hours) accounted for nearly two-thirds of captures, with the remainder consisting of desert pupfish (2.7 desert pupfish/ 10 trapping hours). We suspect this temporary dominance of sailfin mollies was caused by placing traps near a localized aggregation of this species, because all sailfin mollies (64 individuals) were captured in traps set along the south shore of pond 2. Moreover, 59 sailfin mollies (92%) were captured in two of 10 traps, and the two traps were adjacent to each other. By January 2009, and continuing through the end of our study, desert pupfish once again dominated catches.
Surveys using baited minnow traps in 29 agricultural drains along the southern shore of the Salton Sea yielded 44,040 fish, of which >98% were of eight nonnative species and <2% (816 fish) consisted of desert pupfish (M. K. Saiki et al., in litt.). This included 2,270 fish captured from two drains flowing adjacent to the experimental ponds (o Drain, which parallels McDonald Road, and p Drain, which parallels Hazard Road), of which only 0.3% (7 fish) consisted of desert pupfish. Dominance by nonnative species in surveys targeting desert pupfish has been noted by investigators beginning in the late 1970s, particularly after sailfin mollies and tilapias were introduced into the Salton Sea Basin (e.g., Black, 1980; Schoenherr, 1981, 1988; Martin and Saiki, 2005; S. Keeney, pers. comm.). Other investigators elsewhere in the desert Southwest reported invasions by nonnative fishes that subsequently resulted in a decline in populations of native fishes, presumably in response to predation, competition, hybridization, or spread of exotic parasites and diseases (Minckley and Deacon, 1968; Deacon and Minckley, 1974; Moyle, 1976; Rinne and Minckley, 1991). However, this scenario has not occurred in the experimental ponds, although small populations of nonnative fishes have been present for >2 years.
Dominance of desert pupfish over nonnative species was apparent in ponds 1-3 (18.5-66.6 desert pupfish/10 trapping hours), but not in pond 4 where minnow traps failed to catch fish. The most likely reason fish were not captured in pond 4 was the high salinities (127-380) that occurred year-round in this terminal sink (Fig. 1d). However, we observed dead desert pupfish in pond 4 adjacent to the culvert draining pond 3 during July 2008 and July 2009, presumably after live individuals were transported downslope in the discharge from pond 3, then succumbing from osmotic stress after encountering the hypersaline water in pond 4. in general, fishes of diverse taxonomic groups have the capacity to persist, at least briefly, in salinities [less than or equal to] 2-3 times the concentration of seawater (Deacon and Minckley, 1974), although a few species have been found alive in even higher salinities. For example, Bayly (1972) reported that the sheepshead minnow (Cyprinodon variegatus) exhibited a remarkable tolerance to salinities of < 1-140. Higher salinities are almost universally lethal to fishes. Although Coleman (1929) reported desert pupfish inhabiting salt vats at salinities [less than or equal to] 50% saturation, or ca. 200, Barlow (1958) mentioned that these observations should be viewed with caution. Instead, observations at the Salton Sea by Barlow (1958) indicated that the maximum tolerance of salinity by juvenile desert pupfish was ca. 90; whereas, tolerance by adults probably was lower. A review by Schoenherr (1988) concluded that selected life stages of desert pupfish can tolerate the following extremes in salinity: eggs, 0-70; juveniles, 0-90; and adults, 0-70.
Scarcity of nonnative fishes in experimental ponds (mean [+ or -] 95% confidence interval for ponds 1-3, 2.8 [+ or -] 3.5 fish/10 trapping hours) contrasted markedly with higher abundance of nonnative fishes in the 29 agricultural drains (mean [+ or -] 95% confidence interval, 162.6 [+ or -] 44.4 fish/10 trapping hours; M. K. Saiki et al., in litt.). Ponds 1-3 typically exhibited salinities approaching or slightly exceeding those in drains, although, except for pond 3, salinities were well within upper limits of tolerance for western mosquitofish, sailfin mollies, and tilapias determined under laboratory conditions by other investigators (e.g., Chervinski, 1983; Nordlie et al., 1992; Sardella et al., 2004; Sardella and Brauner, 2007). Drains exhibited variable characteristics of water-quality presumably influenced by inflows of irrigation tailwater and subsurface drainage, and intrusion in the lowermost reaches of some drains by hypersaline water from the Salton Sea. According to M. K. Saiki et al. (in litt.), water-quality in the 29 drains during 2005-2009, collectively, exhibited the following characteristics (means; ranges in parentheses): temperature of water, 21.6[degrees]C (8.02-35.9[degrees]C); dissolved oxygen, 6.23 mg/L (0.100-19.2 mg/L); pH, 7.86 (6.14-9.53); specific conductance, 5.10 mS/cm @ 25[degrees]C (1.10-58.6 mS/cm @ 25[degrees]C); salinity, 2.94 (0.575-39.2); and turbidity, 51.9 nephelometric units (0.00-884 nephelometric units).
A thorough assessment of ecological phenomena responsible for the paucity of nonnative fishes (and unusual dominance by desert pupfish) in the experimental ponds was well beyond the scope of our study. Nevertheless, except for pond 4, which was fishless due to excessively high salinities, we suspect that abundance of nonnative fishes in the three remaining ponds was influenced by low temperatures in winter, high salinities, selective predation by fish-eating birds, or a combination of these and other still undetermined factors.
In the Salton Sea, both hybrid Mozambique tilapias and redbelly tilapias, but not western mosquitofish and sailfin mollies, are prone to winter die-offs when ambient temperatures of water decrease to 11-14[degrees]C for more than a few days (Moyle, 2002). During a cold front in November 2009, we recorded temperatures of water as low as 7.66[degrees]C in pond 1 (minimum temperatures were slightly higher in other ponds, ranging from 8.31[degrees]C in pond 2 to 11.4[degrees] C in pond 3). Moreover, temperatures of 11-14[degrees]C or lower occasionally occurred in experimental ponds during or shortly after daybreak during October-April, but temperatures generally increased 5-6[degrees]C or more later in the day. Although we never observed mortalities of tilapia in ponds during cool weather, Moyle (2002) indicated that cold-stressed individuals in the Salton Sea became sluggish and highly susceptible to fungal and parasitic infections that could have adversely affected their survival.
Although high salinities excluded fish from pond 4, considerably lower but still elevated salinities may have influenced abundances of species in other ponds. We measured maximum salinities of 24.2 in pond 1, 32.4 in pond 2, and 70.7 in pond 3 (Table 1). By comparison, western mosquitofish can tolerate salinities ranging from freshwater to 58, although they mostly occur where salinities are <25 (Chervinski, 1983; Moyle, 2002). Laboratory experiments by Nordlie et al. (1992) showed that sailfin mollies inhabiting freshwater were able to tolerate salinities from freshwater to 70, whereas those inhabiting brackish water tolerated salinities from freshwater to 80. These results were similar to observations by Herre (1929), who reported sailfin mollies to be abundant in salt ponds around Manila Bay, Philippine Islands, where salinities were 32-87, but were absent in ponds where salinity had risen to 94. Whitfield et al. (2006) observed Mozambique tilapia (Oreochromis mossambicus) surviving for extended periods in Lake Saint Lucia, South Africa, at salinities >110. By comparison, Sardella et al. (2007) determined that hybrid Mozambique tilapia could tolerate salinities as high as 95 for [less than or equal to]5 days under laboratory conditions when pre-acclimated to seawater. Moreover, hybrid Mozambique tilapia showed no mortality or signs of osmoregulatory disturbance during a 28-day exposure at 25[degrees]C to salinities as high as 65. However, a reduction in temperature to 15[degrees]C greatly reduced the salinity at which sublethal indicators and mortality occurred (mortality increased from 0% at a salinity of 43 to 85.7% at a salinity of 51, and to 100% at a salinity of 60; Sardella et al., 2004; Sardella and Brauner, 2007), presumably due to impaired function of gills that led to osmoregulatory stress. According to Sardella and Brauner (2007), impaired function of gills associated with low temperatures of water might explain mortality in winter among tilapias inhabiting the Salton Sea. These comparisons suggest that occasional episodes of high salinity could temporarily exclude western mosquitofish and possibly sailfin mollies and tilapias from pond 3, especially during cold weather; however, these comparisons do not explain scarcity of these nonnative fishes in ponds 1 and 2 where salinities never exceeded 32.4 (Table 1). Additionally, when catches in ponds 1 and 2 were compared to those in agricultural drains exhibiting a similar range of salinities (5.0-34.7), abundance of nonnative species in ponds (mean [+ or -] 95% confidence interval, 3.9 [+ or -] 5.4 fish/10 trapping hours) was lower than in drains (mean [+ or -] 95% confidence interval, 282.0 [+ or -] 231.0 fish/10 trapping hours; M. K. Saiki et al., in litt.), further suggesting that paucity of nonnative species in these two ponds was not due to high salinity.
Observations of intensive foraging activity by fish-eating birds in experimental ponds raised the possibility that predation by these birds was responsible for scarcity of nonnative fishes. According to observations made by Anderson (2008) during October 2006-September 2007, higher proportions of fish-eating birds (e.g., pelicans, egrets and herons, skimmers, terns) foraged within ponds than in adjacent portions of the Salton Sea (i.e., Morton Bay and shoreline). Although speculative, low percentages of nonnative fishes occurring in ponds might be due to selective predation by fish-eating birds. on several occasions, black skimmers feeding in ponds were observed with relatively large tilapia in their bills, although tilapia comprised only 0.1% of all fish captured with minnow traps during our study (Table 2). Egrets and herons also were observed feeding on aggregations of smaller-bodied fishes in ponds, including the abundant desert pupfish, but it is uncertain if these large wading birds exhibited a preference for nonnative species. Other investigators reported that a variety of avian predators are selective for larger fish and certain species (e.g., Britton and Moser, 1982; Trexler et al., 1994; Shealer, 1998; Gawlik, 2002; Steinmetz et al., 2003). In particular, Britton and Moser (1984) determined that herons selected for larger and more energetically profitable female western mosquitofish; thus, altering the sex ratio of natural populations of this fish. Trexler et al. (1994) also demonstrated that herons selected for larger prey when offered sailfin mollies under experimental conditions. Similar experiments are needed to determine if fish-eating birds at experimental ponds preferentially consumed certain species offish, and whether such selective predation is controlling abundance of nonnative species and allowing desert pupfish to dominate the community of fishes.
We thank the united States Geological Survey (Salton Sea Science office and the Fisheries: Aquatic and Endangered Resources Program) for funding this study, and especially D. A. Barnum for alerting us to the possible presence of desert pupfish in the experimental ponds and for providing useful information on history of the ponds. We also thank D. A. Barnum and three anonymous reviewers for comments on early drafts of the manuscript. Special thanks to B. E. Brussee, C. L. Emerson, J. A. Lawson-Hersch, T. M. Russell, and P. M. Valcarcel for assisting with field work, C. L. Emerson for creating and editing spreadsheets, and F. H. Mejia for translating the abstract into Spanish.
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Submitted 7 April 2010. Accepted 3 May 2011.
Associate Editor was Christopher M. Taylor.
MICHAEL K. SAIKI, * BARBARA A. MARTIN, AND THOMAS W. ANDERSON
United States Geological Survey, Western Fisheries Research Center, Dixon Duty Station, 6924 Tremont Road, Dixon, CA 95620 (MKS, BAM)
United States Geological Survey, Salton Sea Science Office, 78401 Highway 111, Suite R La Quinta, CA 92253 (TWA)
* Correspondent: email@example.com
TABLE 1--Characteristics measured at four experimental ponds in the Salton Sea Basin, Imperial County, California, October 2007- November 2009. Temperature Dissolved oxygen Pond Statistic ([degrees]C) (mg/L) 1 n 360 360 Mean [+ or -] SD 18.85 [+ or -] 5.80 6.37 [+ or -] 4.78 Range 7.66-30.83 0.19-18.86 2 n 375 375 Mean [+ or -] SD 20.14 67.22 5.30 [+ or -] 3.39 Range 8.31-39.77 0.14-15.28 3 n 360 360 Mean [+ or -] SD 21.69 [+ or -] 6.27 7.15 [+ or -] 3.74 Range 11.39-35.93 0.16-18.55 4 n 261 261 Mean [+ or -] SD 21.69 [+ or -] 7.85 5.16 [+ or -] 4.72 Range 9.17-42.31 0.18-20.00 Specific conductance Pond Statistic pH (mS/cm @ 25[degrees]C) 1 n 360 360 Mean [+ or -] SD 8.26 [+ or -] 0.60 19.02 [+ or -] 7.81 Range 6.95-9.45 8.56-38.10 2 n 365 365 Mean [+ or -] SD 8.06 [+ or -] 0.66 27.04 [+ or -] 9.37 Range 6.36-9.36 11.16-42.60 3 n 360 360 Mean [+ or -] SD 8.60 [+ or -] 0.92 58.24 [+ or -] 20.03 Range 6.43-9.84 16.76-100.00 4 n 261 248 Mean [+ or -] SD 7.69 [+ or -] 0.77 358.53 [+ or -] 113.01 Range 5.56-8.79 208.00-557.60 Turbidity (nephelometric Pond Statistic Salinity (a) units) 1 n 360 320 Mean [+ or -] SD 11.41 [+ or -] 5.01 72.9 [+ or -] 97.6 Range 4.80-24.21 0.0-592.0 2 n 365 325 Mean [+ or -] SD 16.74 [+ or -] 6.31 83.8 [+ or -] 127.3 Range 6.35-32.38 0.0-892.0 3 n 360 320 Mean [+ or -] SD 39.13 [+ or -] 14.90 55.7 [+ or -] 118.1 Range 9.82-70.73 0.0-1,000.0 4 n 248 248 Mean [+ or -] SD 234.19 [+ or -] 82.14 67.9 [+ or -] 118.5 Range 126.72-380.00 0.0-1,000.0 Pond Statistic Depth (cm) 1 n 360 Mean [+ or -] SD 31 [+ or -] 8 Range 15-63 2 n 355 Mean [+ or -] SD 30 [+ or -] 13 Range 5-70 3 n 355 Mean [+ or -] SD 46 [+ or -] 26 Range 3-120 4 n 221 Mean [+ or -] SD 18 [+ or -] 8 Range 5-52 (a) Estimated from measurements of specific conductance. TABLE 2--Number (n) and percentage of four taxa of fishes captured with baited minnow traps at four experimental ponds in the Salton Sea Basin, Imperial County, California. Minnow traps were deployed on nine occasions during October 2007-November 2009. Taxon n percentage Oreochromis mossambicus x O. urolepis and Tilapia 3 0.1 Cyprinodon maculariu 3,367 93.0 Gambusia affinis 150 4.1 Poecilia latipinna 100 2.8