Brett P. Fessell, Edward J. Peters, Richard S. Holland, Department of Forestry, Fisheries, and Wildlife, University of Nebraska, Lincoln, NE

INTRODUCTION

Temperature is considered to be a controlling factor and has pervasive influences on the biology, physiology, and ecology of fishes. Direct effects of elevated temperatures may stem from decreased metabolic activity in nervous tissue resulting in asphyxiation; however indirect effects on reproduction, ability to resist disease, and maintain performance in the presence of competition and predation may be more influential on the ultimate success of fishes occupying harsh environments (Brett 1956). Harsh environments typically exhibit extreme fluctuations in temperature, discharge, and turbidity and can have significant effects on fish populations therein (Matthews 1987). Exact mechanisms which allow fluctuating temperatures to play a pivotal role in regulating fish populations remain unclear. Fish either evolve physiologically to tolerate high temperatures or avoid temperatures less conducive to physiological performance through behavioral modifications (Hutchinson 1976; Reynolds and Casterlin 1976; Hutchinson and Maness 1979; Neill 1979; and Matthews 1987; Rutledge and Beitinger 1989). Huey and Stevenson (1979) argue that thermal tolerance measurements have limited ecological significance, and they feel behavioral regulation of optimum body temperature may provide more relevant information for extrapolation to ecological influences. However it is difficult to measure and interpret behavioral responses of fish under laboratory conditions and complications are magnified in the field. Information obtained from thermal tolerance studies appears to be reliable for measuring physiological stress and adaptation of fishes giving researchers the ability to set observable limits of tolerance by measuring physiological performance thresholds (Kowalski et al. 1978; and Paladino et al. 1980).

Critical Thermal Maximum (CTM) has been used extensively as a tool to measure thermal tolerance of ectotherms, primarily in laboratory experiments, since it was defined by Cowells and Bogert (1944), modified for statistical analysis by Lowe and Vance (1955), and standardized by Hutchinson (1961). Critical thermal maximum is defined as the "arithmetic mean of collected thermal points at which locomotor activity becomes disorganized to the point at which the organism loses its ability to escape conditions that will promptly lead to its death" (Cowells and Bogert 1944). This is typically characterized by the inability to right oneself and the onset of muscular spasms. Direct field measurements of CTM have been virtually ignored in the literature, therefore it is difficult to effectively correlate laboratory data with field data. Most previous work has examined CTM's of fish acclimated to stable temperatures in the laboratory where little attempt is made to directly correlate laboratory and field data (Reynolds 1977; Magnuson et al. 1979; and Deacon et al. 1987). Both field and laboratory studies were conducted in this project, however only the field component is reported here.

Rivers of the Great Plains often exhibit declines in summer flow as a result of drought and dewatering for agricultural, domestic, and industrial purposes. Consequently, fish communities may be adversely affected through modification of abiotic factors such as temperature or dissolved oxygen (Matthews 1988). The wide, shallow braided channels of the Platte River are typical of Great Plains Rivers. These characterisitcs make the Platte susceptible to drought and diversion of water for power and irrigation districts contributing further to the depletion of summer flows critical for fish populations during this stressful period. Instream temperatures are susceptible to the effects of insolation and have been known to exceed 39o C under such conditions. Fish kills have been reported along several reaches of the Platte River periodically since the summer of 1988 (Dinan 1992). However temperature could not be conclusively implicated in the fish kills reported. The focus of this research was to determine whether temperature can in fact be implicated in fish kills reported. Using the information obtained from this work we hope to predict periods where fish populations may be susceptible to temperature related mortalities. Finally, thermal tolerance estimates of 16 common species in the Platte River will be presented.

METHODS

Four sites on the Platte River were selected for fish collection and subsequent testing (Fig 1). Sites near Alda and Elm Creek represented the central Platte River and sites near North Bend and Two Rivers State Park represented the lower Platte River. All sites were visited during four periods in 1994 (mid April, late June/early July, mid August, and mid October).

All fish were collected using a 7.62 x 1.22 m seine with 6.4 mm mesh and maintained in a livewell kept in the river throughout the testing period to ensure the fish would remain subject to ambient river conditions. Ten fish of each species, red shiner Cyprinella lutrensis, sand shiner Notropis stramineus, and plains killifish Fundulus zebrinus, were randomly selected after capture for determination of their CTM and death point. Critical Thermal Maxima were measured using a heating rate of 1o C/min (Lowe and Vance 1955; and Hutchinson 1961). Stirring hotplates were calibrated to heat at 10C/min. Fish were placed in 2000 ml beakers containing ambient temperature river water and heated to their CTM and death point. The CTM was recorded when a fish was unable to maintain equilibrium following erratic attempts to escape thermal discomfort. Death point, characterized by cessation of opercular movement, was also be recorded for each specimen. Each specimen was preserved individually in 10% Formaldehyde for length and weight measurements in the lab. Temperature was measured using a digital thermistor. A 4000 W gasoline powered generator served as a remote power source for hot plates and thermistors.

Field temperature collection

River temperature was recorded continuously throughout the sample period. These data were summarized into two week intervals representing the Mean Ambient River Temperature (MART) two weeks prior to field thermal tolerance measurement and used to evaluate the relationships between river temperature and thermal tolerance of the three target species.

RESULTS AND DISCUSSION

Critical thermal maxima for all three species were averaged over the entire study period from April to October 1994 (Table 1). Means are reported with ranges for each species in addition to the overall MART for the entire sample season. As expected, ANOVA indicated all three species have significantly different thermal tolerances when measured under field conditions (p<0.0001). These general differences in thermal tolerance reflect each species relative susceptibility to elevated temperatures and imply directive adaptation in response to existing river temperature regimes (Feminella and Matthews 1984). Plains killifish exhibited the highest measured thermal tolerance. This is most likely indicative of this species habitat selection strategies and ability to maintain efficient physiological functions at high temperatures where physiological functions may be aided through the employment of isoenzymes unique to particular thermal conditions experienced by this species among others in this genera (Soltz and Naiman 1978). Feldmeth et al. (1974) reported that an increased scope of thermal tolerance allows fish to maintain physiological functions over a wider range of temperatures when exposed to fluctuating temperatures as in the Platte River. Although, it has been observed in the Platte River that killifish do utilize different habitats at different periods during the day, and perhaps these "directive movements" into different habitats serve to maximize the thermal scope of this species in its range in the Platte River (pers. obs.). Thermal lability is clearly exhibited by the red and sand shiner in the Platte River with a range even greater than that of killifish. Again, the general susceptibility to high temperatures is reflected in these species thermal tolerance where sand shiners appear to be the most vulnerable. Further, sand shiners were, in fact, commonly the most abundant species reported in the fish kills of the late 80's (Fannin 1988).

Analysis of variance (ANOVA) was employed to investigate differences in thermal tolerance of the target species from different locations (Central and Lower) in the Platte river and also to determine whether any differences in thermal tolerance could be detected between sites within the locations. No significant differences were observed between sites within locations for any species (p=0.7357 Cl; p=0.6837 Ns; p=0.7435 Fz). Consequently, all data collected at sites within locations were pooled and analyzed accordingly to enhance the statistical rigor of the tests. However, killifish were only collected on one occasion in the lower Platte, therefore no reasonable comparison could be made between location for this species. Initially, t-tests performed to assess the effect of location on thermal tolerance of sand and red shiners indicated no significant difference in thermal tolerance for either species between central and lower locations in the Platte River (Table 2). However it was suspected that Mean Ambient River Temperature (MART) has a direct influence on these fishes thermal tolerances since it essentially represents the thermal history of the fish for a period of two weeks prior to thermal tolerance measurement in the field. Analysis of covariance (ANCOVA) did, in fact, indicate that MART plays a significant role in how location effects the thermal tolerance of sand shiners, but not red shiners (Table 2). This indicates that thermal tolerance of sand shiners is influenced to a greater degree by location effects where MART covaries with thermal tolerance. In practical terms this means that sand shiners from the central Platte have different thermal tolerances than sand shiners in the lower Platte relative to the thermal regimes of the different locations in the Platte River. Conversely, red shiners do not exhibit differences in thermal tolerances between locations in the Platte River, and is most likely indicative of this species thermal lability and capacity to adapt physiologically in the context of its environment. Matthews (1986) reported no significant difference between 18 populations of red shiners from south central Texas to north Kansas, which corroborate the findings of this research. Further, Matthews indicated that this species has been characterized as having a "malleable genome" allowing it to occupy a wide range of habitats. Therefore, in the case of Platte River red shiners, the absolute temperature tolerance of this species most likely plays a more significant role in allowing this species to rapidly adapt to local environmental temperature changes than other species it is commonly associated with, such as the sand shiner.

As indicated by ANCOVA, it was evident for all three species that MART was the primary factor influencing the thermal tolerances of the fish tested as it represents the thermal history of those fish prior to CTM measurement. Characterization of the relationship between MART and CTM was attempted through regression analysis to ascertain the significance of linear, quadratic, and cubic terms in the model, and to estimate coefficients of those terms. All terms were found to be significant for sand shiners and plains killifish, however red shiners showed no significant relationship at any level investigated. The rational for investigating the significance of quadratic and cubic terms and their coefficients was to evaluate the presence of a thermal threshold represented by the cubic term in such a relationship. Unfortunately, the coefficients generated from the analysis proved to be inconsistent with observed thermal tolerances when they were used to predict CTM from MART. A likely reason for this inconsistency is that the actual relationship between MART and CTM is, for all practical purposes, linear to a point where MART equals the upper incipient lethal temperature of these fish which were not measured in this study (Houston 1982). As a result, the linear component of this relationship was investigated using instantaneous river temperature data represented by initial temperatures of the water at the beginning of each individual fishes CTM measurement. Alternatively, thermal thresholds were estimated from log-linear graphs by calculating the mean and standard deviation of the data representative of the highest instantaneous temperatures (Fig 2). Rationale for using instantaneous river temperatures to reflect MART is justified in that correlation between instantaneous and Mean Ambient river temperatures is high. Estimated thermal thresholds were then superimposed over the 1994 sample season thermograph (Fig 3). This graph illustrates the general susceptibility of these species to thermal stress related morality during dry summer months and further emphasizes the importance of water management efforts to preserve adequate instream flow during these critical periods. Furthermore, as indicated by the graph, a breech in the thermal threshold of the sand shiner occurred on June 20, 1994, and, in fact, fish kills including sand shiners were reported in both the central and lower Platte on this date (Hutchinson pers. comm.). Red shiners were the only species which displayed no significant statistical relationship with MART, however graphically this appears to be contradicted (Fig 2). Perhaps an explanation lies, once again, in the ability of this species to adjust their thermal tolerance more readily in the context of their environment. For example, red shiners may require a relatively shorter acclimation period (thermal history) than that of sand shiners and plains killifish in a fluctuating environment such as the Platte River. Furthermore, this apparent advantage may consequently contribute to the relative success of this species in harsh environments.

Thermal tolerances of other common species

The following is a brief report on the estimated thermal tolerances of some other common species of the Platte River (Table 3). These species thermal tolerances were measured on an as encountered basis. Consequently, these species, by no means, represent all species in the Platte River, however these 16 species do represent some of the most common fishes occupying the Platte River. In addition, many of these species possess thermal tolerances which may leave them vulnerable to high temperatures. These estimates were measured in late August, 1994, therefore should represent the seasonal maximum thermal tolerance relative to ambient temperature conditions of the Platte River. Furthermore, keeping in mind these estimates only represent a snapshot in a temperature tolerance continuum of these species, they never-the-less may be important indicators to relative susceptibility to high summer instream temperatures, and further accentuate the importance of adequate instream flow maintenance for the life requisites of fish in the Platte River.

ACKNOWLEDGEMENTS

We would like to thank the United States Fish and Wildlife Service for the primary financial support of this research. Kenneth Dinan (USFWS) provided valuable information included in his temperature work on the Platte River. The Water Resources Center at the University of Nebraska-Lincoln for the purchase of Tempmentors. I also extend appreciation to Justin King at the Nebraska Public Power District and the Nebraska Game and Parks for providing field equipment much needed for this research. Finally, I thank the many students and technicians who assisted with the field and laboratory work involved in this project.

LITERATURE CITED

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