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REC-ERC-77-11 OBSERVATIONS ON THE STATUS OF THE DEVIL'S HOLE PUPFISH IN THE HOOVER DAM REFUGIUM Prepared and submitted to the Bureau of Reclamation by J. E. Williams Department of Biological Sciences University of Nevada, Las Vegas September 1977 Applied Sciences Branch Division of General Research Engineering and Research Center Denver, Colorado UNITED STATES DEPARTMENT OF THE INTERIOR SI METRIC BUREAU OF RECLAMATION ACKNOWLEDGMENTS Funding for this project was provided by the Division of General Research of the Bureau of Reclamation. Engineering and Research Center, Denver, Colo. (Purchase Order No. 6-01-ER-03376/Research Project DR-397) and by the Department of Biological Sciences, University of Nevada, Las Vegas. Thanks are due Herb Guenther and Gary Bryant of the Bureau of Reclamation in Boulder City, Nev., and Robert Furtek of the University of Nevada, for their assistance in monthly population counts. Dr. J. E. Deacon of the University of Nevada is thanked for his comprehensive review of this manuscript and the use of unpublished population data from Devil's Hole. A final technical review was performed for the USBR by J. F. LaBounty. CONTENTS Page Introduction Application ........................................................................................................... Summary ............................................................................................................... Conclusions ........................................................................................................... Description of the Hoover Dam refugium Methods ............................................................................................................... Results Discussion of fish refugia Bibliography 1 2 2 2 3 3 3 4 5 TABLES Table 1 2 Morphometric comparison of Hoover Dam refugium and Devil's Hole populations ........................................................................ Meristic comparison of Hoover Dam refugium and Devil's Hole populations 14 15 FIGURES Figure 1 2 3 4 5 6 7 8 Underwater photograph of Cyprinodon diabolis in the Hoover Dam 6 refugium ...................................................................................................................................... 7 The Hoover Dam refugium located just below Hoover Dam ........................... Longitudinal profile of the Hoover Dam refugium ........................................................ 8 9 The tapped hot springs being piped to the refugium ......10 The outflow box (left) of the Hoover Dam refugium Comparison of monthly population fluctuations from Hoover Dam 11 refugium and Devil's Hole populations of Cyprinodon diabolis ................... Comparison of sex ratio fluctuations in Hoover Dam refugium and .... 12 Devil's Hole populations The shallow end of the refugium showing substrate and associated 13 algal mats of Mastigocladus lamiriosa INTRODUCTION Miller [1], in observing the fact that only about 1 00 fish species occur naturally in the United States west of the Rocky Mountains, described this fauna as depauperate, characterized by relicts, monotypic genera, and large amounts of endemism. In recent years, this unique and specialized fish fauna has come under considerable pressure from the introduction and establishment of exotic fish species and a reduction in habitat, usually via human demand on the water supply [1, 2]. This problem has become particularly intense in the southwestern United States where agricultural pumping and diversion of watercourses coupled with the establishment of exotic fish species is threatening many native fishes. During the past 35 years, man's activities have apparently caused the extinction of four species and six subspecies of fish within California, Nevada, and Arizona [2]. Additionally, 50 species of fish, including the Devil's Hole pupfish, within the eight Great Basin states and northern Mexico are listed by the Department of the Interior as threatened or endangered [3]. The one species which has received the most publicity and in many ways has spearheaded endangered species recovery work is the Devil's Hole pupfish, Cyprinodon diabolis. The Devil's Hole pupfish, until the recent transplant into the Hoover Dam refugium, was restricted to a single spring source in Ash Meadows, Nye County, Nev. This single spring, named Devil's Hole, is located approximately 15 m (metres) below the land surface in a collapsed cave. Devil's Hole is part of an underground water cavern system which is known to exceed 100 m in depth. While the cavern system is extensive, the pupfish use a very small portion of it. A small 2- by 3-m shelf area near the surface was found to be the site of virtually all pupfish reproductive and feeding activity [4]. Devil's Hole is, according to the National Park Service, the most restricted habitat of any vertebrate species in the world. Owing to this unique habitat, and the uniqueness of the fish itself, Devil's Hole was designated as a disjunct ' Numbers in brackets identify references in the bibliography. portion of the Death Valley National Monument in 1952. During the 1960's, agricultural development, with its associated pumping of underground water supplies, began affecting Devil's Hole by lowering the water level on the critical shelf area. When the relationship between pumping of ground water and the falling water level in Devil's Hole, and between the falling water level and reduced pupfish reproduction was established, the Department of the Interior requested that pumping be stopped from four nearby wells. The request was ignored; so in July 1973, the Department of Interior requested the Department of Justice to initiate litigation to prevent the continued withdrawal of ground water by pumping. This battle led to the U.S. Supreme Court where, in 1976, the justices upheld a lower court decision which "favored" the pupfish by ordering a water level which would allow the pupfish to survive and reproduce successfully. However, in the early 1 970's (and presently for that matter), the survival of the pupfish in Devil's Hole was in doubt, therefore, a search began for a possible transplant site. In 1971, a spring was located below Hoover Darn which appeared to have a temperature similar to the water of Devil's Hole, 33 °C. By August 1972, a triparty agreement between the U.S. Fish and Wildlife Service, the Bureau of Reclamation, and the Nevada Department of Fish and Game resulted in the completion of a water-filled cement pool, known as the Hoover Dam refugium. In September 1972, substrate material, planarians Dugesia, hydrobiid snails Tryonia, elmid beetles Stenelmis, and the green algae Spirogyra, all from Devil's Hole, were placed in the Hoover Dam refugium. On October 2, 1972, seven Devil's Hole pupfish were placed in the refugium. A 96-hour bioassay period was initiated to examine the pupfish in their new home. Two pupfish were lost shortly after transfer. However, the remaining five apparently thrived, so 20 additional fish were transplanted into the refugium on October 13, 1976. One dead pupfish was removed two days later. On October 21, 1976, reproduction was taking place, indicating the fishes apparent adoption of their new environment. but highly fluctuating population of Tryonia now exists within the refugium. Elmid beetles and planaria are, however, absent. Significant numbers of other aquatic insects are present. Of particular importance are the dragonfly larvae which at times are numerous. APPLICATION The results and conclusions in this report will be of interest to anyone working with desert fishes and/or endangered fish species in particular. Unfortunately, increased human need for land (and water) coupled with the restricted range of many desert fishes has threatened some of the endemic species. This has resulted in an increased interest in fish refugia. With the design of the Hoover Dam refugium being incorporated for other endangered fish species; the Pahrump killifish, Empetrichthys I. latos, for example; it is necessary to determine the effectiveness of this refugium as quickly as possible. Results of this study will also be important in determining the future status of the Devil's Hole pupfish (Cyprinodon diabolis). CONCLUSIONS The following general conclusions resulted from the Hoover Dam refugium studies: 1. The Hoover Dam refugium population of Cyprinodon diabolis has become the first established population of the fish outside of Devil's Hole. 2. Population fluctuations in the Hoover Dam refugium are similar to those observed in Devil's Hole, with lowest numbers observed in late winter and highest numbers in August through October. SUMMARY This report attempts to examine the status of the Hoover Dam refugium population of Cyprinodon diabolis by comparing community structure and complexity, population fluctuations, sex ratio data, morphometrics, and meristics of the Hoover Dam refugium population to the original Devil's Hole population, the source of the transplanted fish (fig. 1). Research reported herein was conducted from October 1975 through March 1977. 3. During the sampling period, population levels varied from a high of 69 during October 1 976 to a low of 48 in February 1977. 4. The male:female sex ratio in the refugium is highly variable, ranging from 2.92:1 to 0.75:1. Monthly population counts showed that fluctuations of pupfish numbers observed in the Hoover Dam refugium were similar to those noted in Devil's Hole, with lowest numbers occuring in late winter and highest numbers in late summer and early fall. The number of fish in the refugium ranged from 48 to 69. Sex ratios are highly variable, as they are in Devil's Hole. 6. Mean values of predorsal and anal to caudal length are very similar for the refugium and Devil's Hole populations. Morphometric examination revealed significant differences between the Hoover Dam refugium and Devil's Hole populations in five of seven measurements taken. An overall increase in length was particularly evident. 7. Mean values of greatest body depth, greatest body width, head length, and least caudal peduncle depth all differed significantly between the two populations. 5. Standard length of the refugium population has increased to a mean of 25.1 mm (millimetres) compared to a mean of 1 9 mm in Devil's Hole. 8. Meristic examinations showed insignificant variation between the two populations. Community diversity has increased within the Hoover Dam refugium since the initial introduction of the green alga Spirogyra, the hydrobiid snail Tryonia, elmid beetles Stenelmis, and planaria Dugesia, from Devil's Hole. Algal diversity has increased from the single species originally planted to at least seven. An often large, 9. Community diversity has increased in the refugium since the initial introduction of algae and invertebrates from Devil's Hole. 2 Morphometric and meristic counts were made as described for cyprinodonts by Miller [5]. Measurements were made with precision dial calipers to the nearest 0.1 mm. All counts and measurements were taken from live specimens at the refugium. Eleven fish were examined for morphometric and meristic characters. In some fin ray counts, less than 11 fish were examined. The necessity of making these counts and measurements on live fish of course raises questions regarding their comparability with preserved specimens. This approach was taken because of restrictions on the collecting permit. 10. Predation by dragonfly larvae may pose a problem for the refugium population. 11. The Hoover Dam refugium has been successful as a temporary environment for the Devil's Hole pupfish, however, the environment which Devil's Hole provides is unique and probably cannot be permanently duplicated. DESCRIPTION OF THE HOOVER DAM REFUGIUM The Hoover Dam refugium was constructed in August 1 972 and is located less than 1 km (kilometre) below Hoover Dam in Clark County, Nev. on land administered by the Bureau of Reclamation (fig. 2). The refugium is approximately 2 by 6 m and ranges from 1 to 3 m in depth (fig. 3). RESULTS During the period from October 1 975 through March 1 977, population numbers ranged from a low of 48 in February 1 977 to a high of 69 in October 1 976. This fluctuation corresponds well with that displayed by Cyprinodon diabolis m Devil's Hole where in 1 976, the lowest population estimate was 1 80 in February and the highest was 410 in September (fig. 6). On March 1 4, 1974, the Hoover Dam refugium population was estimated at between 1 94 and 204. After the initial introduction of fish in October 1 972, the population underwent an explosion in numbers which resulted in the March 1 974 estimate [7]. In November 1 974, the population had decreased to an estimated 110 to 117, and by October 1 975, there was a further reduction to 62 fish. However, since October 1 975, the population has apparently stabilized. Water for the refugium is provided by a nearby hot spring which flows at 40 °C (fig. 4). Within the refugium, water temperature is maintained near 33 °C by controlling the rate of flow into the refugium. In summer months, the flow of hot water is increased. Water enters the refugium at the deep end and exits via an outlfow weir near the surface (fig. 5). METHODS Monthly population counts were made at the Hoover Dam refugium. Counting procedures involved partitioning the refugium into seven sections. In each section, the following categories of fish were counted on at least two occasions (the number of occasions depending upon the consistency of the counts): males, females, juveniles, and all fish. The average counts of males, females, and juveniles were added to any observed larvae to arrive at a final total count which appears in the tables. The counts of "all fish" were used only as a comparison. Beginning in January 1 976, a scuba diver was employed during the population counts. By chasing the fish out otthe deep end, the diver enabled more accurate and consistent counts to be made at the surface. Through the described counting procedure, sex ratios of all adult individuals (those longer than 1 5 mm length) were determined. During population counts, the sex ratio of the population was examined. Monthly male:female sex ratios proved to be highly variable, ranging from 2.92:1 to 0.75:1 (fig. 7). During most of the year, there are slightly more males than females. Reproduction occurs throughout the year in the Hoover Dam refugium with highest reproductive activity from late May through August. Juveniles (fish 1 5 mm or less in length) comprised approximately a third of the total fish observed in June (20 of 63), July (22 of 63), and August (21 of 66) of 1 976. Lowest numbers of juveniles observed was in February 1 977 (1 of 48). The greatest abundance of larval fishes was seen in May through August. Community diversity has increased within the Hoover Dam refugium. Originally in October 3 1 972, hydrobiid snails. Tryonia; elmid beetles, Stenelmis; green algae. Spirogyra; and planaria, Dugesia, were planted into the refugium from Devil's Hole. Examination of the algae during the fall of 1976 revealed the presence of six genera. Dominant algae included the green, Spirogyra, and the blue-green, Mastigocladus laminosa (fig. 8). Two other blue-greens, Oscillatoria and to a lesser extent Aphanothece, were present. Attached to Mastigocladusand Oscillatoria were the diatoms, Achnanthes and Cymbella. The hydrobiid snail, Tryonia, exists as a viable, highly fluctuating population within the refugium. The snails become omnipresent within the refugium from March through August. In winter months, the snail population is reduced to a mere remnant of its summer numbers, with most individuals on rocks in the deep end of the refugium. Elmid beetles and planaria failed to become established within the refugium, perhaps because of the absence of shallow, sloping substrates within the refugium. The refugium walls are vertical and the shallowest depth is 1 m. The addition of a declining shelf, or the simple rearrangement of substrate, might provide a suitable niche for these invertebrates. Various aquatic insects inhabit the refugium. Dragonfly larvae, a known fish-fish egg predator, occur in the refugium. It is not uncommon to remove these larvae from the waters of the refugium during the monthly population counts. Normally, dragonfly larvae would not pose a threat to a fish population; however, with the low numbers of fish in winter and spring, even limited predation could be significant. Such larvae do not exist in Devil's Hole. caudal length were similar in both populations. Meristic data, seeming less susceptable to change, showed little difference between the two populations (table 2). DISCUSSION OF FISH REFUGIA Recent deterioration of habitats and introduction of exotic fishes has led to reduced populations of many native fishes. The Death Valley fishes of eastern California and southwestern Nevada in particular, have been under ever-increasing pressures due to increased use and development of the area. It is here, in the Death Valley system, where interest in fish refugia has often been focused. Refugia for endangered Death Valley fishes exist in a myriad of forms dependent upon land and water availability and the requirements of the fishes involved. Completibn of the Owens Valley Native Fish Sanctuary in October 1969 provided a 2.3 hectare natural refugium in Fish Slough, Owens Valley, including the Owens pupfish, Cyprinodon radiosus;a distinctive speckled dace, Rhinichthys osculus; the Owens River Tui chub, Gila bicolor snyderi; and the Owens sucker, Catostomus fumeiventris [6]. This type of natural refugium located in the same basin in which the fish are endemic to is preferred. However, conditions do not always permit the establishment of such a refugium. This was the case when the Hoover Dam refugium site was chosen. This "cement tank" type of refugium should be considered only as a last resort. With this type of refugium, continuous surveillance and management by qualified personnel is often necessitated. Refugia are not substitutes for the native fishes original habitat, but could act as "insurance" against loss of a population. During October 1 976, morphometric and meristic data were gathered at the Hoover Dam refugium. These data were compared to those gathered by Miller [5] on the Devil's Hole population. Morphometric measurements showed an overall increase in body size of C. diabolisin the refugium (table 1). Standard length increased from a mean of 19 mm in Devil's Hole to a mean of 25.2 mm in the refugium. A maximum length of 31.5 mm was observed for a refugium female (larger fish were seen but were not sampled). The proportion of body depth to standard length was greater in the refugium population. Similar results were observed for body width and caudal peduncle depth. The proportion of head length to standard length was less in the refugium population than in the Devil's Hole population. Predorsal length and anal to Some fish populations exist in highly unique habitats which are nearly impossible or not practical to reproduce elsewhere; Devil's Hole represents such a habitat. The Hoover Dam refugium provides insurance against loss of the Devil's Hole pupfish but does not provide a replacement for Devil's Hole itself, because different environments select for different characters, resulting in genetic drift of the Hoover Dam population away from the Devil's Hole population. 4 BIBLIOGRAPHY [1] Miller, R. R. "Man and the Changing Fish Fauna of the American Southwest," Pap. Mich. Acad. Sci., Arts, Lett. 46: 365-404, 1961. [2] Pister, E. P. "Desert Fishes and Their Habitats," Trans. Amer. Fish. Soc., 103(3): 531-540, 1974. [3] U.S. Dept. of Interior. "Threatened Wildlife of the United States," Bur. Sport Fish. Wild. Resource Publ. 114 Wash., D.C., March, 1973. [4] James, C. J. "Aspects of the Ecology of the Devil's Hole Pupfish, Cyprinodon diabolis Wales," Unpubl. M.S. Thesis, University of Nevada-L.V., 1969. Miller, R. R. "The Cyprindodont Fishes of the Death Valley System of Eastern California and Southeastern Nevada," Misc. Publ. Mus. Zool. Un iv. Mich., 68: 1-155, 1948. [5] [6] Miller, R. R. and E. P. Pister. "Management of the Owens Pupfish, Cyprinodon radiosus, in Mono County, California," Trans. Amer. Fish. Soc., 100(3): 502-509, 1971. [7] Williams, J. E. "1975 Status Report on the Devil's Hole Pupfish, Cyprinodon diabolis, at the Hoover Dam Refugium," in, Preceedings of the Seventh Annual Symposium of the Desert Fishes Council, in press. 5 Figure 1.—Underwater photograph of Cyprinodon diabolis in the Hoover Dam refugium. Males are blue with black fin banding; females appear as a straw-green color. Photo P45-D-77921, P45-D-77925, and P45-D-77926 6 Figure 2.—The Hoover Dam refugium located just below Hoover Dam. Shown in the foreground, covered by screens. Photo P45-D-77920 7 5.79 m T I 0 I JL — A. • Z/• Th ouTFLOW „ „I. -1 A./ .a. — /1 1-, 1.75 m Figure 3.-Longitudinal profile of the Hoover Dam refugium. 8 Inflow pipe ---- Figure 4.-The tapped hot springs being piped to the refugium. Photo P45-D-77923 9 Figure 5.—The outflow box (left) of the Hoover Dam refugium. Photo P45-D-77922 10 80 • / / N / 400 \ N N N 60 AL ,, / V) 0 \. ct _J a. 0 40 a_ cc , V V \ ,.../ Hoover Dam -- — —Devil's Hole 0 300 200 HOLE POPULATION SIZE) N..., Refugium 20 100 0 OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR 1 975 1 976 1 977 OCTOBER 1 975 THROUGH MARCH 1977 SAMPLING INTERVALS Figure 6.—Comparison of monthly population fluctuations from Hoover Dam refugium and Devil's Hole populations of Cyprinodon diabolis. Refugium (solid line) and Devil's Hole (dashed line) data were collected from October 1975 to February 1977. 11 3:1 / \ \ I / / / 2:1 ‘\ X r'• 4 / / / \. \ \ , V Hoover Darn Refug urn — Devi I 's Hole OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY 1 975 1 976 1 977 SAMPLING INTERVALS Figure 7.—Comparison of sex ratio fluctuations in Hoover Dam refugium and Devil's Hole populations. Refugium (solid line) and Devil's Hole (dashed line) data were gathered from October 1975 to March 1977 and October 1967 to August 1 968, respectively. Devil's Hole data from James [4]. 12 Figure 8.-The shallow end of the refugium showing substrate and associated algal mats of Mastigocladus laminosa. Photo P45-D-77924 13 Table 1.—Morphometric comparison of Hoover Dam refugium and Devil's Hole populations. Refugium data were gathered in October 1976. Devil's Hole data from Miller (5). Population Sex No. Standard length ( mm) range mean Body dimensions in thousandths of standard length: range (mean) Predorsal length Anal to caudal length Body depth Body width Head length Peduncle depth Hoover Dam M 4 20.5-29.9 25.2 559-702 (606) 317-378 (349) 361401 (386) 204235 (225) 288334 (316) 167-186 (178) Hoover Dam F 7 16.2-31.5 25.0 546-691 (632) 277-395 (330) 313399 (359) 200295 (246) 294385 (334) 154-177 (167) Devil's Hole M&F 34 14-28 19 — — 335375 (354) Devil's Hole Devil's Hole M F 10-18 10-12 — — — 601-654 (631) 329-355 (343) 309407 (355) 226255 (237) — 159-177 (165) 617-645 (631) 315-332 (325) 304353 (329) 220249 (236) — 150-165 (157) Table 2.—Meristic comparison of Hoover Dam refugium and Devil's Hole populations. Refugium data were gathered in October 1976. Devil's Hole data from Miller (5). Devil's Hole Hoover Dam Dorsal rays Anal rays Caudal rays Pectoral rays Pelvic rays Range Mean Range Mean 11 11-12 14-18 1 4-19 0 11.0 11.5 15.2 16.5 0.0 1 0-13 1 0-12 13-20 14-18 0-6 11.13 11.24 16.83 16.86 0.02 15