Temperature, Dissolved Oxygen, and Salinity Tolerances ... - CiteSeerX

Transactions of the American Fisheries Society 130:742–749, 2001 q Copyright by the American Fisheries Society 2001

Temperature, Dissolved Oxygen, and Salinity Tolerances of Five Prairie Stream Fishes and Their Role in Explaining Fish Assemblage Patterns KENNETH G. OSTRAND*1

AND

GENE R. WILDE

Wildlife and Fisheries Management Institute, Mail Stop 2125, Texas Tech University, Lubbock, Texas 79409, USA Abstract.—We compared the maximum temperature, maximum salinity, and minimum dissolved oxygen tolerances of two cyprinodontids and three cyprinids to identify the abiotic factors that determine assemblage structure in drying streambed pools. Red River pupfish Cyprinodon rubrofluviatilis, plains killifish Fundulus zebrinus, plains minnow Hybognathus placitus, smalleye shiner Notropis buccula, and sharpnose shiner N. oxyrhynchus all have high thermal, low dissolved oxygen, and high salinity tolerances. Cyprinodontids were able to tolerate temperatures between 398C and 428C, salinities up to 40‰, and dissolved oxygen concentrations as low as 0.95 mg/L, whereas cyprinids were tolerant of temperatures between 378C and 418C, salinities up to 14‰, and dissolved oxygen concentrations as low as 2.1 mg/L. Our laboratory results provide compelling evidence that the greater salinity tolerances of cyprinodontids can explain temporal changes in fish assemblages in evaporating streambed pools and may in part explain the spatial zonation of species within the upper Brazos River Basin, Texas.

The headwater reaches of many prairie streams are characterized by extreme and rapidly changing physical and chemical conditions (Matthews and Maness 1979; Smith and Fausch 1997). Changing environmental conditions often are the result of normal hydrologic variability. Prairie streams become swollen and swift after periods of moderate to heavy rainfall (Echelle et al. 1972; Fausch and Bramblett 1991) and then recede, often becoming intermittent and ultimately comprising a series of isolated pools in the streambed (Rutledge and Beitinger 1989; Taylor et al. 1996; Wilde and Ostrand 1999). Formation of isolated pools may accentuate variability in environmental conditions. As water evaporates from these pools, temperature and salinity increase (Hill and Holland 1971), and diel changes in temperature, pH, and dissolved oxygen concentrations become more pronounced (Feldmeth et al. 1974; Mundahl 1990; Capone and Kushlan 1991). Fishes confined to isolated pools may be subject to extreme and rapidly changing physical and chemical conditions (Rutledge and Beitinger 1989) that are capable of causing stress or death (Tramer 1977; Matthews et al. 1982; Matthews and Zimmerman 1990; Mundahl 1990). Temperature,

* Corresponding author: [email protected] 1 Present address: Center for Aquatic Ecology, Illinois Natural History Survey, Sam Parr Biological Station, 6401 Meacham Road, Kinmundy, Illinois 62854, USA Received October 16, 2000; accepted March 30, 2001

salinity, and dissolved oxygen concentrations that are outside tolerable limits have been identified as causal factors in the mortality of stream fishes confined to pools (Huntsman 1942, 1946; Bailey 1955; Barlow 1958; Tramer 1977; Matthews et al. 1982; Mundahl 1990). Ostrand (2000) examined fish assemblages in isolated pools in the upper Brazos River, Texas, from the time the pools became isolated until either the surface connections were reestablished after a rain event or the pools dried out completely. As the pools diminished in volume, dissolved oxygen concentration decreased, whereas temperature and salinity increased. The abundance and composition of fish assemblages in pools underwent systematic changes that coincided with changes in environmental conditions. Sharpnose shiner Notropis oxyrhynchus was the first species to succumb to changing habitat conditions in pools, followed by smalleye shiner N. buccula, plains minnow Hybognathus placitus, plains killifish Fundulus zebrinus, and Red River pupfish Cyprinodon rubrofluviatilis. Here, we report the results of laboratory tests designed to determine whether differences in physiological tolerances among fish species could explain observed changes in the composition of fish assemblages in isolated streambed pools. We hypothesized that the smalleye shiner would have least tolerance to high temperature, low dissolved oxygen, and high salinity—followed by smalleye shiner, plains minnow, plains killifish, and Red

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FACTORS AFFECTING FIVE PRAIRIE STREAM FISHES

River pupfish. Therefore, we tested for differences in tolerances to high temperatures and salinities and low dissolved oxygen concentrations among Red River pupfish, plains killifish, plains minnow, smalleye shiner, and sharpnose shiner, which represent 94% of all specimens (N 5 24,986) captured in isolated pools in the upper Brazos River. Methods Fish collection and maintenance.—Red River pupfish (mean 5 28.5 6 0.5 total length [TL], mm), plains killifish (mean 5 49.7 6 5.5 TL, mm), plains minnow (mean 5 58.1 6 8.9 TL, mm), smalleye shiner (mean 5 38.6 6 4.2 TL, mm), and sharpnose shiner (mean 5 48.0 6 8.7 TL, mm) were collected from the Salt Fork (Garza County, Texas) and Double Mountain Fork (Kent County, Texas) of the Brazos River, on 20 May 1998. Fish were held in the laboratory for at least 2 weeks at a photoperiod of 12 h light:12 h dark in aerated 75-L glass aquaria and fed Wardleys tropical flakefood ad libitum, except that food was withheld for 48 h before tolerance tests. Aquaria were maintained at a desired acclimation temperature: 258C for minimum dissolved oxygen and maximum salinity tolerance tests, and 258C and 308C for critical thermal maxima (CTM) tests. Fish were acclimated at 258C because this temperature allows for direct comparisons with previous studies (e.g., Matthews and Maness 1979; Smale and Rabeni 1995; Bennett and Beitinger 1996), which reported on fish acclimated to this temperature. An acclimation temperature of 308C was used for CTMs to approximate the average daily summer temperature encountered by fish inhabiting pools in the upper Brazos River (mean 5 30.5 6 0.28C; Ostrand 2000). Critical thermal maxima.—Thermal tolerance was determined by the CTM method of Hutchison (1961). An individual of each species (N $ 15 per species) from each acclimation temperature was transferred to a 500-mL beaker held in a water bath, and temperature was increased at a rate of 0.58C/min. Each beaker was aerated to ensure uniformity of heat transfer and to preclude oxygen depletion during tests. The CTM was defined as the temperature at which a fish lost equilibrium and failed to right itself. Interspecific group comparisons were made by using analysis of variance (ANOVA). Significant ANOVAs (P , 0.05) were followed by pairwise tests using Fisher’s leastsignificant-difference mean separation (SAS 1996). Salinity tolerance.—Salinity tolerance was de-

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termined by the introduction of five fish of each species into 75-L aquaria adjusted previously to five specific conductances (20, 25, 35, 40, and 61 6 4.0 mS/cm). Specific conductance was used as a surrogate of salinity and nominal concentrations (;11, 15, 22, 25, and 42‰) of sodium chloride. The range of specific conductance tested encompassed the average (mean 5 22.8 mS/cm) and the upper concentration (95% quantile 5 58.4 mS/cm) observed in the field. Each treatment was replicated at least three times for each species. Fish mortality was observed in each aquarium at 48 h and dead fish were removed daily. Fish that survived 48 h were returned to a holding tank to recover and were not used in subsequent trials. We used logistic regression to assess the effect of specific conductance on the probability of mortality at 48 h. The logistic response function form is Y i 5 e (b01b1 Xj ) /[1 1 e (b01b1 Xj ) ], where Yi 5 mortality probability of species i, b0 5 regression intercept, b1 5 regression slope, and Xj 5 independent variable (specific conductance) in aquarium j (Neter et al. 1996). We tested the appropriateness of the logistic response function between species mortality at 48 h and specific conductance, using a full/reduced model likelihood ratio x2 test (SAS 1996). Logistic regression analysis was performed with SAS (1996), and the salt concentration that resulted in 50% mortality of each test species (LC50) was estimated from logistic regressions. Minimum dissolved oxygen.—Minimum oxygen tolerances (N 5 15 per species) were determined in a test chamber, described by Hlohowskyj and Wissing (1987), through which nitrogen gas was bubbled to remove dissolved oxygen. The chamber was constructed from a stoppered 500-mL Erlenmeyer flask. Two 1-mL glass pipettes were inserted through the rubber stopper to within 5 mm of the bottom of the flask. One glass pipette was used to provide nitrogen to the system. The second was connected to a 100-mL syringe that was used to remove water samples for oxygen determinations. A piece of rubber tubing was inserted through the rubber stopper to serve as a pressure release. A single fish, introduced into 400-mL of water from the holding tank, was placed within the chamber. Once the chamber was sealed, a 50-mL sample was withdrawn for determination of initial oxygen concentration. Nitrogen gas was then bubbled into the chamber at a pressure of approximately 1.8

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FIGURE 1.—Critical thermal maxima for Red River pupfish, plains killifish, plains minnow, smalleye shiner, and sharpnose shiner collected from the upper Brazos River, Texas. For each species the rectangle box encompasses the median (represented by a horizontal line through the box), the 25th percentile, and the 75th percentile. Above and below the box are lines extending to values within 1.5 times the interquartile range; filled circles represent outliers. Species maxima not sharing the same letter are significantly different.

psi, which drove dissolved oxygen from the water at an average rate of 0.62 6 0.22 mg·L–1min–1. Nitrogen was bubbled through the chamber until the endpoint was reached, defined as the point at which the fish lost equilibrium and was unable to maintain an upright position. Once the fish lost equilibrium, 50 mL of water was collected for determination of minimum dissolved oxygen. Oxygen concentrations were measured with the azidemodified Winkler method according to the procedure described by Hornbach et al. (1983). Interspecific group comparisons for minimum dissolved oxygen concentrations were made using ANOVA and followed by pairwise tests using Fisher’s least-significant-difference mean separation (SAS 1996). Results

FIGURE 2.—Logistic regressions showing the relationships between percent mortality and salinity concentration for Red River pupfish, plains killifish, plains minnow, smalleye shiner, and sharpnose shiner. Open circles represent percent mortality per aquarium (five fish per aquarium).

5 38.8 6 0.88C). CTMs for both cyprinodontids were greater than those for all three cyprinid species (mean 5 37.9–36.58C), which did not differ from one another. At an acclimation temperature of 308C, plains killifish had a significantly greater (F4,70 5 4.87, P 5 0.0016) CTM (mean 5 42.0 6 0.28C) than smalleye shiner (mean 5 40.6 6 0.48C), Red River pupfish (mean 5 40.3 6 0.58C), plains minnow (mean 5 39.7 6 0.78C), and sharpnose shiner (mean 5 39.2 6 0.28C; Figure 1).

Critical Thermal Maxima

Salinity Tolerance

The CTM values of cyprinodontids were generally greater than those of cyprinids at both acclimation temperatures (Figure 1). At an acclimation temperature of 258C, CTMs differed significantly among the five species (F4,67 5 7.89, P , 0.0001). Among the cyprinodontids, plains killifish (mean 5 40.5 6 0.48C) had a significantly greater CTM than did the Red River pupfish (mean

Salinity tolerance differed among the species. Red River pupfish (LC50 5 46 6 0.03‰) had the greatest tolerance (Figure 2), followed by the plains killifish (LC50 5 43 6 0.05‰), smalleye shiner (LC50 5 18 6 2.52‰), plains minnow (LC50 5 16 6 1.94‰), and sharpnose shiner (LC50 5 15 6 0.72‰). There was a positive relationship between mortality and salinity concen-


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TABLE 1.—Logistic regressions describing species mortality and salt concentrations among dominant fishes collected in upper Brazos River pools, Texas. Species

b0

b1

x2

df

P

Red River pupfish Plains killifish Plains minnow Smalleye shiner Sharpnose shiner

26.46 29.64 27.50 242.32 227.57

0.10 0.15 0.28 1.37 1.07

14.23 17.22 32.91 45.99 29.52

1 1 1 1 1

0.0002 0.0001 0.0001 0.0001 0.0001

tration for Red River pupfish (x2 5 14.23, P 5 0.0002), plains killifish (x2 5 17.23, P 5 0.0001), smalleye shiner (x2 5 45.99, P 5 0.0001), plains minnow (x2 5 32.92, P 5 0.0001), and sharpnose shiner (x2 5 29.52, P 5 0.0001; Table 1). Plains minnow, smalleye shiner, and sharpnose shiner incurred 100% mortality at salinities less than 22‰ (Figure 2).

FIGURE 3.—Box plots showing the minimum dissolved oxygen concentrations at which Red River pupfish, plains killifish, plains minnow, smalleye shiner, and sharpnose shiner lost equilibrium. Plot symbols are as in Figure 1. Species minima not sharing the same letter are significantly different.

Minimum Dissolved Oxygen Minimum oxygen tolerances differed significantly (F4,70 5 47.32, P 5 0.0001) among species (Figure 3). Red River pupfish was able to tolerate the lowest (mean 5 0.95 6 0.07 mg/L) dissolved oxygen concentration, followed by plains killifish (mean 5 1.25 6 0.09 mg/L), plains minnow (mean 5 2.08 6 0.14 mg/L), smalleye shiner (mean 5 2.11 6 0.08 mg/L), and sharpnose shiner (mean 5 2.66 6 0.12 mg/L). Discussion Red River pupfish, plains killifish, plains minnow, smalleye shiner, and sharpnose shiner all have high thermal, low dissolved oxygen, and high salinity tolerances in comparison with other intermittent stream fishes (Matthews and Hill 1977; Rutledge and Beitinger 1989; Smale and Rabeni 1995). Cyprinodontids collected from the upper Brazos River, Texas, were more tolerant of high temperatures, high salt concentrations, and low dissolved oxygen concentrations than were cyprinids from the same location. Our laboratory results support field observations that show the less tolerant species decrease in abundance and succumb to mortality as environmental conditions in isolated pools reach and exceed our laboratory determined tolerances. Sharpnose shiner were the least tolerant species and disappeared first in pools, followed by plains minnow, and smalleye shiner, whereas Red River pupfish and plains killifish were the most tolerant and were able to maintain their populations until the next flood event (Ostrand 2000).

Fish trapped in pools experience high temperatures (Huntsman 1942, 1946; Bailey 1955; Matthews et al. 1982; Mundahl 1990) and low dissolved oxygen concentrations (Tramer 1977; Ostrand and Marks 2000), which may cause fish kills. However, none of the field observations reported by Ostrand (2000) suggest that high temperature and low dissolved oxygen resulted in species-specific mortality capable of affecting species assemblage changes observed in pools. Thermal tolerances of Brazos River fish exceeded those for all species reported by Matthews (1987) and Smale and Rabeni (1995) common in headwater reaches and Brazos River fish had CTMs greater than average (mean 5 30.5 6 0.28C) and maximum (max 5 39.28C) daily summer temperatures observed in pools (Ostrand 2000). Minimum dissolved oxygen tolerances of Brazos River cyprinodontids were intermediate to, and those of cyprinids lower than, those presented by Smale and Rabeni (1995) for 36 species. However, lower dissolved oxygen tolerances were similar to those reported by Matthews and Maness (1979) for the plains minnow, red shiner, Arkansas River shiner, and emerald shiner, which are commonly collected in prairie streams. Canton et al. (1984) and Capone and Kushlan (1991) suggest that low dissolved oxygen concentrations may become increasingly important as pool volume shrinks from water evaporation, particularly when fish density increases. However, during our field observations, dissolved oxygen concentrations below tolerable levels for cyprinids occurred only in 8% of our samples, dur-

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ing which no fish kills were observed; this suggests these species may have the capacity to acclimate to chronic exposure to low dissolved oxygen concentrations (Ostrand 2000). High temperatures and low dissolved oxygen concentrations occurred stochastically and were beyond tolerable limits in only a few pools. Thus, high temperatures and low dissolved oxygen concentrations acting stochastically did not affect all pools in the same manner or at the same time and therefore may have little effect on fish assemblage patterns on a local scale. If the majority of a local population(s) is not negatively affected (e.g., reduced reproduction, growth, survival), local assemblage patterns presumably will not change. Differences in maximum salinity tolerance among cyprinodontids and cyprinids shown in our laboratory results are consistent with fish assemblage changes observed in upper Brazos River pools, where salinity increases in a deterministic fashion as water evaporates (Ostrand 2000). Members of the genus Fundulus, including the plains killifish, have been reported to have upper salinity tolerance of 74–114‰ (Griffith 1974), and Red River pupfish have been observed in waters having a salinity of 150‰ (Echelle et al. 1972). The salinity tolerance of cyprinids tested from the upper Brazos River was greater than that of other cyprinids (i.e., red shiner Cyprinella lutrensis, emerald shiner N. atherinoides, Arkansas River shiner N. girardi, and speckled chub Macrhybopsis aestivalis) reported by Matthews and Hill (1977) but significantly less than that of the cyprinodontids we tested. In Brazos River pools, fish assemblages became less diverse and species composition shifted from primarily cyprinids to cyprinodontids as salinity in pools increased through time (Ostrand 2000). Cyprinodontids were collected from pools with salinities ranging from 5‰ to 44‰, whereas cyprinids were absent from pools when salinity exceeded 21‰; sharpnose shiner disappeared from pools first, followed by smalleye shiner and plains minnow (Ostrand 2000). Because cyprinodontids were able to maintain their populations in pools until the next flood event, we expect the composition of the fish assemblages after pool confinement and a prolonged drought to be dominated locally by more euryhaline cyprinodontids. The maintenance of fish populations within a stream reach is influenced by the availability of pools and their ability to hold water during droughts (Paloumpis 1958; Lowe-McConnell 1964; Griswold et al. 1982; Schlosser 1987; Capone and Kushlan 1991; Meador and Matthews

1992). However, not only the availability of pools but also the environmental conditions of pools ultimately determine the maintenance of fish populations. Canton et al.’s (1984) review of the effects of late summer reduction in streamflow on fish emphasized that crowding in shallow, hot, stagnant pools led to decreases in catostomids and salmonids in an upland Colorado stream. Attributing the differential survival of species to differences in physiological tolerances (Bailey 1955; Matthews and Maness 1979; Matthews et al. 1982; Mundahl 1990) can therefore explain the success of tolerant species versus less tolerant species during periods of drought. However, for changes in local assemblage structure to occur, all or the majority of pools serving as refugia must undergo similar environmental changes that exceed species-specific tolerances. Pools are more likely to form in headwater reaches of prairie streams (Taylor et al. 1996); thus, tolerances to environmental conditions such as high temperatures and salinities and low dissolved oxygen concentrations may influence fish composition over larger geographic scales (Ostrand 2000). Variable environmental conditions upstream, which become less variable downstream, have been suggested to be a common causal mechanism for longitudinal zonation of fish species (Matthews and Styron 1981; Taylor et al. 1993; Williams et al. 1996). Headwater stream reaches that are not spring-fed are often intermittent and environmentally harsher than the larger tributaries and mainstreams (Starrett 1950; Neel 1951; Metcalf 1959; Whiteside and McNatt 1972), particularly during periods of low flow and drought. Prolonged confinement and differential mortality of stream fishes confined to pools may negatively impact species richness, diversity, colonization rates, and reproductive success of fish (Paloumpis 1958; Poff and Ward 1990; Capone and Kushlan 1991). As a result, headwaters support fewer species than downstream reaches do (Tramer 1977), the species that are more tolerant of environmental variability and extreme conditions being more likely to occur in upstream reaches (Thompson and Hunt 1930; Burton and Odum 1945; Starrett 1950; Matthews 1987). For example, Matthews and Stryon (1981) and Smale and Rabeni (1995) found significantly greater temperature, oxygen, and pH tolerance among headwater species than among congeneric or cofamilial mainstream taxa. Conversely, less tolerant species are most likely to inhabit downstream reaches (Harrel et al. 1967; Matthews and Styron 1981; Rutledge


and Beitinger 1989), resulting in longitudinal zonation of fish assemblages along an upstream– downstream gradient (Sheldon 1967). Fishes that survive droughts in isolated pools might be expected to regain their former population levels more rapidly than those that must continually recolonize from downstream sites (Tramer 1977). Therefore, environmental conditions that exceed physiologically tolerable limits, accentuated by pool formation, may act as a filter (Smith and Powell 1971; Tonn 1990) that sorts fishes in streams into clearly distributed assemblages, particularly if headwater reaches are more ephemeral than downstream reaches (Matthews 1998). Tolerances to temperature, dissolved oxygen, and particularly salinity as determined here are congruent with the longitudinal zonation in species composition in the Brazos River reported by Echelle et al. (1972) and in the Red River reported by Taylor et al. (1993). Three species complexes were described by Echelle et al. (1972) from the middle and upper Brazos River. The first, Red River pupfish and plains killifish, were more commonly collected upstream and occurred over a wide salinity range (0.4 to nearly 150‰; Echelle et al. 1972). The second complex—plains minnow, smalleye shiner, and sharpnose shiner—most commonly occurred in downstream reaches at salinities ranging from 10 to 20‰. Fish in the third complex, red shiner and mosquitofish Gambusia affinis, were ubiquitous, although they were most abundant at salinities less than 10‰ (Ostrand 2000). A similar distributional pattern was observed by Taylor et al. (1993) in the upper Red River drainage of southwestern Oklahoma. The abundance and species distribution patterns identified by Echelle et al. (1972) and Taylor et al. (1993) agree with our laboratory results and show that upper salinity tolerance predicts cyprinid presence in the field. In these systems the greater probability of isolated pool formation in upstream reaches and the abiotic changes accompanying pool formation, which create extreme environmental conditions beyond tolerable levels, are the most likely factors creating longitudinal zonation of fish assemblages. Physiological tolerances are directly related to largescale spatial patterns of fish distributions where only a subset (e.g., Red River pupfish and plains killifish) of the regionally available species is able to successfully maintain populations upstream (Ostrand 2000). Acknowledgments We thank D. E. Marks and T. H. Bonner for help in the field and are grateful to R. Patin˜o for tech-

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