Relative Sensitivity of Three Endangered Fishes, Colorado Squawfish, Bonytail, and Razorback Sucker, to Selected Metal Pollutants

ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY ARTICLE NO. ES971543

37, 186–192 (1997)

Relative Sensitivity of Three Endangered Fishes, Colorado Squawfish, Bonytail, and Razorback Sucker, to Selected Metal Pollutants1 Kevin J. Buhl U.S. Geological Survey, Biological Resources Division, Midwest Science Center, Ecotoxicology Research Station, RR1 Box 295, Yankton, South Dakota 57078-9214 Received December 24, 1996

The acute toxicity of four metal pollutants to larval and juvenile stages of endangered Colorado squawfish (Ptychocheilus lucius), bonytail (Gila elegans), and razorback sucker (Xyrauchen texanus) were determined in a water quality representative of that in the Green River, Utah. The rank order of toxicity (96-hr LC50) of the metals to all species and life stages from most toxic to least toxic was mercury (57–168 µg/liter) > cadmium (78–168 µg/liter) > hexavalent chromium (32,000–123,000 µg/liter) > lead (>170,000 µg/liter). In tests with lead, a precipitate formed in all test solutions and no mortalities occurred in these treatments. The larvae of each species were as sensitive or more sensitive than the juveniles to cadmium, hexavalent chromium, and mercury. Overall, the three species exhibited similar sensitivities to cadmium, hexavalent chromium, and mercury. Comparison of test results for the juveniles with toxicity values reported for other freshwater fishes tested in different water qualities indicates that the endangered fishes are more sensitive to cadmium than other cyprinids and centrarchids and less sensitive than salmonids, whereas their sensitivity to hexavalent chromium and mercury is similar to that of other cyprinids, centrarchids, and salmonids. © 1997 Academic Press

INTRODUCTION

The Green River of Utah is critical habitat for several endangered fishes in the United States, including two large cyprinids, the Colorado squawfish (Ptychocheilus lucius) and bonytail (Gila elegans), and one catostomid, the razorback sucker (Xyrauchen texanus) (U.S. Fish and Wildlife Service, 1994). Populations of these endemic Colorado River fishes have declined as a result of habitat alteration and loss, competition and predation by nonnative fish, and other factors (U.S. Fish and Wildlife Service, 1987). Recent investigations on the quality of irrigation drainwater entering the Green River near Jensen, Utah, have implicated inorganic contaminants as a possible adverse influence on endangered fishes (Stephens et al., 1988; Peltz and Waddell, 1991). Current water quality criteria for priority pollutants were 1 References to trade names of commercial products or manufacturers do not imply or constitute government endorsement or recommendation for use.

186 0147-6513/97 $25.00 Copyright © 1997 by Academic Press All rights of reproduction in any form reserved.

derived primarily from laboratory toxicity tests using standard test species exposed in generic water qualities. The most commonly used fish species in toxicity testing programs include rainbow trout (Oncorhynchus mykiss), fathead minnow (Pimephales promelas), and bluegill (Lepomis macrochirus). Toxicity data derived for these fishes is believed to provide a reasonable estimate of the response of species in the same or closely related taxonomic family. However, it is recognized that some pollutants may be more or less toxic to native species in specific water bodies compared to the standard species because of interspecific differences in sensitivity and the effects of site-water characteristics on the bioavailability and toxicity of pollutants (U.S. Environmental Protection Agency [USEPA], 1983). The establishment of realistic water quality criteria for pollutants entering critical habitats requires information on the response of native endangered species to these pollutants. Few data are available on the relative sensitivity of Colorado squawfish, bonytail, and razorback sucker to priority pollutants because of their reduced numbers and limited distribution. Of the 14 inorganic pollutants designated as toxic under section 307(a)(1) of the Clean Water Act (USEPA, 1980), the only published toxicity data found for all three species was for copper, selenium, and zinc (Hamilton, 1995; Buhl and Hamilton, 1996). Toxicity information on cadmium and organic mercury is available for Colorado squawfish (Beleau and Bartosz, 1982), but not for the other two species. Results of these studies demonstrated that the relative sensitivity of these endangered fishes to copper, selenium, and zinc was similar to or less than that of standard test fishes. However, it is not known if toxicity data derived for other priority inorganic pollutants using standard test fishes is predictive of the sensitivity of these endangered Colorado River fishes. The purpose of this study was to determine the relative sensitivity of early life stages of Colorado squawfish, bonytail, and razorback sucker to four priority metal pollutants, cadmium, hexavalent chromium, lead, and mercury, in an environmentally relevant water quality. The acute toxicity values were compared to those reported in the literature for other freshwater fishes routinely used in toxicity tests.

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MATERIALS AND METHODS

Experimental Fish The size and age of the fishes used in acute toxicity tests are given in Table 1. Life stage terminology follows that of Snyder (1976). All fish tested at the larval stage and bonytail tested as juveniles were hatched from eggs obtained from Dexter National Fish Hatchery (DNFH), Dexter, New Mexico. Razorback sucker tested as juveniles were reared from 5-day-old larvae obtained from DNFH. Colorado squawfish tested as juveniles were obtained as 21-week-old fish from the Midwest Science Center (MSC), Columbia, Missouri, and were from the same spawn as those tested as larvae. The fish were cultured in aerated well water maintained at 20 ± 2°C and the larvae and juveniles were placed under a photoperiod of 16:8 hr light:dark. The eggs were hatched in a vertical-flow (Heath) incubator. The larvae were cultured in 70-liter glass aquaria for 4 weeks and then transferred to 757liter fiberglass circular tanks. Colorado squawfish obtained as juveniles (from the MSC) were held in the laboratory for at least 2 weeks before being used in tests. The fish were fed a commercial salmon diet (Bioproducts, Inc., Warrenton, Oregon) ad libitum at least three times a day. Larvae were also fed live nauplii of brine shrimp Artemia sp. Dilution Water Tests were conducted in a nonstandard reconstituted water designed to simulate the major water quality characteristics (without the inorganic contaminants) of the Green River near TABLE 1 Age and Size of Endangered Fishes Used in Each Set of Acute Toxicity Tests with Metals Total lengthb (mm)

Weightb (mg)

961 961 43 6 2

4c 4c 499 6 69

Cadmium and mercury Chromium (VI) and lead Cadmium, chromium (VI), lead, and mercury

4 5 11 100

761 861 10 6 1 39 6 2

2c 2c 5c 378 6 60

Mercury Chromium (VI) and lead Cadmium Cadmium, Chromium (VI), lead, and mercury

Razorback sucker Larva 6 7 Juvenile 102 109

11 6 1 11 6 1 33 6 1 34 6 3

3c 4c 351 6 82 394 6 104

Cadmium and mercury Chromium (VI) and lead Chromium (VI) Cadmium, lead, and mercury

Species and life stage

Age (dph)a

Colorado squawfish Larva 8 9 Juvenile 155 Bonytail Larva

Juvenile

Metal

Jensen, Utah (Table 2; ReMillard et al., 1989). The dilution water was prepared by adding mineral salts (calcium chloride, calcium sulfate dihydrate, magnesium sulfate heptahydrate, and sodium bicarbonate) to deionized (DI) water in a polyethylene tank equipped with a recirculation pump to mix and aerate the water. Water quality characteristics were measured on each tank of dilution water prepared following standard procedures (APHA et al., 1989). Test Chemicals Chemicals used as toxicants were either reagent grade or of the highest purity available from the supplier (Aldrich Chemical Co., Milwaukee, WI). The chemicals tested (percent metal toxicant) were as follows: cadmium chloride (61.1% Cd), lead nitrate (61.2% Pb), mercury (II) chloride (72.5% Hg), and sodium chromate tetrahydrate (22.5% Cr). The percentage of metal toxicant in each compound was determined from the certificate of analysis provided by the supplier. Test solutions were formulated by either pipetting appropriate aliquants of stock solution (prepared in DI water) or adding the chemical directly to the test vessel. Test Methods Larvae were acclimated to the test temperature and dilution water over a period of 1–2 days and then held in the tempered dilution water for 1–2 days before being tested. Larvae were fed nauplii of brine shrimp during acclimation, but not during the test. Juveniles were acclimated to the test temperature and dilution water over a 2-day period and then held in the tempered dilution water for 2 days before they were tested. They were not fed during acclimation and testing. Static test procedures used in this study closely followed those recommended by the American Society for Testing and Materials (1989). With one exception, each test consisted of exposing groups of 10 fish to a geometric series of five to nine toxicant concentrations and a control treatment for 96 hr. Due to a shortage of fish, only one concentration of lead (170,000 mg/liter) was tested with larval Colorado squawfish. Larvae TABLE 2 Characteristics of Nonstandardized Reconstituted Water Used in Acute Toxicity Tests with Endangered Fishes Characteristic (unit)

Mean 6 SDa

Range

pH Conductivity (mmhos/cm at 25°C) Hardness (mg/liter as CaCO3) Calcium (mg/liter) Magnesium (mg/liter) Sodium (mg/liter) Alkalinity (mg/liter as CaCO3) Chloride (mg/liter) Sulfate (mg/liter)

8.0 6 0.3 603 6 8 199 6 2 46 6 1 20 6 1 49b 106 6 1 21 6 1 174 6 5

7.5–8.3 597–618 196–203 46–47 20–21

a

dph, days posthatch. Mean 6 SD of control fish. c Weight determined by dividing the pooled weight by the number of fish in the sample. b

105–108 20–21 168–184

n 4 7. Nominal concentration based on the amount of NaHCO3 added to deionized water in tanks. a b

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were tested in 3.8-liter glass jars containing 3 liters of solution and juveniles were tested in 19.6-liter glass jars containing 15 liters of solution. Temperature was maintained at 25 ± 1°C by immersing the jars in temperature-controlled water baths. Mortality and overt behavioral alterations were recorded, and all dead fish were removed at 24-hr intervals. Weight and total length were measured on the control fish at the end of the test. Dissolved oxygen was measured in the control, low, medium, and high treatments at 0, 48, and 96 hr of exposure, and pH was measured in these treatments at 0 and 96 hr of exposure. Data Analysis The 96-hr LC50 values and their 95% confidence intervals were calculated by the moving average-angle method (Peltier and Weber, 1985). All 96-hr LC50 values reported here are based on nominal concentrations of the metal toxicant in each compound tested. In tests where no partial kills occurred, the 95% confidence intervals were estimated as follows: the lower limit was the highest concentration with 0% mortality and the upper limit was the lowest concentration with 100% mortality. The criterion of nonoverlapping 95% confidence intervals was used to determine significant differences (a 4 0.05) between 96-hr LC50 values (APHA et al., 1989). To determine overall differences in toxicity of the metals to each species and life stage and differences in sensitivity of the three species to each metal, the 96-hr LC50s were ranked and the rank sums compared by the Friedman test (a 4 0.05; Conover, 1980).

TABLE 3 Relative Sensitivity of Two Life Stages of Endangered Fishes to Three Metals 96-hr LC50 in mg/liter (95% confidence interval)a Species and life stage Colorado squawfish Larva Juvenile Bonytail Larva Juvenile Razorback sucker Larva Juvenile

Mercury

Cadmium

Chromium (VI)

57A1 (46–70) 168C1 (130–216)b

78A1 (64–102) 108AB1 (88–135)

66,000B2 (50,000–84,000) 123,000C2 (98,000–153,000)

61A1 (47–78)b 108BC1 (88–135)

148BC2 (111–187)b 168C2 (136–207)

81,000BC3 (66,000–101,000) 104,000BC3 (80,000–128,000)

128BC1 (102–177) 90AB1 (68–114)

139BC1 (116–174) 160BC2 (128–197)

32,000A2 (26,000–41,000) 70,000B3 (56,000–86,000)

a

96-hr LC50 values sharing the same letter within a column and 96-hr LC50 values sharing the same number within a row are not significantly different (a 4 0.05). b No partial kills; 95% confidence intervals: lower limit, highest concentration tested with 0% mortality; upper limit, lowest concentration tested with 100% mortality.

RESULTS

96-hr LC50s) to all species and life stages from most toxic to least toxic was (‘‘>’’ denotes significant difference at a 4 0.05): mercury > cadmium > hexavalent chromium > lead.

There were no mortalities in any of the control treatments. Dissolved oxygen concentrations after 96 hr were ù72% saturation in tests with the larvae and ù50% saturation in tests with the juveniles. All pH values in tests with cadmium and mercury were within 0.2 unit of the controls. The addition of sodium chromate tetrahydrate to the dilution water increased the pH and the addition of lead nitrate decreased the pH of the test solutions. For chromium, the pH of test solutions was within 0.4 unit of the controls; whereas for lead, the pH of the highest test concentration (170,000 mg/liter) was 1.5–2.1 units lower than the controls. In tests with lead, a precipitate formed in all test solutions and no mortalities occurred in these treatments after 96 hr. Consequently, 96-hr LC50 values for lead in this dilution water are greater than the highest concentration tested (170,000 mg/ liter).

Comparative Sensitivity The larvae of each species were as sensitive or more sensitive than the juveniles to mercury, cadmium, and hexavalent chromium (Table 3). Larvae of Colorado squawfish and bonytail were more sensitive to mercury and larvae of Colorado squawfish and razorback sucker were more sensitive to hexavalent chromium than the juveniles. Among the larvae, Colorado squawfish were more sensitive to cadmium than bonytail and razorback sucker; whereas razorback sucker were more sensitive to hexavalent chromium and less sensitive to mercury than Colorado squawfish and bonytail (Table 3). Among the juveniles, no one species was more or less sensitive than the other two to any metal. Rank sum comparisons of 96-hr LC50s for both life stages across species indicated that there were no differences in sensitivity to the three metals among these species-life stage combinations.

Relative Toxicity Comparison of 96-hr LC50s for a given species and life stage indicated that mercury and cadmium were about 230–1330 times more toxic than hexavalent chromium and at least 1000 times more toxic than lead (Table 3). Mercury was more toxic to both life stages of bonytail and juvenile razorback sucker than cadmium. The overall rank order of toxicity (based on

DISCUSSION

Relative Sensitivity Although some statistical differences in 96-hr LC50s among species and between life stages were observed for cadmium, hexavalent chromium, and mercury, the magnitude of these differences was not great. For a given metal, differences in

SENSITIVITY OF ENDANGERED FISHES TO METALS

96-hr LC50 values were ø2.5-fold for the larvae and ø1.9-fold for the juveniles. The interspecific sensitivity differences observed in this study generally fall within the range of intraspecific variation in acute LC50s reported for repeated tests conducted with a single species-toxicant combination in the same laboratory. In tests with Colorado squawfish, bonytail, and razorback sucker, Buhl and Hamilton (1996) reported that their 96-hr LC50s for zinc and selenite were within a factor of two and their 96-hr LC50s for selenate were within a factor six of those reported by Hamilton (1995) for the same species from an earlier year class. Lemke (1981) conducted a round-robin study with fathead minnow and rainbow trout exposed to silver nitrate and endosulfan. Using his data, differences in 96-hr LC50 values calculated from duplicate static and flow-through tests with both toxicants were within a factor of two for each laboratory that maintained similar test conditions between repeated tests. In a similar round-robin study with estuarine animals, Schimmel (1981) concluded that LC50 values obtained from repeated tests (for a given species–toxicant combination) at the same laboratory generally fall within a factor of two; whereas those obtained from different laboratories should fall within a factor of four. Interlaboratory Comparisons Direct comparisons of toxicity values obtained in this study with those in the literature is difficult because of differences in the characteristics (primarily hardness, alkalinity, pH, and temperature) of the test waters. Also, most investigators do not report the concentrations of other major ions (i.e., calcium, magnesium, sulfate, and chloride) in their test water. Water hardness is the only characteristic currently used by the USEPA to adjust criterion values of some metals in relation to the quality of the dilution water. Of the metals tested here, hardness has been found to have a greater effect on cadmium and lead toxicity than on mercury and hexavalent chromium toxicity to fish (USEPA, 1986). Cadmium. Juvenile Colorado squawfish in this study are more sensitive to cadmium than those tested by Beleau and Bartosz (1982). Their 96-hr LC50s for <5-g juveniles of 5316– 7617 mg/liter in river water (hardness, 32 mg/liter as CaCO3), 7376 mg/liter in well water (hardness, 100 mg/liter as CaCO3), and 27,809 mg/liter in fortified well water (hardness, 316 mg/ liter as CaCO3) are considerably higher (49- to 257-fold) than that obtained in this study (108 mg/liter). This large intraspecific variation in cadmium toxicity between the two studies is difficult to explain. Two of the waters used by Beleau and Bartosz (1982) had a lower hardness than that used in this study, but their 96-hr LC50 values in these waters were 49–71 times higher than that in this study. Beleau and Bartosz (1982) tested juvenile Colorado squawfish at 12, 17, and 22°C compared to 25°C in this study. In their study, cadmium was only 1.4 times more toxic at 22°C than at 12°C within a given water type. Based on this information, it is doubtful that these differences under test conditions between studies could account

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for the large interlaboratory variation in cadmium toxicity to Colorado squawfish. Andros and Garton (1980) exposed juvenile northern squawfish (P. oregonensis; 1.44–2.49 g) to cadmium in soft water (hardness, 20–30 mg/liter as CaCO3) and obtained 96-hr LC50s of 1092 and 1104 mg/liter. Their toxicity values for northern squawfish in soft water are an order of magnitude higher than that obtained for juvenile Colorado squawfish (108 mg/liter) in this hard water study. In tests with postlarval and <5-g juvenile northern squawfish exposed in hard water (hardness, 316–347 mg/liter as CaCO3), Beleau and Bartosz (1982) reported higher 96-hr LC50s of 5555 and 5518 mg/liter respectively. These results indicate that Colorado squawfish in this study are more sensitive to cadmium than northern squawfish. The range of 96-hr LC50s of cadmium obtained for larvae of the three endangered fishes (78–148 mg/liter) encompasses the values reported by Hall et al. (1986) for larval fathead minnow (80–90 mg/liter) tested in water of similar hardness (200 mg/ liter as CaCO3). Schubauer-Berigan et al. (1993) obtained slightly lower values of 60–73 mg/liter for ø1-day-old fathead minnow tested in hard water (hardness, 280–300 mg/liter as CaCO3). Sensitivity differences between larvae of the three endangered fishes in this study and fathead minnow is ø2.5fold, which falls within the expected interlaboratory variation in LC50s of fourfold for a given species-toxicant combination (Schimmel, 1981). In contrast to the larvae, toxic concentrations of cadmium reported for juvenile fathead minnow in hard water are one to two orders of magnitude higher than those obtained for juveniles of the endangered fishes in this study (108–168 mg/liter). In tests with cadmium and 2-g immature fathead minnow conducted in water of similar hardness (201–204 mg/liter as CaCO3), Pickering and Gast (1972) reported 96-hr LC50s of 30,000–32,000 mg/liter for static exposures and 3200–19,000 mg/liter (2000–12,000 mg/liter based on measured concentrations) for flow-through exposures. The authors attributed part of this variation in cadmium toxicity to the precipitation of cadmium in their test solutions. No visible precipitation of cadmium was observed in this study. These results indicate that juvenile Colorado squawfish, bonytail, and razorback sucker are more sensitive to cadmium than juvenile fathead minnow. In comparisons with fish in other taxonomic families, the three endangered species tested in this study seem to be more acutely sensitive to cadmium than centrachids and less sensitive than salmonids. The 96-hr LC50s of cadmium reported by Pickering and Henderson (1966) for 1- to 2-g bluegill (1940 mg/liter) and 1- to 2-g green sunfish (L. cyanellus; 2840 mg/ liter) tested in soft water (hardness, 20 mg/liter as CaCO3) are at least an order of magnitude higher than those obtained in this study for the three endangered species in hard water. Using the empirical relation between cadmium toxicity (4and 7- to 10-day LC50s) and total water hardness developed for salmonids by Sprague (1987), the predicted lethal cadmium concentration to salmonids in water of hardness 200 mg/liter as

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CaCO3 is 17 mg/liter. Hamilton and Buhl (1990) reported a similar 96-hr LC50 of 26 mg/liter for 1-g chinook salmon (O. tshawytscha) exposed in water of hardness 211 mg/liter as CaCO3. The 96-hr LC50s of cadmium obtained in this study are 3–10 times higher than those given above for salmonids. Chromium (VI). Concentrations of hexavalent chromium that were toxic to larvae of the endangered fishes in this study(32,000–81,000 mg/liter) encompass the 96-hr LC50 of 43,300 mg/liter for 30-day-old fathead minnow tested in soft water (hardness, 44 mg/liter as CaCO3) by Spehar and Fiandt (1986). Acute toxicity values of hexavalent chromium obtained for juveniles of the endangered fishes in this study (70,000– 123,000 mg/liter) are about 2.6–4.5 times higher than those reported by Pickering and Henderson (1966) for 1- to 2-g fathead minnow tested in hard water (96-hr LC50, 27,300 mg/ liter; hardness, 360 mg/liter as CaCO3). These sensitivity differences are close to the expected interlaboratory variability for a given species–toxicant combination of fourfold (Schimmel, 1981). Sensitivity of juvenile Colorado squawfish, bonytail, and razorback sucker to hexavalent chromium in this study (70,000–123,000 mg/liter) is similar to that of chinook salmon (96-hr LC50, 111,000 mg/liter) tested in hard water (hardness, 211 mg/liter as CaCO3) by Hamilton and Buhl (1990) and bluegill (96-hr LC50, 118,000–133,000 mg/liter) tested in both soft water (hardness, 20 mg/liter as CaCO3) and hard water (hardness, 360 mg/liter as CaCO3) by Pickering and Henderson (1966). Lead. No toxicity data for lead could be derived in this study (96-hr LC50s > 170,000 mg/liter) because of the low solubility of lead in the dilution water and lack of toxic effects in the fish after 96 hr. Beleau and Bartosz (1982) reported high 96-hr LC50s of 168,000 and 173,000 mg/liter for organic lead (as lead acetate) tested with northern squawfish in hard water (hardness, 347 mg/liter as CaCO3). They did not report the occurrence of a precipitate in their test solutions as was observed in this study. The solubility of lead is strongly dependent on the alkalinity of the water (Davies et al., 1976). Consequently, the precipitate that formed in the lead tests was probably PbCO3. Davies et al. (1976) also observed lead precipitation in hard water tests (hardness, 290–385 mg/liter as CaCO3) with rainbow trout. The 96-hr LC50s they obtained in hard water were 471,000 and 542,000 mg/liter based on total lead, but were only 1470 and 1320 mg/liter, respectively, based on dissolved lead. Mercury. The concentration of inorganic mercury (as mercuric chloride) found to be toxic to larval Colorado squawfish in this study (57 mg/liter) is five times higher than the 96-hr LC50 of 11 mg/liter reported by Beleau and Bartosz (1982) for organic mercury (as mercuric acetate) tested with postlarval Colorado squawfish. Conversely, the range of 96-hr LC50s of organic mercury reported by Beleau and Bartosz (1982) for <5-g juvenile Colorado squawfish (127–312 mg/liter) encom-

passes the 96-hr LC50 obtained for inorganic mercury and juvenile Colorado squawfish in this study (168 mg/liter). The difference in toxicity between the two forms of mercury to larval and postlarval Colorado squawfish is close to that reported in the literature. Based on toxicity data given in USEPA (1985), fathead minnow are about four times more sensitive to mercuric acetate than to mercuric chloride. The acute sensitivity of larval Colorado squawfish, bonytail, and razorback sucker to inorganic mercury in this study (96-hr LC50s, 57–128 mg/liter) is greater than that reported for 30day-old fathead minnow (96-hr LC50, 172 mg/liter; Spehar and Fiandt, 1986). In tests with 3-month-old fathead minnow, Snarski and Olson (1982) reported a mean 96-hr LC50 of 168 mg/ liter, which is identical to that obtained for juvenile Colorado squawfish and slightly higher than those obtained for juvenile bonytail (108 mg/liter) and razorback sucker (90 mg/liter). Differences in 96-hr LC50s of inorganic mercury obtained in this study and those reported by Snarski and Olson (1982) and Spehar and Fiandt (1986) for fathead minnow are within a factor of three, which is less than the expected interlaboratory variability of fourfold for a given species–toxicant combination (Schimmel, 1981). In contrast to cadmium, the sensitivity of juvenile Colorado squawfish, bonytail, and razorback sucker to mercury in this study is similar to that reported for salmonids (96-hr LC50s, 101–420 mg/liter) and bluegill (96-hr LC50, 160 mg/liter) tested in freshwater (USEPA, 1985; Hamilton and Buhl, 1990). In tests with 18-g white sucker (Catostomus commersoni), Duncan and Klaverkamp (1983) reported a 96-hr LC50 of 687 mg/liter, which is about eight times higher than that obtained for 0.4-g razorback sucker in this study (90 mg/liter). Comparison to Criterion Concentrations The USEPA has proposed national water quality criterion concentrations for cadmium, hexavalent chromium, lead, and mercury (Table 4; USEPA, 1986). Each criterion consists of two numbers, the Criterion Continuous Concentration and the Criterion Maximum Concentration. The Criterion Continuous Concentration is the 4-day average concentration that should TABLE 4 Comparison of Acute Toxicity Values (96-hr LC50s) for Endangered Fishes with National Water Quality Criteria for Freshwater Aquatic Organisms

Metal Cadmium Chromium (VI) Lead Mercury a

Lowest acute valuea (mg/liter) 78 32,000 >170,000 57

Criterion Concentration (mg/liter)b Maximum c

8.6 16 200c 2.4

This study. U.S. Environmental Protection Agency (1986). c Adjusted to a hardness of 200 mg/liter as CaCO3. b

Continuous 2.0c 11 7.7c 0.012

191

SENSITIVITY OF ENDANGERED FISHES TO METALS

not be exceeded more than once every 3 years and is derived primarily from chronic toxicity tests. The Criterion Maximum Concentration is the 1-hr average concentration that should not be exceeded more than once every 3 years and is derived from acute toxicity data (48- and 96-hr LC50s) with representative organisms. Guidelines for deriving these criterion concentrations are given in Stephan et al. (1985). Current water quality criteria for these metals are based on total recoverable concentrations and do not take into account the speciation of the soluble forms in different dilution waters. Although it has been demonstrated that metal toxicity is related to the free ion, it is not known how much of the observed toxicity is contributed by the complexed forms (Allen and Hansen, 1996). Total hardness is the only parameter used to adjust the criterion values in relation to the quality of the dilution water. Of the metals tested here, only cadmium and lead criterion values are adjusted for a hardness of 200 mg/liter as CaCO3 using the formulas given in USEPA (1986). For each metal tested in this study, all 96-hr LC50 values are considerably higher (ù9-fold) than their Criterion Maximum Concentration (Table 4). Consequently, the Criterion Maximum Concentrations of cadmium, hexavalent chromium, lead, and mercury seem to be protective of these endangered fishes in water qualities similar to that in the Green River. Comparison to Environmental Concentrations Young-of-year Colorado squawfish and razorback sucker utilize slack water habitats at or below the confluence of Ashley Creek and Stewart Lake outlet with the Green River near Jensen, Utah (Tyus, 1987; Tyus and Haines, 1991). Little is known about the distribution and abundance of bonytail in the Green River (Tyus and Karp, 1989). Both of these tributaries receive irrigation drainwater containing elevated concentrations of selenium and other inorganics. Measured concentrations of cadmium, chromium, lead, and mercury in Ashley Creek and Stewart Lake outlet were below the reporting limit for the methods used (1 mg/liter for cadmium, 10 mg/liter for chromium, 1 mg/liter for lead, and 0.1 mg/liter for mercury; Stephens et al., 1988). These ambient concentrations are at least two to five orders of magnitude lower than those found to be acutely lethal to early life stages of endangered fishes in this study. These results indicate that concentrations of cadmium, chromium, lead, and mercury in these tributaries of the Green River do not pose an acute hazard to early life stages of these endangered Colorado River fishes. However, native fishes are exposed to mixtures of these and other inorganics in irrigation drainwater, which may be more toxic than predicted based on the individual toxicities of each inorganic (e.g., see Buhl and Hamilton, 1996). Additional studies on the chronic toxicity of these inorganic mixtures are needed to determine if current water quality criteria based on individual chemicals are protective of endangered fishes in the Colorado River basin.

CONCLUSION

This study demonstrated that early life stages of Colorado squawfish, bonytail, and razorback sucker were similar in their relative sensitivities to cadmium, chromium (VI), and mercury. Moreover, the range of acute sensitivities exhibited by the endangered fishes to these metals falls within those reported for other species. The 96-hr LC50s obtained in this study are considerably higher than USEPA water quality criterion concentrations for these metals. Consequently, current Criterion Maximum Concentrations of these metals seem to be protective of Colorado squawfish, bonytail, and razorback sucker in water qualities similar to that used in this study. ACKNOWLEDGMENTS The author would like to thank Roger Hamman, Dexter National Fish Hatchery, Dexter, MN, for providing the fish and F. Art Bullard for his valuable technical assistance.

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