Publications (WR) Water Resources
5-1990
Enhancement and monitoring of the Procambarus clarkii population in Lake Mead
Mikell Beth Hager
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ENHANCEMENT AND MONITORING OF THE LJ at
Procambarus clarkii
.POPULATION IN
LAKE MEAD *J
by
Mikell Beth Hager
A thesis submitted in partial fulfillment of the requirements for the degree of
Master of Science in
Biological Sciences
Department of Biological Sciences University of Nevada, Las Vegas May 1990 The thesis of Mikell Beth Hager for the degree of Master of Science in the
Biological Sciences is approved.
Chairperson, Larry J. Paulson
Examining Committee Me/mber,u SMnley D. Hillyard
Examining Comrmttee/Member, Joseph R. McAuliffe
Gradame Faculty Representative, Stephen M. Rowland
Graduate Dean, Ronald W. Smith
University of Nevada Las Vegas, Nevada May, 1990
11 TABLE OF CONTENTS
TITLE PAGE i APPROVAL PAGE ii
TABLE OF CONTENTS iii
LIST OF FIGURES vii
LIST OF TABLES k
ABSTRACT xi
ACKNOWLEDGEMENTS xii INTRODUCTION 1
General History 1
Distribution 1
Life History 3
Habitat Requirements 4
Impacts of Introductions 10 Lake Mead History 12
Literature Citations 12
Lake-wide Survey (1986-1987) 13
Statement of Research Hypothesis and Objectives 16
DESCRIPTION OF STUDY AREA 19
Lake Mead 19
Saddle Island 19
METHODS 23
111 Data Survey from 1987 23
Mean CPTD Value 23
Sex Ratio 23
NDOW/LMLRC Crayfish Introductions 23
Collection 23
Marking 24
Release 26
Monitoring 30
Mean CPTD Values 30
Sex Ratios 30
Fall Trapping 1989 33
Monitoring 33
Mean CPTD Value 33
Sex Ratio 33
Statistical Testing 34
Comparison of Mean CPTD Values 34
Comparison of Sex Ratios 34
RESULTS 35
Data Survey from 1987 Survey 35
Mean CPTD Value 35
Sex Ratio 35
: NDOW/LMLRC Crayfish Introductions 35
IV Introduction 1 (January 1988) 35
Mean CPTD Value 35
Sex Ratio 35
Introduction 2 (April 1988) 39
Mean CPTD Value 39
Sex Ratio 39
Introduction 3 (July 1988) 39
Mean CPTD Value 39
Sex Ratio 42
Introduction 4 (November 1988) 42
Mean CPTD Value 42
Sex Ratio 42
Fall Trapping 1989 42
Mean CPTD Value 42
Sex Ratio 45
Comparison of Mean CPTD Values 45
Comparison of Sex Ratios 45
DISCUSSION 49
Mean CPTD Values 49
Sex Ratios 51
Suitability of Crayfish Species in Lake Mead 53
Environmental Impacts of Introduction 54
Conclusions 55
v REFERENCES 57 APPENDIX 62
VI LIST OF FIGURES
Figure Page
1 Distribution of Procambarus clarkii in the continental
United States. The actual range within the state is not
shown (modified from Huner and Barr 1984).
Dorsal and ventral view of crayfish. Standard
length measurements shown on dorsal view
(modified from Huner and Barr 1984).
Distribution of Procambarus clarkii in
Lake Mead from Lake Mead Limnological
Research Center 1987 trap survey.
Limnological zones are shown. 14
4 Location of study cove on Saddle Island. 22
5 Markings on stocked crayfish. Uropods
were clipped to represent a different
introduction. The type of mark
differentiated between lots. 25
Diagram of study cove. Each lot of
crayfish was released in a different
habitat or substrate in the cove. 27
vn 7 Lake Mead water elevations (in feet)
from January 1988 through December
1988. The August 1989 elevation is
marked by the X. 29
8 Diagram of trap lines set in study cove. 31
9 Mean CPTD values compared using a One-Way
ANOVA and LSD Mean test. Actual CPTD values
were multiplied by 1000 for improved
graphic presentation. * represents
significance with g < 0.05. 46
10 Quadrant descriptions as denoted by PCI
and PC2 values. 64
11 Bivariate plot of Principal Component
1 and Principal Component 2. This is
a model of crayfish genera distribution
in regard to environmental conditions of lakes. 65
vm LIST OF TABLES
Table Page
1 Summary of preference or range of
environmental conditions for
Procambarus clarkii.
2 Catch per trap day (CPTD) values for
Procambarus clarkii in nine
limnological zones of Lake Mead (see
Figure 3) form 1987-1988 survey. 17
3 Morphometric characteristics of Lake Mead,
AZ-NV at maximum operating level of
374.0 meters. 20
4 Trap data from LMLRC 1987 trap survey of
Saddle Island. Mean CPTD value, standard
deviation, and standard error calculated
for this trapping period. 36
5 Summary of captured crayfish for each of
the four introductions. 37
6 Trap data from Introduction 1 (January -
February 1988). Mean CPTD value, standard
deviation, and standard error calculated
for this trapping period. 38
IX 7 Trap data from Introduction 2 (April -
May 1988). Mean CPTD value, standard
deviation, and standard error calculated
for this trapping period. 40
8 Trap data from Introduction 3 (July -
August 1988). Mean CPTD value, standard
deviation, and standard error calculated
for this trapping period. 41
9 Trap data from Introduction 4 (November -
December 1988). Mean CPTD value, standard
deviation, and standard error calculated
for this trapping period. 43
10 Trap data from Fall Trapping 1989
of Saddle Island. Mean CPTD value,
standard deviation, and standard error
calculated for this trapping period. 44
11 Summary of adult sex distributions for
the six trapping periods. Chi-square
test results for comparison of all
distributions and chi-square test
results for comparison of pre-stocking
and post-stocking distributions. 47 ABSTRACT
Procambarus clarkii are found in extremely low numbers throughout Lake
Mead, AZ-NV. The crayfish are an important dietary component for game fish.
Enhancement of the crayfish population would broaden the fishery forage base.
Crayfish were stocked and monitored in a study cove on Saddle Island to determine if the Procambarus clarkii population could be enhanced. A trapping survey of the area after the following reproductive season yielded low numbers of crayfish. A comparison of pre-stocking and post-stocking catch per trap day (CPTD) values revealed no significant increase in the population. Procambarus clarkii population growth is limited by environmental factors in Lake Mead. If management plans indicate that a large crayfish population is needed for the Lake Mead fishery, stocking of Qrconectes virilis is recommended.
XI ACKNOWLEDGEMENTS I would like to thank a number of people who provided help and encouragement during this thesis and degree work. Dr. Larry J. Paulson provided the opportunity to work in the Lake Mead Limnological Research Center and the financial support during my graduate studies. I am grateful for his advice and for allowing me to use LMLRC data. The Nevada Department of Wildlife and the Barrick Fellowship Foundation provided funding for this research and graduate studies. I am indebted to the following people for technical help and invaluable discussions: Gene Wilde, Joe McAuliffe, Stan Hillyard, Jennifer Haley, and Peter Vaux. Without the tremendous field support of the LMLRC crew, Suzanne Leavitt, Larry Shepard, Kathy Longshore, Kim (Bowdridge) Hunter, Mary Saethre, Cathy Baker, and Joe Hentz this research would never have been completed. Finally, I would especially like to thank my parents. They have always been there for me. Without their love, immeasurable support, and encouragement I would have never finished.
1
Xll INTRODUCTION GENERAL HISTORY The red swamp crayfish, Procambarus clarkii. is one of the most widely recognized species of crayfish in the world. It is a major commercial crop for human consumption along the Gulf Coast and southeastern United States and is raised as fish bait in the Midwest and parts of the West Coast. Procambarus clarkii have been extensively studied by physiologists, aquaculturists, and ecologists.
Distribution The natural range of P. clarkii has been documented over the last fifty years. Perm (1943) described the range as principally the Gulf Coastal Plain from Alabama to western Texas, extending north to Little Rock, Arkansas. More recent descriptions of the natural range (Huner and Barr 1984, Hobbs 1988) encompass the Mississippi River Valley up to the Great Lakes, reaching southeasterly into Florida, and west into New Mexico (Figure 1). P. clarkii has been introduced into areas outside its natural range (Figure 1). In the United States it has been introduced along the Eastern Seaboard up to New York and in the western states of Arizona, Nevada, California, Oregon, and as far as Hawaii (Bonnot 1930, Huner 1977, 1978, Bouchard 1978, Sommer and Goldman 1983, Sommer 1984, Huner and Barr 1984, Hobbs 1988). It has been conjectured that many of the introductions in the United States have been "accidental". As live crayfish are a popular fish bait, fishermen have been the vector of many Introduced
Figure 1. Distribution of Procambarus clarkii in the continental United States. The acutual range within the state is not shown (modified from Huner and Barr 1984). introductions. Many states have restricted the use of live crayfish as bait because endemic crayfish species have been out-competed or endangered by the exotic crayfish (Rach and Bills 1989).
Life History
Introduced P. clarkii usually thrive in their new environment (Huner 1977,
Lowery and Mendes 1977). This is often attributed to their life history. Perm (1943) and Huner and Barr (1984) have developed detailed life histories for this species, so I will briefly describe their findings and observations. P. clarkii have peak mating seasons in early-spring and in fall and sporadic mating occurs in the summer. In
Southern Nevada, I have observed that P. clarkii mated throughout the year. This species produces one to two generations of offspring per year (Perm 1943, Huner and Barr 1984, Momot 1984).
Under favorable environmental conditions, eggs will be extruded on to the abdomen of the female six weeks after a successful mating. If conditions are not favorable, the eggs may "over-winter" in the ovary for up to eight months. P. clarkii usually produce twelve to seven hundred eggs per successful mating (Momot 1984).
According to Romaire (1976), twenty-four to forty-seven percent of these eggs will survive to adult stages. The eggs will hatch into larval crayfish which continue to be attached to the female. Larval crayfish will become independent from the female after two to three molts.
Growth of P. clarkii juveniles is dependent on environmental factors.
Juveniles molt at least nine more times before they reach maturity (Huner and Barr 1984). In favorable conditions, this may occur in as little as two to three months
(Huner and Avault 1976). Carapace lengths (Figure 2) of mature crayfish measure
30 millimeters or greater.
Crayfish continue to grow throughout their life span by molting. Average carapace lengths measure 45 millimeters after the first year and 65 millimeters after one and one-half years (Momot 1984). Like other cambarids, adult male P. clarkii molt from a sexually active form (Form I) to a sexually inactive form (Form II) and vice versa. Form I may be differentiated from Form II by the presence of large ischial spines on the second pair of walking legs. Both forms may try to mate with females, but only Form I males are capable of producing viable sperm. The P. (it*' clarkii life span generally lasts from one to two years (Penn 1943, Huner and Barr
1984, Momot 1984).
Habitat Requirements
Crayfish growth and longevity depends upon many environmental factors
(Table 1). Many studies have investigated crayfish habitats and their physical, chemical, and biological parameters and/or preferences. Studies of their natural habitats in Louisiana (Penn 1943, Konikoff 1977, Pollard et.al. 1983) show that P. clarkii prefer shallow , eutrophic, slow-moving water or swamps. As a burrower, this species is well-adapted to alternating periods of flooding and receding water levels.
Much of the literature (Penn 1943, Konikoff 1977, Welles 1982, Huner and
Barr 1984) cites that P. clarkii prefer shallow water approximately one-half meter in
depth. When dewatering occurs, this crayfish forms burrows in the mud with CL
TL = Total length CL = Carapace length
Figure 2. Dorsal and ventral view of crayfish. Standard length measurements shown on dorsal view (modified from Huner and Barr 1984). Table 1. Summary of preference or range of environmental conditions for Procambarus clarkii.
Environmental Preference* or Range Condition
Water Depth -0.5 meters *
Cover Rock, Plants, Mud
Nutrients Plant detritus * Thin-stemmed aquatic plants Animal detritus Benthic animals
pH 6.5 - 8.5 *
Dissolved Oxygen > 2.0 ppm
Water Temperature 0 - 37 ° C 22 - 30 ° C
Hardness and Alkalinity > 50 ppm as CaCO3 100-150 ppm as CaCO3 *
Salinity < 5 ppt 7 chimneys extending above the ground five to ten centimeters. In studies in lakes, this species has been trapped at greater depths, expecially when conditions in the
littoral zone become unfavorable. Witzig et.al. (1983) found crayfish showed no
preference for shallow or deep water when littoral vegetation was not available for
f cover or sustenance. I Crayfish are crepuscular to nocturnal (Perm 1956). These opportunistic hunters are polytrophic in that they consume detritus, plants, and animals. Plant detritus comprises the major part of their diet, followed by living plant material and
animals (Williamson 1979, Huner and Barr 1984, Lodge and Lorman 1987). Most
thin-stemmed aquatic plants, algae, and periphyton are consumed by crayfish. Lorman and Magnuson (1978) support stocking crayfish to control aquatic
macrophytes. Plants are a major nutrient; but when crayfish are inactive, plants also
serve as cover.
Crayfish seek shelter not only in vegetation, but also in mud, gravel, and
rocks. The origin of the word "crayfish" is thought to be from the Old French
"ecrevisse" meaning crevice (Perm 1943). Stein examined crayfish cover selections
in the presence of predators. In tests using sand and gravel, crayfish migrated into
" gravel areas. Vulnerability to predators increased use of cover. Juveniles, being
small and most vulnerable, are most likely to seek shelter. Recent molts (soft-
shelled) and berried females (carrying eggs on abdomen) are the most susceptible
adult forms in terms of predation. They seek shelter and reduce activity until they
change life stages. Crayfish will utilize cover closely related to a food source (Flint
and Goldman 1977), thus reducing vulnerability when feeding. 8 Beingesser and Copp (1985) observed diurnal distributions of P. clarkii in a clear California stream. Adults took cover in sheltered areas such as undercut banks, while juveniles occupied open, exposed portions of the stream. This study does not agree with Stein's study (1977), but predation may have been less severe or color patterns may have been a factor. In a stream bed of mud, the juvenile color morph of greenish-brown may serve as camouflage. Adult P. clarkii color may vary from orange-red to a red-black. With this coloration, rock or vegetation would provide better cover for adults. Stein (1977) also noted that with the exception of berried females, the most aggressive crayfish -- the adults with the largest chelae -
- out-competed all others for shelter.
Color variation in P. clarkii is linked to the pH and turbidity of the water.
Clear, acidic waters produce dark red or black crayfish. Murky, alkaline waters have tannish-orange to orange-red colored adults (Huner and Barr 1984). Studies involving low environmental pH (France 1984, 1985, Berrill et.al. 1985) show that crayfish will tolerate a large range of pH. A range of 6.5 to 8.5 is preferred.
Juveniles and molting crayfish are the most sensitive to adverse pH as low pH inhibits calcium uptake across the gills.
In soft water with a pH 6.1 or less, juvenile mortality is observed. Adults will avoid an environment with a pH of 4.5 or lower. France (1984) maintained that an average annual pH below 5.5 could result in eventual population extinction due to mortality of juveniles.
Juveniles are also affected by low dissolved oxygen levels. Melancon and
Avault (1977) determined LC50 values for acute dissolved oxygen depletion in two size classes of juveniles. Newly released P. clarkii with a total length (TL) of nine to twelve millimeters had a LC50 for 96 hour range of 0.75-1.10 parts per million oxygen. Larger juveniles (TL= 31-35 mm) had a lower tolerance with an LC50 value for 96 hours of 0.49 parts per million oxygen. Crayfish that are stressed for oxygen will move to more highly oxygenated water or will turn on their sides at the water surface and utilize atmospheric oxygen (Penn 1943, Konikoff 1977, Huner and
Barr 1984). Huner and Barr (1984) recommend pond aeration any time water temperature exceeds 18 ° C to insure that oxygen levels are above 2.0 parts per million.
As water temperature affects the solubility of exygen and many other molecules in water, it plays an important role in crayfish habitat. Even with all other environmental factors being favorable, ambient water temperature can affect crayfish behavior and rate of activity (Peck 1985). Most commercial crayfisheries ship live crayfish packed in ice or ice water. The metabolism is slowed for transport to reduce mortality rates. If gradually warmed, crayfish revive with no harmful side- effects (Huner and Barr 1984). A temperature preferendum for P. clarkii was determined to be 21.96 +. 0.65 ° C (Taylor 1984). Huner (1988) reports that molting rates increase in temperatures between 22 ° C to 30 ° C.
Other environmental conditions of crayfish habitat have been determined.
Huner and Linquist (1985) observed that a total hardness and alkalinity of greater than fifty parts per million as calcium carbonate was needed for successful molting.
A range of one-hundred to one-hundred fifty parts per million of calcium carbonate is recommended. Other recommendations of water quality include: salinity of less 10 than twelve parts per thousand for survival and less than five parts per thousand for reproduction (Malley 1980); above three parts per million carbon dioxide; below five parts per million hydrogen sulfide; and less than three parts per million iron (Huner
1988). As a variety of freshwater systems have suitable ranges of the previously mentioned environmental factors, P. clarkii has developed a large natural and introduced distribution.
Impacts of introductions
Introductions of crayfish have been studied to determine their impacts (Huner
1977, Lowery and Mendes 1977, Grigarick and Way 1982, Pickett and Sloan 1985).
The purpose for many introductions has been to establish a new food crop or provide forage for game fish or other consumers. These introductions often fulfilled their purposes; unfortunately, many introductions created unexpected problems.
Introductions of Orconectes rusticus into the Upper Mississippi Valley has caused crayfish species displacements, macrophyte reductions (resulting in elimination of fish
cover and nesting sites), elimination or devestation of benthic insects, and reduction
of fish populations due to consumption of eggs (Capelli 1982, Flynn and Hobbs 1984,
Butler and Stein 1985, Lodge et.al. 1985). Lodge etal. (1985) concluded that this
introduction has "short-circuited" the food chain at two levels; replacing other
invertebrates as top grazers and small fish as secondary predators. They suggest that
this crayfish will eventually replace the game fish as top predators. Pickett and
Sloan (1985) observed similar results from introductions of Procambarus acutus
acutus in Glass Lake, Rensselaer County, New York. 11 Introductions of Procambarus clarkii have predominated over endemic
Pacifastacus species in California (Goldman 1973), however crop damage to the rice fields is the most reported problem. Sommer (1984) documented that introduced
P. clarkii adapted well to California rice fields and were beneficial as a high-yield food crop. Conversely, in earlier papers (Grigarick and Way 1982, Sommer and Goldman 1983) he and others described P. clarkii as "pests". Grigarick and Way
(1982) found that infestations of P. clarkii caused rice seedling reductions of seventy to one-hundred percent. P. clarkii have also caused damage to levees and irrigation boxes in rice fields. Sommer and Goldman (1983) estimated that the crayfish may cost growers in excess of one-hundred fifty thousand dollars per year.
Huner (1977) reveals that P. clarkii introductions in Japan have created serious problems in rice fields, and in Hawaii the introductions have resulted in significant losses to taro crops. Huner (1977) also reports that introductions into
Uganda, Sudan, Costa Rica, Nicaragua, and Spain have been successful and had no adverse repercussions.
Lowery and Mendes (1977) examined the effects of P. clarkii introductions to fisheries in Kenya. A crayfish fishery in Kenya had not been established as of their paper, but they discussed potential yields. They did find that existing fisheries were effected by the crayfish. In a positive aspect, P. clarkii provided up to seventy percent of black bass (Micropterus salmonoides) forage. Adversely, the crayfish damage fish after they are caught in the gill nets. Fishermen claim that up to thirty percent of their bass harvest is spoiled and the crayfish also cause severe damage to the gill nets. 12 Unestam (1975) discussed dangers of crayfish introductions and proposed restrictions. He expressed concern over the spread of the crayfish plague or other diseases via introductions. Unestam felt that the negative aspects outweigh the benefits. Huner and Barr (1984) contend that "one would never think twice about moving cattle, sheep, and goats around the world, even though native herbivores might be suitable for domestication." Crayfish introductions must be examined with care to determine the possible effects on established species and the potential benefit to fisheries.
LAKE MEAD HISTORY
Literature Citations
Procambarus clarkii are not native to Southern Nevada. The earliest literature citation is entitled Notes on the Las Vegas Crayfish by T.D.A. Cockerell dated 1900. This article is listed in a Smithsonian Institute crayfish bibliography
(Hart and Clark 1987), however a copy of this article was unobtainable. Soon after
Lake Mead was formed, Moffett (1943) reported that he had heard of the presence of crayfish in the lake, but had not observed any. Zinn collected several specimens of P. clarkii in the Las Vegas River (presently called the Las Vegas Wash), Las
Vegas, Nevada in the fall of 1944. Investigations of how or when the crayfish was introduced yielded no answers (Hobbs and Zinn 1948).
Jonez and Sumner (1954) surveyed Lake Mead for crayfish but found no specimens. Crayfish sightings in Lake Mead were unconfirmed until Allan and 13 Romero (1975) sampled bass stomachs and found that crayfish were a predominant
forage. Allan and Roden (1978) collected several specimens of crayfish in Lake
Mead and sent them to the California Academy of Science for identification. All adult members of the collection were identified as Procambarus clarkii. It was not until 1986 that enough interest in the Lake Mead crayfish population was generated to support a study.
Lake-wide Survey (1986-1987')
A lake-wide survey for crayfish was conducted by the Lake Mead
Limnological Research Center (LMLRC), University of Nevada, Las Vegas, from
October 1986 through July 1987. Quarterly trapping was performed in Lake Mead to determine the abundance of P. clarkii. During the 1834 trap days, a total of
eighty crayfish were collected. Crayfish were distributed throughout the lake (Figure
3) and were reproductively active; however, densities were extremely low (Peck et.al.
1987).
The quantifying unit of relative abundance for crayfish populations used in
crayfish literature is the Catch Per Trap Day (CPTD). This is the total number of
crayfish caught divided by the number of traps set over a twelve-hour period.
Total number of crayfish in traps CPTD = Total number of traps set
The lake-wide CPTD value was 0.06. The study divided the lake into nine
limnological zones (Figure 3) and calculated the CPTD for each zone. The relative Figure 3. Distribution of Procambarus clarkii in Lake Mead from Lake Mead Limnological Research Center trap survey of 1986-1987. Limnological zones are shown.
14 LEGEND Shoreline Elevation 366 m 0 5 km
Colorado River Arm 16 abundance in the zones revealed that distribution was not homogeneous throughout
Lake Mead. The CPTD values for the zones (Table 2) ranged from 0.01 in the Colorado Arm to 0.10 in Outer Las Vegas Bay (Peck et.al. 1987). These CPTD values are extremely low in comparison to CPTD values in Louisiana where P. clarkii are endemic. Konikoff (1977) observed peak CPTD values of sixty-two in the
Atchafalaya Basin of Louisiana but considered a CPTD range of ten to thirty-two to be a good relative abundance. The LMLRC study showed that P. clarkii had established a foothold in Lake Mead but not a strong one.
STATEMENT OF RESEARCH HYPOTHESIS AND OBJECTIVES
After the LMLRC study was completed, the Nevada Department of Wildlife (NDOW) expressed an interest in crayfish population dynamics and crayfish stockings or introductions in Lake Mead. The NDOW creel census determined that
Procambarus clarkii serve as an important food source for largemouth bass
(Micropterus salmoides) and striped bass (Morone saxatilis). The bass fishery in
Lake Mead has declined since 1978 and small and emaciated fish have been observed since the early 1980's (Paulson and Baker 1984). Augmenting the crayfish population would result in a larger forage base for game fish and bolster a fishery which brings in revenues of over seventy million dollars per year (W. Molini, personal communication). The NDOW contracted the LMLRC to study the potential for stocking Procambarus clarkii in Lake Mead to improve the forage base.
This thesis research was a part of that NDOW/LMLRC study. My goal was
to determine if the P. clarkii population could be increased through stocking. My 1
17
Table 2. Catch per trap day (CPTD) values for Procambarus clarkii in nine limnological zones of Lake Mead (see Figure 3) from 1986-1987 survey.
Limnological Zone CPTD Value
1 Inner Las Vegas Bay 0.02
2 Outer Las Vegas Bay 0.10
3 Boulder Basin 0.02
4 Virgin Basin 0.02
5 Lower Overton Arm 0.02
6 Upper Overton Arm 0.03
7 Lower Colorado River Arm 0.01
8 Middle Colorado River Arm 0.01
9 Upper Colorado River Arm 0.02 18 research hypothesis is that a stocking program is successful if the mean CPTD value after stocking is significantly increased in comparison to the mean CPTD value before stocking.
The objectives of this study were to:
1. Determine the CPTD value of the proposed study area from the
data of the LMLRC lake-wide trapping survey. 2. Stock crayfish in the proposed study area and determine the CPTD for each introduction.
3. Execute a follow-up survey of the proposed study area after the
reproductive season following the last introduction and determine
the CPTD.
4. Evaluate the success of the stocking program in regard to the
research hypothesis and discuss future management plans for the
crayfish of Lake Mead. INTRODUCTION
GENERAL HISTORY
The red swamp crayfish, Procambarus clarkii. is one of the most widely
recognized species of crayfish in the world. It is a major commercial crop for human
consumption along the Gulf Coast and southeastern United States and is raised as
fish bait in the Midwest and parts of the West Coast. Procambarus clarkii have been
extensively studied by physiologists, aquaculturists, and ecologists.
Distribution
The natural range of P. clarkii has been documented over the last fifty years.
Penn (1943) described the range as principally the Gulf Coastal Plain from
Alabama to western Texas, extending north to Little Rock, Arkansas. More recent
descriptions of the natural range (Huner and Barr 1984, Hobbs 1988) encompass the
!' Mississippi River Valley up to the Great Lakes, reaching southeasterly into Florida,
and west into New Mexico (Figure 1).
P. clarkii has been introduced into areas outside its natural range (Figure 1).
In the United States it has been introduced along the Eastern Seaboard up to New
York and in the western states of Arizona, Nevada, California, Oregon, and as far
as Hawaii (Bonnot 1930, Huner 1977, 1978, Bouchard 1978, Sommer and Goldman
1983, Sommer 1984, Huner and Barr 1984, Hobbs 1988). It has been conjectured
that many of the introductions in the United States have been "accidental". As live
crayfish are a popular fish bait, fishermen have been the vector of many introductions. Many states have restricted the use of live crayfish as bait because 19
DESCRIPTION OF STUDY AREA
LAKE MEAD
Lake Mead (Figure 3, Table 3) was created by the completion of Hoover
Dam in 1935. Located on the lower Colorado River, it forms a border between
Mohave County, Arizona and Clark County, Nevada. Lake Mead is one of the largest man-made lakes in the United States with a volume of 36.0 km3 and a surface area of 660.0 km2 at maximum operating level of 374.0 meters. Its maximum length and width are 183.0 km and 28.0 km, respectively. It has a
maximum depth of 1.80 meters and a mean depth of 55 meters. Retention time is
3.7 years (Paulson and Baker 1984).
Lake Mead lies with highly dissected fault block ranges bordered by alluvial fans (Thornbury 1965). It has a rugged topography and a highly convoluted shoreline. The shoreline is comprised of numerous coves and points and measures
885 km (at maximum operating level). Limnological conditions have been
thoroughly documented (for example: Paulson et.al. 1980, Paulson and Baker 1984)
and in general, Lake Mead is classified as extremely oligotrophic with measured
orthophosphorus levels below 0.001 mg/L (Paulson and Baker 1984).
SADDLE ISLAND
As Lake Mead covers such a large area, a smaller stocking area was
designated. Saddle Island (Boulder Beach Quadrangle, Nevada-Arizona, T22S,R64E,
52,3,10,11) was chosen with the following justifications: 20
Table 3. Morphometric characteristecs of Lake Mead, AZ-NV at a maximum operating level of 374.0 meters.
Parameter Lake Mead
Volume 36.0 km3
Surface Area 660.0 km2
Maximum Length 183.0 km
Maximum Width 28.0 km
Maximum Depth 180.0 m
Mean Depth 55.0 m
Shoreline 885.0 km 1. The LMLRC lake-wide survey CPTD values were highest in this area
indicating that environmental conditions were favorable for a crayfish
population.
2. The area had a wide range of habitats available for crayfish, including mud,
gravel, rocks, submerged brush, aquatic plants, emergent plants, and
inundated terrestrial plants.
3. Distance from the source of crayfish stock allowed for successful
transportation of stock (low mortality rates).
4. The area was inaccessible to recreational swimmers and water skiers
(Unfortunately all areas of the lake are accessible by recreational boaters
and jet skiers, so equipment can be lost.)
5. Previous LMLRC studies of the area provided references for identification
of plants and limnological characteristics.
Saddle Island (Figure 4) is located in the Boulder Basin of Lake Mead in the region of the Outer Las Vegas Bay. This area of the lake is oligotrophic. The island runs north-south with a maximum length of approximately three kilometers and a maximum width of approximately 0.7 kilometers. Saddle Island is connected to the shore by a causeway. The shoreline of the west side of the island has a gradual slope of rock, with light vegetation and mud. The shoreline of the east side has a sharp drop-off, very rocky terrain, and sparse vegetation. Figure 4. Location of study cove on Saddle Island.
22 23
METHODS
DATA SURVEY FROM 1987 SURVEY
Mean CPTD Value
Unpublished raw data from the LMLRC lake-wide crayfish trapping survey was used to determine the mean CPTD value of the study area before stocking
occurred. The area was trapped from October 1986 through July 1987. A total of
two-hundred twenty-five traps were used over nine calendar days. A CPTD for each of the nine calendar days was determined by dividing the number of crayfish caught
that day by the total number of traps set that day. The mean CPTD for Saddle
Island for 1987 was calculated by taking the mean of the nine CPTD values. The
standard deviation and standard error were also calculated.
Sex Ratio
The ratio of adult females to adult males was determined for the trapping
period of October 1986 through July 1987 in the area of Saddle Island. This ratio
was used to monitor crayfish reproduction dynamics.
NDOW/LMLRC CRAYFISH INTRODUCTIONS
Collection
It was decided that crayfish were to be stocked in the study cove on a
quarterly basis during 1988. The four introductions of one-thousand six-hundred
crayfish each would aid in monitoring potential seasonal effects on introductions that
might not be seen in one major introduction. The NDOW expressed concern over 24 the origins of the stock of crayfish. NDOW requested that, if feasible, the crayfish should be collected from the Lake Mead drainage basin. Procambarns clarkii are common in the Las Vegas Wash system (Las Vegas, Clark County, Nevada) which drains into the Inner Las Vegas Bay (Figure 3). Various sites in the wash were trapped to find optimal collection sites. Crayfish were trapped using minnow traps
(41.9 cm long, 22.7 cm diameter) with the openings on each end enlarged to approximately six centimeters. The traps were baited with chicken or fish pieces and set for twenty-four hours. The collected crayfish were measured and sexed, then transported to a laboratory pond to be processed for stocking. Collection for each quarterly stocking took approximately one week.
Marking
The processing of the crayfish entailed marking them so they could be identified after stocking in the study cove. Markings had to: 1) be easy to apply; 2) not disturb behavior; and 3) withstand molting. Several methods including fluorescent dyeing, branding, and clipping and punching were tried prior to the initial stocking. Clipping and punching the uropods was found to be the most suitable method. As crayfish have four uropods, each stocking could be represented by a specifically clipped or punched uropod (Figure 5). Crayfish stocked in different introductions could be differentiated. ' The first intoduction was represented by clipping the most distal uropod on the left when looking on the ventral side. Each successive stocking was denoted by clipping or punching the next uropod toward the right. 111'"!1 lid!
4 3 2 1
Introduction Number
Lot 1 Lot 2 Lot 3
Figure 5. Markings on stocked crayfish. Uropods were clipped to represent a different introduction. The type of mark differentiated between crayfish lots.
25 26
The markings also denoted three different lots as a way to monitor dispersal
of crayfish after they were introduced into the study cove. Each lot would be
released in a different area of the study cove. The lot markings would show
movement of the crayfish in the cove by how far the crayfish was trapped from its
lot release area. The collected crayfish were separated into three separate lots with
each lot having the same number of adult females, Form I males, Form II males,
and juveniles of each sex. Lot 1 crayfish were marked with one hole in the
appropriate uropod using a leather punch. Lot 2 crayfish were punched twice, and •4 Lot 3 crayfish received a diagonal clip with scissors (Figure 5). I«• Release
HlHI Crayfish were transported to the study cove on Saddle Island (Figures 4, 6). inn"
IIIUI] "'"I To ensure low mortality rates, their metabolism was lowered by transporting the Illlj IIM II*. crayfish in ice water. The crayfish were released in three lots.
Each lot was released in a different habitat (Figure 6). Lot 1 crayfish were HI"5M1 released on a mud bank with some aquatic vegetation (Najas marina and
Potamogeton pectinatus). As water levels receded during the year (Figure 7), Lot
1 habitat was comprised of sand, gravel, and aquatic vegetation. Lot 2 crayfish
were introduced in a cattail (Typha latafolia) stand with mud substrate. In later
stockings the water levels dropped below the stand, and Lot 2 habitat was primarily
aquatic plants with mud. Lot 3 crayfish were released on a gravel and rock
embankment. Lot 3 habitat was not significantly altered with the falling water levels. Figure 6. Diagram of study cove. Each lot of crayfish was released in a different habitat or substrate in the cove.
I Illl
'I'll
27 oo cs 1216 i1210 •• "'I I ml m
1190 F M A M J J A 0 N D
Figure 7. Lake Mead water elevations (in feet) from January 1988 through December 1988. The August 1989 elevation is marked by the X.
29 30
Monitoring
Monitoring of the crayfish began two days after each stocking. Fifteen sets
of eighty meter trap lines were placed in the study area (Figure 8). Eight standard
minnow traps (41.9 cm long, 22.7 cm diameter) With enlarged openings were
attached to each line with a spacing of ten meters between each trap. The traps were baited with chicken.
The traps were checked twenty-four hours after being set. Crayfish captured were checked for their introduction and lot marking. They were also classified in
regard to age (adult or juvenile) and sex. Forms of adult males were also noted.
The crayfish were released back into the general area of their capture. The traplines
were reset and retrieved for four days and then once a week until the number of
crayfish caught was ten percent or less of the initial trap day catch.
Mean CPTD Values
CPTD values were determined for each trap day for each of the quarterly
stockings. The mean CPTD value of each introduction was calculated by averaging
the CPTD values within the introduction. The standard deviation and standard error
were also calculated for each of the four mean CPTD values.
Sex Ratio
The sex ratio of mature females to mature males was determined for each
quarterly stocking or introduction. The ratios were used to monitor reproduction
dynamics. Figure 8. Diagram of trap lines set in study cove.
31 to 33 FALL TRAPPING 1989
Monitoring
After the 1989 peak reproductive season was completed, a trap survey of the area around Saddle Island was done to determine the CPTD value after slocking of crayfish. The original trapping pattern could not be employed because water levels were too low (Figure 7). Trap lines were set in two patterns, parallel to the shore and perpendicular to the shore. Trap lines were eighty meters in length with eight minnow traps, each trap attached every ten meters. Traps were baited with chicken.
The depths of the set traps were determined using echo sounding.
The traps lines were retrieved after twenty-four hours. The captured crayfish were checked for markings and identified with regard to age (juvenile or adult) and sex. Crayfish were released in the general area of capture. Trapping was continued until the entire area around Saddle Island was surveyed.
Mean CPTD Value
For each trap day of the Fall 1989 survey, a CPTD value was calculated. The mean CPTD value, standard deviation, standard error were calculated.
Sex Ratio
The sex ratio of adult females to adult males was determined for the Fall
1989 survey. This ratio was used to monitor sexual-state relatioships of the crayfish population. 34 STATISTICAL TESTING
Comparison of Mean CPTD Values
The six mean CPTD values (one pre-stocking value, four stocking values, and one post-stocking value) were compared using a One-Way Analysis of Variance
(ANOVA). If the ANOVA showed that the difference in the mean CPTD values was significant with a p_ -value less than 0,05, further comparison of the means was completed using the Least Significant Difference (LSD) Mean test.
Comparison of Sex Ratios
A chi-square test (alpha level = 0.05) was used to see if the distribution of the sexes was significantly changed during the six different trapping periods. A chi- square test (alpha level = 0.05) was also used to compare the pre-stocking (1987) sex ratio with the post-stocking (1989) sex ratio. The latter test would determine if stocking affected the sex ratio of the Saddle Island crayfish population. 35
RESULTS
DATA SURVEY FORM 1987 SURVEY i^' Mean CPTD Value ,1-o4. Data from the LMLRC 1987 lake-wide crayfish study yielded nine CPTD
values (Table 4). The mean CPTD value was 0.107.
Sex Ratio
In the 1987 survey, a total of twenty-one crayfish were caught. Ten of these
crayfish were adult females and eleven were adult males (10 AF: 11 AM). No ftV. juveniles were caught in this trapping period.
f**4 "*$
NDOW/LMLRC CRAYFISH INTRODUCTIONS
Introduction 1 (January 1988)
The total number of crayfish trapped in the first introduction was one-
hundred fifty-six. Ten crayfish were unmarked (Table 5) which signifies that they
I were established in the study area prior to the stocking. After the late January
introduction, trapping lasted until ten percent of the initial catch was reached. This
was accomplished in late February, encompassing six trap days.
Mean CPTD Value
A CPTD value for each trap day was determined (Table 6). The mean of the
six CPTD values yielded a mean CPTD value of 0.217.
Sex Ratio
Of the one-hundred fifty-six crayfish collected in Introduction 1, one-hundred
forty-nine were mature (Table 5). The majority were females, giving a sex ratio of 36
Table 4. Trap data from LMLRC 1987 trap survey of Saddle Island. Mean CPTD value, standard deviation, and standard error calculated for this trapping period.
Number of Number of Trap Day Crayfish Traps CPTD (mo/dy/yr) Captured Set *.'
12/9-10/86 1 10 0.100
12/10-11/86 0 10 0.000
12/17-18/86 7 16 0.438
1/8-8/87 -i 32 0.094
2/18-19/87 3 48 0.063
3/19-20/87 3 48 0.063
4/13-14/87 0 21 0.000
5/6-7/87 2 16 0.125
7/27-28/87 2 24 0.083
Mean CPTD = 0.107; Standard Deviation = 0.36; Standard Error - 0.12. 37
Table 5. Summary of crayfish captured for each of the four introductions.
CRAYFISH UNMARK INTRO 1 INTRO2 INTRO3 INTRO4 INTRO 1 AF 3 87 1 /27/88- AM 3 56 2/25/88 JF 7 2 JM 2 1
10 146
INTRO 2 AF 6 17 254 5/2/88- AM 8 23 357 5/24/88 JF 4 1 4 JM 1 1 1
19 42 616
INTRO 3 AF 1 0 8 160 7/15/88- AM 3 0 3 170 8/25/88 JF 9 0 0 19 JM 0 0 14 5 18 0 11 363
INTRO 4 AF 4 0 0 2 294 11/16/88- AM 6 0 0 0 12/13/88 JF 9 0 0 0 3 JM 1 0 0 0 2
13 0 0 2 571
AF = Adult Female; AM = Adult Male; JF = Juvenile Female; JM = Juvenile Male. 38
Table 6. Trap data from Introduction 1 (January - February 1988). Mean CPTD value, standard deviation, and standard error calculated for "this trapping'period.
Number of Number of Trap Day Crayfish Traps CPTD (mo/dy/yr) Captured Set
1/27-28/88 54 120 0.450
1/28-29/88 34 120 0.283
2/1-2/88 39 120 0.325
2/2-3/88 14 120 0.117
2/1:1-12/88 10 120 0.083
2/24-25/88 5 120 0.042
Mean CPTD = 0.21.7; Standard Deviation = 0.40: Standard Error = 0 16 39 ninety adult females to fifty-nine adult males (90 AF: 59 AM).
Introduction 2 (April 1988)
Recapture of stocked crayfish was high with a total catch of six-hundred seventy-seven. Nineteen had no markings (Table 5). These unmarked crayfish were either native to the study area or early-release offspring of Introduction 1 crayfish.
Forty-two of the crayfish had markings identifying them as first introduction crayfish.
Mean CPTD Value
Six trap days were necessary to have a recapture of ten percent or less of the first day catch. The mean CPTD value for Introduction 2 was 0.940 (Table 7).
Sex Ratio
More adult males were trapped in the second introduction. Of the six- hundred fifty-five total adult crayfish, three-hundred eighty-eight were male. The sex ratio was two-hundred seventy-seven were adult females to three-hundred eighty- eight adult males (277 AF: 388 AM).
Introduction 3 (July 1988)
The trapping total for the third introduction was three-hundred ninety-two crayfish (Table 5). No crayfish displayed Introduction 1 markings, but eleven had
Introduction 2 markings. Eighteen crayfish were unmarked.
Mean CPTD Value
The number of trap days increased to eight as capture rates remained slightly over ten percent (Table 8). The mean of the eight trap day CPTD values was 0.408. 40
Table 7. Trap data from Introduction 2 (April - May 1988). Mean CPTD value, standard deviation, and standard error calculated for this trapping period.
Number of Number of Trap Day Crayfish Traps CPTD (mo/dy/yr) Captured Set
5/1-2/88 183 120 1.525
5/2-3/88 143 120 1.192
5/3-4/88 99 120 0.825
5/4-6/88 105 120 0.875
5/11-12/88 134 120 1.117
5/25-26/88 13 120 0.108
Mean CPTD = 0.940; Standard Deviation = 0.35; Standard Error = 0.14. 41
Table 8. Trap data from Introduction 3 (July - August 1988). Mean CPTD value, standard deviation, and standard error calculated for this trapping period.
Number of Number of Trap Day Crayfish Traps CPTD (mo/dy/yr) Captured Set
7/15-16/88 80 120 0.667 •
7/16-17/88 86 120 0.717
7/17-18/88 75 120 0.625 4J 7/18-19/88 61 120 0.508 I 7/19-20/88 68 120 0.567 7/27-28/88 9 120 0.075
1 8/10-11/88 9 120 0.075
8/24-25/88 4 120 0.033 1m: .1 Mean CPTD = 0.408; Standard Deviation = 0.48; Standard Error = 0.17. 42 Sex Ratio The number of adult males was slightly higher than the number of adult females captured after the third introduction. With one-hundred sixty-nine females out of three-hundred forty-five mature crayfish, the sex ratio was one-hundred sixty- nine adult females to one-hundred seventy-six adult males (169 AF: 176 AM).
Introduction 4.(November 1988)
In the fourth introduction, trapping yielded five-hundred eighty-six crayfish.
Two specimens had been introduced in the previous stocking, and thirteen were unmarked. There were no crayfish trapped from the January or April introductions
(Table 5). Mean CPTD
The five-hundred eighty-six crayfish were captured in five trap clays. The mean CPTD value was 0.977 for the final introduction (Table 9).
Sex Ratio
After the last introduction, five-hundred seventy-eight of the crayfish were mature. Of this total, two-hundred seventy-eight crayfish were adult males and three-hundred were adult females (300 AP: 278 AM) (Table 5).
FALL TRAPPING 1989
Mean CPTD Value
Five trap clays were necessary to trap the area around Saddle Island. A total of seventy-three crayfish were collected. The mean CPTD value for the Fall
Trapping 1989 was 0.117 (Table 10). Table 9. Trap data from Introduction 4 (November - December 1988). Mean CPTD value, standard deviation, and standard error calculated for this trapping period.
Number of Number of Trap Day Crayfish Traps CPTD (mo/dy/yr) Captured Set
11/16-17/88 250 120 2.083
11/17-19/88 191 120 1.592
11/19-20/88 104 120 0.867
11/28-29/88 27 120 0.225
12/12-13/88 14 120 0.117
Mean CPTD = 0.977; Standard Deviation = 0.77; Standard Error = 0.34. 44
Table 10. Trap data from Fall Trapping 1989 of Saddle Island. Mean CPTD value, standard deviation, and standard error calculated for this trapping period.
Number of Number of Trap Day Crayfish Traps CPTD (mo/dy/yr) Captured Set
8/6-7/89 4 128 0.031
8/7-8/89 9 128 0.016 8/8-9/89 3 128 0.023
8/9-10/89 37 128 0.289
9/27-28/89 27 120 0.225
Mean CPTD - 0.117; Standard Deviation = 0.36; Standard Error = 0.16. 45 f Vw* i, Sex Ratio V •<1 Out of a total of seventy-three crayfish captured in the last trapping period, forty-two were adult females and thirty-one were adult males (42 AF: 31 AM).
COMPARISON OF THE MEAN CPTD VALUES
The six mean CPTD values were compared using a One-Way Analysis of
Variance (ANOVA). The means were found to be significantly different (Figure 9).
The F-value was 6.277 with of g-value of 0.5326 x 10 E-03. As this p-value is much
below 0.05, a Least Significant Difference by Student's T (LSD) test was completed
to compare individual CPTD means to each other. The LSD test (alpha level =
0.05) revealed that the CPTD mean values of the second and fourth introductions
were significantly diffferent from the other mean CPTD values, however they were
not significantly different from each other. The mean CPTD values from the 1987
survey, the first and third introductions, and the 1989 survey were not significantly
different.
COMPARISON OF SEX RATIOS
The sex ratios of adult females to adult males over the six trapping periods
were compared using a chi-square test. The chi-square value was 26.002, which was
much greater than the critical value of 11.1 (alpha level = 0.05) (Table 11). The
distribution of adult females to adult males changed significantly over the six
trapping periods. CATCH PER TRAP DAY (CPTD) Means for each Trapping Period
1500 * P< 0.05 One Way ANOVA 1200 and LSD Mean Test 900 •- o«— x 600- O
300-
1987 INTR01 INTR02 INTR03 INTR04 1989
Trapping Period Figure 9. Mean CPTD values compared using a One-Wav ANOVA anH test. Actual CPTO values were muSplied by fooo for * represents significance with g-value < 0.05.
46 47
Table 11. Summary of adult sex distributions for the six trapping periods. Chi-square test results for comparison of all distributions and chi-square test results for comparison of pre-stocking and post-stocking distributions.
Trapping Number of Number of Period Adult IVlales Adult Females
1987 11 10
. Introduction 1 59 90
Introduction 2 388 277
Introduction 3 176 169
Introduction 4 278 300
Fall 1989 31 42
Chi-square test of six distributions: Chi-square value = 26.002; Critical value = 11.1; Degrees of freedom = 5. Significant.
Chi-square test of pre-stocking and post-stocking distributions: Chi-square value = 0.635; Critical value - 3.84; Degrees of freedom = 1. Not signficant. 48 Another chi-square test (alpha -level = 0.05) was used to compare the distributions of the sexes before and after the introductions. The chi-square value was 0.635, much less than the critical value of 3.84. The pre-stocking and post- stocking sex ratios of adult females to adult males did not differ significantly (Table
11). 49
DISCUSSION
MEAN CPTD VALUES
The mean CPTD values for 1987 and 1989 did not differ significantly (Figure
9) indicating that the crayfish stockings were not successful in increasing the crayfish population around Saddle Island.
The introductions only increased the populations on a short-term basis. The second and fourth introductions had significantly greater mean CPTD values. The mean water column temperatures at these stocking periods were at or near optimal temperatures (19 - 22 ° C). At these temperatures crayfish would be more active and more likely to be trapped. In colder temperatures, as in the first stocking in
January (mean water column temperature = 12.12 ° C), crayfish would have slower metabolism rates and be less active. The warmer water temperatures (27.98 ° C) of the third introduction in July may have caused crayfish to migrate to cooler, more- oxygenated waters (Konikoff 1977).
- In successive introductions, lake levels dropped (Figure 7), resulting in loss of cover and food. The crayfish may have migrated to more favorable habitat.
Some of the stocked crayfish were trapped over two-hundred meters away from the area of introduction only thirty-six hours after the stocking. Lowery and Mendes
(1977) traced the colonization of Procambarus clarkii in Lake Naivasha, Kenya by movement of crayfish along the littoral zone. During a two year period, crayfish extended their shoreline distribution twenty to twenty-five kilometers. None of the crayfish trapped in 1989 had markings from the stockings, but it is possible that crayfish could migrate around the island. 50
During the trapping survey in the fall of 1989, a majority of the crayfish were trapped on the east side of Saddle Island (stockings were on the west side) at depths of up to 29 meters. Most of the crayfish literature report crayfish in shallow waters
(less than ten meters). However, Abrahamsson and Goldman (1970) found
Pacifastacus leniusculus as deep as two-hundred meters, but report the majority of the population was located at depths between ten and twenty meters. Pacifastacus leniusculus is a cold water crayfish, Procambarns clarkii is a warm water crayfish.
That P. clarkii was found at depths of 29 meters is remarkable.
As P. clarkii were reported to inhabit shallow water, traps were never set very deep in previous studies of Lake Mead (LMLRC unpublished data 1987). Therefore it is not certain that Procambarus clarkii were located in depths up to twenty-nine meters prior to the stockings. Food habit studies of striped bass in Lake Mead support that the crayfish have lived at these depths. NDOW (1983) and Wilde and
Paulson (1989) reported that crayfish comprised a major component of Lake Mead striped bass (Morone saxatilis) diets from April through June. During this season,
Lake Mead is stratifying. As epilimnetic temperatures and levels of dissolved oxygen become intolerable, the striped bass are restricted to depths of fifteen to twenty- five meters (Paulson and Baker 1987). The crayfish must have been this deep in order for these predators to consume them. As the peak predation on crayfish by striped bass coincides with a peak crayfish reproduction season, crayfish population growth would be affected by predation. This indicates that predalion is a limiting factor to crayfish populations at this depth. 51 The food sources in Lake Mead at these depths (twelve 10 twenty-nine meters) are primarily periphyton (Phormidium sp_. and Cladophora sp_.). The lower temperatures and paucity of food would result in lower growth and reproduction rates (Momot 1984).
P. clarkii inhabiting shallow waters may be limited by warm, anoxic conditions and lack of food and cover. P. clarkii inhabiting deeper waters may be limited by predation, low nutrients, and reduced growth and reproduction rates due to low temperatures. Low productivity is the common factor of both shallow and deep waters around Saddle Island.
Most introductions of P. clarkii have been extremely successful (Huner 1977, Lowery and Mendes 1977), but the introductions to augment the P. clarkii population at Saddle Island was not. The environment of this area evidently will not support a large Procambariis clarkii population.
SEX RATIOS
Chi-square testing of the distributions of adult males to adult females revealed significant differences over the six trapping periods (Table 11). Average sex ratios are approximately one adult female to one adult male (1 AF: 1 AM) in native P. clarkii populations (Pena 1943, Sheppard 1974, Rhodes and Avault 1987).
The 1987 sex ratio encompassed approximately a year and was close to a one to one ratio. The other trapping periods were shorter and reflected seasonal changes in the sexual state of the crayfish population. 52 In Introduction 1 more adult females than adult males were trapped. At this
time of year (January), water temperatures limit activity of crayfish (Huner and Burr
1984). Adult males are likely to be Form II males, which are less aggressive and less active than Form I males. Adult females are iess likely to be in berry and are more active than when ovigerous (Stein 1977). Females are in a more active life stage, males are not. Thus, females are more likely to be searching for food and be trapped.
More adult males than adult females were trapped in the second introduction
(April). In the beginning of the spring reproductive peaks, a majority of males would be Form I and be actively hunting and mating. Females that are in. berry would be less active and more likely to seek shelter (Stein 1977). Thus spring reproduction would result in the number of adult males trapped to exceed the number of adult females trapped.
In Introduction 3 (July), the sex ratio was nearly one to one. Many females would no longer be in berry and would be active. With both sexes in active life
stages, a one to one sex ratio is to be expected.
By late fall (Introduction 4), the mature females trapped out-numbered the
mature males trapped. Penn (1943) reported a "die off of males in the fall due to
reproductive exhaustion. This may account for the fall ratio.
In the 1989 trapping survey, more adult females were captured than adult
males. As in the final stocking, this trapping period was in the fall. The adult male
population may have been decreased due to reproductive exhaustion. 53 A chi-square test was also used to compare the pre-stocking and post-stocking distributions of adult females to adult males. There was no significant difference in the distributions. The overall sex ratio of mature males to mature females was not significantly altered by the stockings. The sex ratios throughout the study indicate that Lake Mead crayfish reproduction is similar to reproductive cycles in native P. clarkii habitats.
SUITABILITY OF CRAYFISH SPECIES IN LAKE MEAD
Other Procambarus clarkii introductions have resulted in significant increases in populations (Huner 1977, Lowery and Mendes 1977). Introductions of P. clarkii did not significantly increase the population in this study. The crayfish population in Lake Mead is very low; nevertheless, it is surviving. Several factors may be limiting the Lake Mead P. clarkii population. Some biologists believe other species may be more suited to Lake Mead.
I surveyed crayfish distributional literature to create a model of crayfish genera in relation to lake habitats (see Appendix). This model indicated that
Procambarus is not a suitable crayfish genus for Lake Mead.
The model show that Orconectes sp_. are distributed in widely varied lake habitats. Distributional maps (Hobbs 1988) show that Orconectes have the largest
North American distribution in comparison to other crayfish genera. Their distribution extends north to east-central Alberta, Canada, south to the Gulf of
Mexico, west to western Montana, and east to the Atlantic Coast. Orconectes has been introduced in Oregon and California (Momot 1988). Orconectes is a 54 euryhabitat genera and might be successful in Lake Mead.
Orconectes virilis and Orconectes rusticus are commonly introduced species
(Momot 1988). O. rusticus are extremely aggressive and successfully compete with
Q. virilis (Capelli 1982). Unfortunately O. rusticus is so aggressive it is destroying the food chain in lakes in Wisconsin (Lodge et.al. 1985). It eliminates macrophytes and consumes fish eggs. Lodge et.al. (1985) observed that this species even attacked game fish predators! Introduction of Q. rusticus into Lake Mead might be deleterious to the declining game fishery.
Q. virilis is much less aggressive than Q. rusticus. Momot (1984) reports that
Q. virilis produce larger yields and bio mass in low-nutrient lakes than in high- nutrient lakes of similar size. This species lives approximately three years and has a slower growth rate and fecundity than P. clarkii. Llowever, the smaller crayfish may provide better forage for subadult game fish as they would be easier to swallow.
The combination of increased biomass in low-nutrient waters and a slower, longer growth rate may enable Q. virilis to be the best-suited crayfish species for Lake
Mead.
ENVIRONMENTAL IMPACTS OF INTRODUCED SPECIES
Introductions of exotic crayfish must be considered carefully. Crayfish can be vectors for viruses or parasites which may eliminate other species. American crayfish are immune to the crayfish plague, but European crayfish are not. When
Pacifastacus leniusculus was brought into Sweden, the European species, Astacus astacus was partially eliminated (Unestam 1975). Unestam (1975) also proposed 55 that crayfish may be mechanical vectors of viruses harmful to game fish.
Destruction of habitat is also a major concern when introducing crayfish.
Crayfish may disturb fish nests, spawning areas, or drastically reduce cover for other species (Huner 1977, Lodge et.al. 1985, Unestam 1985). Crayfish can also damage structures such as levees and earthen dams (Penn 1956, Grigarick and Way 1982,
Sommer and Goldman 1983, Sommer 1984).
Lake Mead game fish could be susceptible to diseases carried by exotic crayfish. Destruction of habitat would also affect the fishery. However, the fishery has survived the introduction of Procambarus clarkii and may benefit with an introduced better-suited forage species.
Another concern would be the possible spread of an exotic crayfish species into river drainages such as the Virgin River. The Virgin River drains into the
Upper Overton Arm of Lake Mead. The Virgin River is the remaining natural habitat for rare and endangered fish such as the woundfin (Plagopterus argentissimus), the Virgin roundtail chub (Gila robusta seminuda). and the Virgin spinedace (Lepidomeda mollispinis) (Deacon 1988). Any disease or destruction of habitat in this river system may eliminate these species.
Before introducing a new species such as Orconectes virilis into Lake Mead, more research must be done to determine the impacts of such an introduction.
Enclosure experiments with environmental conditions mirroring Lake Mead conditions should be used to determine how well the new species can adapt to its new environment. 56
Pathological investigations should be completed to control or eradicate any diseases or parasites that might be transmitted by the crayfish. Food habit studies should examine the impact on Lake Mead macrophytes, detritus, and benthic organisms. These food habit studies should also investiagate how the exotic species affects the food chain and how it competes with other grazers.
Current or irrigation clams may prevent the crayfish from migrating up the drainage system. Laboratory research involving current velocity would aid in determining the probability of upstream migration in the Virgin River System. An assessment of any impacts should be made before introducing a new species and the possible consequences must be weighed with care.
CONCLUSIONS
The following are the conclusions of this study:
1. Introductions of Procambarus clarkii did not increase the population.
The crayfish population growth is limited by environmental conditions and
further stockings of this species are not recommended.
2. A panel of management agency officials, fishery biologists, and concerned
ecologists should determine if a large crayfish population is desirable for
the game fishery of Lake Mead and if the benefits of introducing an exotic
crayfish population outweigh any possible ecological effects on the river
system.
3. If the panel decides to stock a new crayfish species, Orconectes virilis
appears to be the species best-suited for Lake Mead. 57
REFERENCES
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APPENDIX
Procambarus clarkii are not an endemic species in Lake Mead. These
crayfish have a wide tolerance of habitat conditions. The environmental conditions
of Lake Mead support a P. clarkii population, but not a thriving population. Other
species of crayfish require or tolerate different habitats. It is possible that a species
of crayfish other than P. clarkii is more suited to the Lake Mead habitat. An
ecological model of chemical and physical conditions of lakes could aid in predicting what crayfish would be suitable for a specific lake based on those conditions.
The literature was surveyed to find specific citations of species of crayfish in
specific or named lakes. (A list of references reviewed is at the end of the appendix.)
Phone surveys and letter surveys were given to agencies that would have limnological
data on these lakes. (A list of responding individuals at those agencies is included
with the references at the end of the appendix.) As many of the lakes had not been
studied, not all of the information could be obtained. Data was returned on a total
of seventy-seven lakes. There is no national standard of measuring lake data or even
what parameters are to be examined. Because of this inconsistency, many data
points were missing.
Many physical or chemical factors could not be examined due to missing data
points. Trophic status indices (TSI), were determined on available secchi depth,
chlorophyll-a, and total phosphorus levels (Carlson 1977). A mean TSI was then
calculated for each lake. To analyze the maximum number of lakes, the factors used
were latitude, elevation, maximum depth, mean depth, and mean TSI. A principal
component analysis (PCA) was used to analyze these factors in sixty-six lakes. 63 Principal component one (PCI) and principal component two (PC2) comprised 78.5 percent of the variance. High positive values of PCI represented southern, high elevation, deep, less productive lakes. Negative values of PCI represented northern, low elevation, shallow, productive lakes. High positive values of PC2 represented southern, high elevation, shallow, productive lakes; while negative values tended toward northern, low elevation, deep, less productive lakes.
A bivariate plot of PCI and PC2 has four quadrants (Figure 10). In quadrant one (Ql), as points become more distant from the origin, lakes tend to be southern, high in elevation, deep, and productive. In quadrant two (Q2), lakes are further north in latitude, high in elevation, deep, and less productive. Quadrant three (Q3) lakes are northern, low elevation, shallow and less productive. Quadrant four (Q4) represents southern, low elevation, shallow, productive lakes.
In the bivariate plot of PCI and PC2, most of the lakes with Procambarus srx are found in the fourth quadrant (Figure 11). The points are near the origin which reflects that this genus of crayfish are located in shallow, low elevation lakes, but are not limited to extremely productive southern areas. Lake Mead lies in quandrant one. Its location near the x-axis reflects that it is not as productive or south in latitude as other points in this quadrant. Its position in regard to lakes with
Procambarus sp_. is distant. Lakes with Orconectes. Pacifastacus. or Cambarus species are much closer in position.
One-Way ANOVA tests were used to test differences in PCI and PC2 in the different crayfish genera. PCI was significantly different (F = 28.5, p_ < 0.0005), however PC2 was not significantly different between genera (F = 1.33, p_ > 0.25). Q4 Ql Low Elevation High Elevation Shallow Deep Southern Northern High Production High Production
« PCI
Q3 Q2 Low Elevation High Elevation Shallow Deep Northern Northern Low Production Low Production
PC2
Figure 10. Quadrant descriptions as denoted by PCI and PC2 values.
64 A • Procambarus 3.500- A D Cambarus * Orconectes 2.500- • v Pacifastacus • <• Lake Mead A 1.500- A
°A HA T 0.500- ' 5 o 0.500- ^LAO"* .i snn. U 1 1 -2.000 0.000 2.000 4.000 6.000 PC 1
Figure 11. Bivariate plot of Principal Component 1 and Principal Component 2. This is a model of crayfish general distribution in regard to environmental conditions of lakes.
65 1
66 The significance of PCI indicates that this model can aid .in predicting suitability of
crayfish genera in lake environments. A greater number of data points would
improve the model, but points were not available at this time. According to this
model, Procambarus srj. populations would not be suitable for Lake Mead.
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