Factors Affecting Reproductive Success of Bonytail Chubs and Razorback Suckers in Lake Mohave
Publications (WR) Water Resources
1-1984
Factors affecting reproductive success of bonytail chubs and razorback suckers in Lake Mohave
Michael A. Bozek University of Nevada, Las Vegas
Larry J. Paulson University of Nevada, Las Vegas
James E. Deacon University of Nevada, Las Vegas
U.S. Department of Interior, Fish and Wildlife Service
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Repository Citation Bozek, M. A., Paulson, L. J., Deacon, J. E., U.S. Department of Interior, Fish and Wildlife Service (1984). Factors affecting reproductive success of bonytail chubs and razorback suckers in Lake Mohave. Available at: https://digitalscholarship.unlv.edu/water_pubs/71
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This Technical Report has been accepted for inclusion in Publications (WR) by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. Factors Affecting Reproductive Success of Bonytail Chubs and Razorback Suckers in Lake Mohave
Michael A. Bozek, Larry J. Paulson and James E. Deacon
Technical Report No. 12
Lake Mead Limnological Research Center Department of Biological Sciences University of Nevada, Las Vegas
Final Report to U. S. Department of Interior, Fish and Wildlife Service Contract No. U-16-0002-81-251
January 1984 TABLE OF CONTENTS
LIST OF FIGURES i i i
LIST OF TABLES v
ACKNOWLEDGEMENTS vi
1.0 INTRODUCTION 1
1.1 Status of the Razorback Suckers and Bonytail Chubs in the Colorado River 1
1 .2 History of the Lake Mohave Area 3
2.0 DESCRIPTION OF THE STUDY AREA 5
3.0 METHODS 8
3.1 Cove Surveys 8
3.2 Sampling Locations 8
3.3 Trammel Netting 10
3.4 Measurements 10
3.5 Larval Collections 11
3.6 Temperature Development Analyses 12
3.7 Physical Hydrodynamics 13
3.8 Statistical Analyses 13
4.0 RESULTS 14
4.1 Cove Surveys 14
4.2 Catch Rates 16
4.3 Length and Weight Distributions, Condition Factors, Sex Ratios, and Disease 23
4.4 Spawning 43
4.41 Spawning Habitat 43
4.42 Spawning Duration and Relationship to Physical Factors 45 TABLE OF CONTENTS (continued)
Page
4.43 Spawning Behavior 48 k.44 Spawning Success 50
4.45 Larval Collection and Development 52 4.46 Factors Affecting Spawning Success 55
4.5 Population Size and Status 63
5.0 DISCUSSION 65
6.0 LITERATURE CITED 78
7.0 APPENDICES 86
7.1 Lake Mohave Individual Species Data 87 7.2 Lake Mohave Cove Survey 116
7-3 Lake Mohave Trammel Netting Catch Data 125
7.4 Razorback Sucker Redd Size, Depth, and Particle Size Distributions 130 7.5 Dexter National Fish Hatchery Two Year Old Razorback Sucker Lengths and Weights 133
i i LIST OF FIGURES Figure No. Page
1 Map of trammel netting locations in .Lake Mohave during 1982-1983 ...... 9
2 Map of Lake Mohave razorback sucker spawning areas ...... 15 3 Lake Mohave species catch rates in the Arizona Bay area during 1982-1983 ...... 17 4 Lake Mohave species catch rates .in the Six Mile Coves during 1982 ...... 19 5 Lake Mohave species catch rates in the Cottonwood Basin during 1983 ...... 21 6 Length frequency histogram of razorback suckers collected in 1982 and 1983 . '...... 27 . 7 Monthly total lengths of razorback sucker males collected during 1982-1983 ...... 28 8 Monthly total lengths of razorback sucker females collected during 1982-1983 ...... 29 9 Monthly weights of razorback sucker males collected during 1982-1983 ...... 31 10 Monthly weights of razorback sucker females collected during 1982-1983 ...... 32 11 Mean monthly condition factors of male and female razorback suckers collected during 1982-1983 ...... 33- 12 Morphological variation in some characters of razorback suckers from Lake Mohave, Senator Wash Reservoir, and the upper Colorado River ...... 34
13 Seasonal sex ratios of razorback suckers in Lake Mohave spawning areas 1982-1983 ...... 38
14 Monthly incidence of gross bacterial infections on razorback suckers in Lake Mohave, 1982-1983 ...... 41 15 Monthly incidence of Lernaea cyprinacea on razorback suckers during 1982-1983 in Lake Mohave ...... 42
16 Length of the Lake Mohave razorback sucker spawning season during 1982 and 1983 ...... 46
i i i LIST OF FIGURES (continued)
Figure No. Page
17 Monthly ripeness of razorback suckers collected from Lake Mohave ...... kj
18 Mean weekly water level elevations in Lake Mohave during the 1982-1983 spawning seasons ...... ^9 19 Estimated percent hatch decimation of razorback sucker eggs due to water level fluctuations, Dec. 1981 - Feb. 1982 ...... 56
20 Estimated percent hatch decimation of razorback sucker eggs due to water level fluctuations, March - May 1982 ...... 57 21 Estimated percent hatch decimation of razorback sucker eggs due to water level fluctations, Dec. 1982 - Feb. 1983 ...... 58 22 Estimated percent hatch decimation of razorback sucker eggs due to water level fluctuations, March - May 1983 ...... 59
IV LIST OF TABLES Table No. Paqe
1 Recalculated catch rates excluding non-razorback sucker spawning areas ...... 22
2 Razorback sucker. length weight summary for netted individuals from Lake Mohave 1982-1983 ...... 2k 3 Bonytail chub length-weight summary for individuals from Lake Mohave, 1982-1983 ...... 26 3a Dorsal fin ray counts of razorback suckers from Lake Mohave and other populations ...... 36
4 Incidence of eye afflictions in Lake Mohave razorback suckers during 1982-1983 ...... 39 5 Substrate particle size composition of the Yuma Cove spawning area, 1982 ...... ^
6 Razorback sucker hatching success and larval development at ambient spawning season temperatures in Lake Mohave. . . 51
7 1983 Razorback sucker larval collections ...... 53 8 Percentages of razorback sucker spawning effort related to depth ...... 60 9 Predation summary on razorback sucker egg and larval stages ...... 62 10 Razorback sucker recaptures from trammel netting efforts in Lake Mohave 1982-1983 ...... 64 ACKNOWLEDGEMENTS
We are extremely grateful to Gene Wilde for information on morpholo- gical variation in razorback suckers that is presented in the text and for his assistance and insight on many ideas that aided in the completion of both the study and this report. Gordon Mueller was most helpful in pro- viding details and information on redd distributions and particle size composition of razorback sucker spawning areas in Lake Mohave. Special thanks goes to Mickey Reese, Pat Sollberger, John Plaskett, Trudi Frautschi and many others who helped in the collection of data in the field. We would also like to thank Dorothy Adashefski and Laurie Vincent who helped type much of this report.
VI 1.0 INTRODUCTION
1.1 Status of Razorback Suckers and BonytaiI Chubs In the Co Iorado River
Razorback suckers (Xyrauchen texanus) and bonytaiI chubs (Gila elegans) were once widespread throughout the Colorado River system. The ranges and populations of these native species and others have declined in the past 50 years (Miller 1961; Minckley and Deacon 1968; Johnson and Rinne 1982; Minckley 1983). BonytaiI chubs appear to be extremely rare and possibly nearing extinction in the Upper Colorado River Basin. The razorback sucker is widely distributed in the upper river but is considered rare throughout most of that range (Vanicek et al. 1970; Holden and Stalnaker 1975 a,b; Seethaler 1979; McAda and Wydoski 1980; Lanigan and Berry 1981; Valdez 1982; Tyus 1982) and is also rare in the Grand Canyon (Suttkus and Clemmer 1979). Razorback suckers are abundant only in a few habitats in the upper river. They congregate and spawn over a cobble bar in an eddy at the mouth of the Yampa River (Holden and Stalnaker 1975; McAda and Wydoski 1980). Tyus (1982) collected a large number of razorback suckers in a similar habitat downstream from that location at the mouth of Ashley Creek. The Walter Walker Wildlife Refuge and Clifton Pond are also sites where razorback suckers are known to congregate for spawning (McAda and Wydoski 1980; Seethaler 1979; Valdez 1982). Recent specimens collected from the upper river all have been large adults.
BonytaiI chubs apparently were abundant throughout the lower Colorado River early in this century. Locals believed bonytaiI chubs once were the most abundant fish in the river reach from Needles to Yuma. BonytaiI chubs and razorback suckers were also common in Lake Mead during early impoundment (Moffett 1943; Wai I is 1951) and below Hoover Dam (Moffett 1942; Dill 1944). BonytalI chubs have not been reported from Lake Mead since 1967, but razorback suckers are frequently sighted in that reservoir (Allan and Roden 1978). Bonytail chubs are now thought to be extirpated throughout most of the lower river. Small populations of razorback suckers still occur in Lake Havasu and Senator Wash Reservoir and scattered individuals inhabit the river and irrigation canals below Parker Dam (Minckley 1973, 1983; Ulmer 1981). Large populations of bonytaiI chubs and razorback suckers were observed spawning over gravel shelf areas of Lake Mohave during the early 1950's (Jonez and Sumner 1954). Bonytail chubs are now rare in Lake Mohave, but a small population still persists in the reservoir. Razorback suckers are presently considered abundant in Lake Mohave (Minckley 1983).
Reservoir spawning activity of razorback suckers has been observed in Lake Mead (Wall is 1951, Jonez and Sumner 1954), Lake Havasu (Douglas 1952), Senator Wash Reservoir (Ulmer 1981), and Lake Mohave (Jonez and Sumner 1954). Jonez and Sumner (1954) observed what they believed to be larval razorbacks near spawning areas south of Eldorado Canyon, and Ulmer (pers. comm.) collected larvae near Arizona Bay in 1981. One larval razorback sucker was collected below Hoover Dam in an invertebrate sampler during 1979 (Paulson et al. 1980a). Several razorback eggs and larvae were collected in drift samples in the same area in 1984 (G. Mueller, pers. comm.). Bonytail chubs also have been observed spawning in Lake Mohave, but larvae have never been collected from that reservoir (Jonez and Sumner 1954). A large number of young-of-the-year razorback suckers were collected from the shores of Lake Mohave prior to filling in the early fifties (Sigler and Miller 1963). Juvenile or subadult fish of either species have not been observed or collected In recent years.
The apparent lack of ,young individuals in the bonytail chub and razorback sucker populations suggests that recruitment is limited (Gustafson 1975a,b; Minckley 1973; Minckley 1983). Jonez and Sumner (1954) noted that predators were numerous in the vicinity of razorback sucker and bonytail chub spawning areas. Predation on early life stages of both species is believed to be the principal factor limiting recruitment in these reservoir populations (Minckley 1973; Minckley 1983).
The U. S. Fish and Wildlife Service initiated this investigation in order to further evaluate the population status and reproductive success of razorback suckers and bonytail chubs in Lake Mohave. The principal objectives were to determine (i) the distributions and abundances of both species, (ii) whether successful spawning occurs for either species, and (iii) the primary factors affecting spawning success.
1.2 History of the Lake Mohave Area
The construction of Hoover Dam in 1935 stabilized flows and drastically reduced suspended sediment loads in the Colorado River below the dam (Dill 1944). The river was transformed from a seasonally warm, silt laden, turbulent stream to a cold, crystal-clear regulated river (Jonez and Sumner 1954). The swift currents degraded the river bottom and exposed large, stable substrates to 70 km below the dam by 1947 (Jonez and Sumner 1954). This allowed for the extensive colonization of Cladophora sp. in shoal and riffle areas (Moffett 1942). Mayflies (CalIibaeti s sp.) were abundant in the river while midge larvae, caddisflies, dragonflles, damselflies, and microcrustaceans (Daphnia sp. 4 and Diaptomus sp.) were common (Moffett 1942). Gammarus fasciatus was introduced into the river In 1941 (Moffett 1942) and was abundant from Willow Beach to Searchlight (Cottonwood Landing) by 1950 (Jonez and Sumner 1954). An excellent rainbow trout (Salmo gairdneri) fishery was established following stocking in 1935 (Moffett 1942; Jonez and Sumner 1954). Largemouth bass (Micropterus sal mo ides), channel catfish (Ictalurus punctatus), and carp (Cyprinus carpio) were also collected or sighted below Hoover Dam in the early 1940 s (Moffett 1942), although the exact time of their introductions is unknown.
Further alterations in this section of the river occurred during the formation of Lake Mohave with the construction of Davis Dam in 1950. 2 Lake Mohave flooded 73,815 km of desert area and developed 322 km of new shoreline (Jonez and Sumner 1954). Lentic habitats were thus formed in the 75 km stretch from Eldorado Canyon to Davis Dam. Extensive stands of mesquite (Prosopsis sp.), willow (Salix sp.), cottonwood (Populus deltoides) and other shoreline vegetation were inundated by rising water levels (Dill 1944) in present-day Cottonwood Basin. Extensive areas of gravel from proximal washes became the predominant shoreline substrate, while the inundated vegetation provided submerged zones of cover. River conditions still persisted below Hoover Dam, but currents were reduced as Lake Mohave receded into Black Canyon.
Periphyton and macrophyte growth were immediately reduced in the lower reaches but persisted in most areas of Black Canyon (Jonez and Sumner 1954). Aquatic insect abundances declined, apparently due to the destruction of breeding habitats in downstream areas (Jonez and Sumner 1954). Centrarchids and channel catfish became abundant in the reservoir. Trout were caught occasionally throughout the reservoir but were primarily confined to the cold water area between Hoover Dam and Eldorado Canyon (Jonez and Sumner 1954).
Hoover Dam was frequently operated from the upper intake gates (1045 ft. elevation ) until 1954. Discharge temperatures were lower than historic river temperatures, but seasonal temperature cycles were still maintained in the river (Baker and Paulson 1980). Since 1954, Hoover Dam has been operated continuously from the lower intake gates (880 ft. elevation), and discharge temperatures remain 12-13°C year-round. Aquatic insects have disappeared from the river since the Jonez and Sumner (I954) investigations (Paulson et al. I980a). This apparently resulted from the loss of breeding habitats (Jonez and Sumner 1954), but was probably also affected by the lack of seasonal temperature cycles in the river. These often are required to trigger reproduction of many aquatic insects (Paulson et al. I980a). 01igochaetes, midges, and amphipods now comprise the dominant invertebrate fauna in Black Canyon (Bryant 1977; Paulson et al. I980a).
The physical and biological features of the Colorado River between Hoover Dam and Davis Dam have been changed considerably since the 1930 s. The changes in habitat, trophic interrelationships, and species interactions could directly or indirectly influence the abundance of bonytaiI chubs and razorback suckers in Lake Mohave. In this study, we attempt to evaluate how changes in temperature regimes, habitat, and predation could be affecting reproductive success of the native species.
2.0 DESCRIPTION OF THE STUDY AREA
Lake Mohave was formed in 1951 by the construction of Davis Dam. The reservoir extends 108 km south from Hoover Dam to Davis Dam. The first 32 km, which includes Black Canyon, Is characterized primarily by river conditions. Water temperatures within Black Canyon are virtually constant at 12-13°C due to the hypolimnetic discharges from Hoover Dam. Current velocities may reach 2.1 m/sec (6.5 ft/sec ) during periods of high discharge. There is considerable seasonal and daily variation in water levels (1-3 m) due to discharge cycles from Hoover Dam.
The only significant inflow to Lake Mohave is from the Colorado River via discharges from Hoover Dam. There are some warm springs located in Black Canyon, but these inflows are insignificant relative to the Colorado River. The Willow Beach Trout Hatchery, located 18 km below the dam, uses river water which is returned to the river. There are no major diversions of water from Lake Mohave.
The river is bordered by the Black Mountains to the east and the Eldorado Mountains on the west. The canyon walls extend several hundred meters above the river and shade the water for most of the day, greatly reducing incident solar radiation. The riparian community in Black Canyon is sparse except for a few stands of saltcedar (Tamarix sp.) and creosote (Larrea tridentata).
The transition between river and true reservoir conditions in Lake Mohave occurs in Eldorado Canyon. This is especially evident in the summer when a sharp interface (convergence) develops as cold, nutrient-rich, river-water flows under warm lake-water. Mixing at the interface promotes high phytopIankton productivity in Eldorado Canyon, resulting in a marked change in water clarity between river and lake-water (Paulson et al. 1980b). Below Eldorado Canyon, Lake Mohave expands Into Eldorado, Little, and Cottonwood Basins. Cottonwood Basin, located 54 km downstream from Hoover Dam, Is the widest point In the reservoir (6.4 km). There are few large bays or coves In Lake Mohave, and It Is best characterized as a "run of the river" reservoir.
Small coves are formed in some areas of the basins by the series of desert washes that have been inundated by the reservoir. These washes are major sources of the shoreline alluvial substrates that are washed into the reservoir. Generally larger cobble and gravel substrates are found along shore and finer materials are located in deeper waters. The reservoir has a long north-south fetch and is subject to high winds from the predominately south-southwest direction in summer and north-northeast direction in winter. This can result in severe seasonal wind and wave action (Minckley 1983) particularly in these larger flat basin areas. Water levels fluctuate approximately 5 m annually.
Inundated terrestrial vegetation is particularly common in the submerged basin areas during winter and spring when water levels are high. Terrestrial vegetation is typical of that in the Mojave Desert and consists of creosote bush, mesquite (Prosopsis sp.), eriogonum (Erlogonum sp.), salt bush (Atrip lex sp.), arroweed (Pluchea sericea), and barrel cactus (Echinocactus polycephalus). Extensive growth of salt cedar (Tamarix sp.), occurs along the shoreline. Large beds of submergent vegetation, MyriophyI I urn sp., and thin leafed Potomogeton sp. are found along these shorelines afteh draw-downs during summer months.
Climate is arid with an annual precipitation about 8 cm. Mean annual temperature is about 19°C ranging from 45°C in summer to -1°C in winter.
In this report, the Six Mile Coves area refers to the area of the Nevada shoreline in the Cottonwood Basin that is located above Hog Farm Cove. The Arizona Bay area refers to the combined areas of Arizona Bay, Yuma Cove, and Gold Cove In Little Basin. All other areas Indicate specific locations.
3.0 METHODS
3.1 Cove Surveys.
Cove surveys were conducted along representative shoreline sections of Lake Mohave to determine distributions, relative abundances, and habitat preferences of spawning razorback suckers and bonytail chubs. The surveys were conducted in February 1982 and included the area of the reservoir lying south of Great West Cove, Arizona. The surveys were made from the boat with two observers, one located at the bow and the other on the stern. Each had excel lent views of the shore Iine areas by standing on platforms 1 m above the water. Clarity of the water usually allowed for visibility to a depth of 3-4 m allowing for positive fish species identification. Areas were resurveyed when visibility was reduced. Relative abundances of fish, substrate types, and vegetation, when present, were later compared and compiled between both observers.
3.2 Samp I ing Locations.
Sampling of razorback suckers was conducted in twelve locations of Lake Mohave during the 1982 and 1983 spawning seasons. Efforts were concentrated primarily in the two major spawning areas identified during the cove surveys. These were the Arizona Bay area, and the Six Mile Coves area (Fig. 1). These major spawning areas were netted monthly from January through May 1982. Other sampling sites were sampled less frequently during the spawning season. Netting intensity was reduced In 1983, and work was concentrated In the Six Mile Coves area. This area PLACER COVE
LAKE MOHAVE TYEE •CREAT WEST COVE TRAMMEL NETTING COVE LOCATIONS 1982-83
f.'GOLD COVE YUMA COVE 'ARIZONA BAY
COTTONWOOO COVE' cifir-eove TEQUILA C -corroN*ooo EAST AREA
SIX MILE AREA I
NINE MILE COVESX
1 SIDEWINDER COVE
TRAMMEL NETTING AREAS
•••MAJOR EFFORT • MINOR EFFORT C3 1382 U3FWS BROOD STOCK COLLECTION SITE
• STOP SIGN COVE
DAVIS 0AM
Figure 1. Trammel netting locations on Lake Mohave, AZ-NV during 1982 and 1983 for razorback suckers and bonytail chubs, 10 also was sampled In November 1982. Arizona Bay was sampled only In January during 1983, but observations on spawning activites In both major areas were conducted monthly throughout the entire season.
3.3 TrammeI Netting
Routine netting was conducted with trammel nets 91.4 m long and 1.8 m deep. The trammel nets had inner walI bar measures of 3.8 cm and outer wall bar measures of 25.4 cm. Experimental gill and trammel nets were also used on two sampling dates. These nets were 45.7 m long and 1.8 m deep, and mesh sizes ranged from .6 cm to 3.8 cm.
Nets were intentionally set in spawning areas or areas that appeared suitable for spawning. These areas Included coves, point bars, and open basin shoreline areas. Nets were most often set perpendicular from the shore along the bottom. Nets were set for 12 to 20 hours, and efforts were concentrated on twilight and evening hours. Nets were run every four hours, cleared of fish and reset during each sampling period. 2 Catch rates are expressed as number of fish per 100 m net per 24 hours.
3.4 Measurements
Standard length (snout to end of hypural plate), fork length (snout to fork of caudal fin), total length (snout to tip of compressed caudal fin), and weight were recorded for each razorback sucker and bonytail chub collected during netting. Condition factors were calculated using the formula: KF=WT3/TL X 100,000.
More extensive measurements were made on 334 live razorback suckers collected from Tequila Cove, Six Mile Coves, and Arizona Bay and on 14 11 razorback suckers from Senator Wash Reservoir, California. Measurements were made in standard ways except for body depth and lip length (Hubbs and Miller 1953). Body depth was measured just posterior to the apex of the nuchal keel, and lip length was measured as the distance from the front of the upper lip to the posterior-most extension of the right lower lip. All measurements are expressed as perm!Mage standard length. Counts of principle dorsal fin rays were made on a I I razorback suckers with undamaged dorsal fins.
Condition of eyes, ripeness, incidences of bacterial infections, parasites, and other notable conditions were also recorded. Razorback suckers were tagged below the dorsal fin with individually numbered Carl in tags or caudal fin clipped and released near the site of capture. Car I in tags were furnished by the Nevada Department of Wildlife. Special note was made of recaptures throughout the study in an effort to estimate population size. Stomachs were examined on carp and gamefish netted in razorback sucker spawning areas to evaluate predation on egg and larval stages of razorback suckers. Contents were preserved in \0% formalin when eggs or egg material were found in the stomachs. Eggs were enumerated volumetrically.
3.5 Larval Col lections
Razorback sucker larvae were collected at night in spawning areas, using a hand dip net and dive lights. Monthly sampling for larval razorback suckers was conducted in 1983 in the Six Mile Coves area. Sample sites were 45 m apart on a transect following the 1 m contour over a shoreline spawning area. The collector illuminated a sample site with a dive light and dipped up all larvae seen during a 15 minute sample period with a hand net. The collector remained stationary during 12 all sampling periods. During some months, transects were also run at depths of 2.7 m and 5.2 m. Substrate types and temperatures were recorded at each sample location. Positive identification and stages of development were determined for each larvae collected (Snyder 1981). Only 50% of the larvae were processed when collections exceeded 300 individuals. Total lengths of larvae were recorded for 20-30 larvae per sample. Most larvae were returned to the lake while some were brought back to UNLV for behavioral observations in an artificial stream.
3.6 Temperature Development Ana Iyses.
Temperature-development analyses of egg and larval stages of razorback suckers were conducted in the laboratory. Eggs were collected from gravid females and fertilized in the field using a 6 : 2 male/female ratio. Upon fertilization, eggs were counted and transferred to the laboratory in lake water on natural, gravel substrates. Adults were released back into the reservoir. Ambient reservoir temperature was maintained during transport to the laboratory. Eggs were acclimated from ambient lake temperatures to experimental laboratory temperatures at the rate of I°C per hour and kept within I.5°C throughout the experiments. Rates of development were followed through various egg and larval stages, and hatching success was recorded at each experimental temperature.
Laboratory development data were used to determine spawning dates of field-collected larvae. Development'times at particular stages were converted to standardised thermal units using the formula: (Heming et al. 1982) TU = days at temp. X °C 24 13
Thermal units were then Integrated into the monthly lake cove temperatures to determine the approximate spawning dates. Fertilization and 0°C were used as thresholds of time and temperature.
3.7 Physical Hydrodynamics
The effects of water level fluctuations on egg survival were estimated by examining the intensity of spawning effort at various depths in spawning areas. Observations were made from a bluff overlooking a 40 m section of shoreline in Yuma Cove. Additional data on the depth distribution of redds were obtained from Mueller et al. (1982). Percent mortality of the hatch was estimated for each day of the spawning season from the maximum water level decrease occurring during the period required for hatching at respective monthly lake temperatures. These water level decreases were then plotted against depth distributions of redds to estimate the extent of dessication. Hatching times were derived from laboratory studies, and water level fluctuation data were provided by the U.S. Bureau of Reclamation. Reservoir temperatures were again converted to thermal units (Heming et al. I982) for comparison with developmental thermal units calculated from experimental hatching data. This aided in an assessment of the interaction of development rates, temperature, and water level fluctuations throughout the season.
3.8 Statistical Analyses.
Statistical analyses were performed with the SPSS (Nie et al. I975) statistical programs. Non-parametric statistics were used in length and weight analyses. Kruskal-WalI is analysis of variance, Mann-Whitney U-tests and Chi-square analyses were used where appropriate. 4.0 RESULTS
4.1 Cove Surveys
Razorback sucker spawning activity was widespread throughout Lake Mohave during the February 1982 cove surveys (Fig. 2). Razorback suckers were most abundant in shallow water ( <2 m) over flat, shelf-like areas or terraces with gravel substrates. Spawning habitats of these types were located principally along flat shelf-like gravel beach areas or in the mouths of inundated gravel washes. The largest concentrations of spawning razorback suckers were located at and adjacent to the Six Mile Coves area of the Cottowood Basin, and in the Arizona Bay area of the Little Basin.
Thousands of razorback suckers in dense congregations were observed in the extensive gravel flats of the Six Mile Coves area and adjacent shorelines. Razorback sucker spawning densities decreased south of Hog Farm Cove due to a steeper shoreline slope that limited the extent of appropriate shelf-like habitats. Spawning distributions north and south of the basin area became scattered due to the discontinuous nature of the preferred substrate and morphometry of the coves. Arizona Bay and adjoining Yuma Cove and Gold Cove were the site of the second largest spawning population. Spawning intensity was as great as that found in the Six Mile Coves area, but the overall size of the area was smaller. Razorback suckers utilized gravel substrates in the western half of Arizona Bay where a gravel wash enters the reservoir. A gravel bench, formed and terraced by wave action provided extensive spawning habitat at Yuma Cove.
Other smaller more localized populations of razorback suckers 15
LAKE MOHAVE
COVE SURVEY I GREAT V/E3TCOVE FEBRUARY 1982
N
'ARIZONA BAY
COTTONWOOD COVE
(CARP COVE
HOG FARM COVE {
NINE MILE COVES'
RAZORBACK SUCKER SPAWNIN8 AREAS
• < 50 FISH O > SO FISH JCWU PEPPER COVE
SS> 500 FISH
JT LARVAE
JKATHERIMES I.ANOIMO
DAVIS 0AM
Figure 2. Cove survey map of Lake Mohave, AZ-NV showing major and minor areas of razorback sucker abundance in spawning areas during February 1982. 16 occurred throughout the reservoir. Several small coves across from Cottonwood Landing In the Painted Canyon contained remarkably high numbers of razorback suckers. Spawning occurred over submerged areas of eroded gravel talus. The Three Mile Flat area on the Arizona shore of the Cottonwood Basin contained spawning razorback suckers, but it was difficult to survey this area due to extensive inundated terrestrial vegetation. This area probably contained more razorbacks than indicated by the survey map. The Windmill Coves area (southeast of Carp Cove) was netted for brood stock in January 1982 by the USFWS, but no razorbacks were apparent in this area at the time of the cove survey. Spawning has also been reported from below Hoover Dam (Gustafson I975b). This area was not included in our survey. Many other locations contained various sized local populations while some areas of the reservoir were not surveyed at all. The areas and numbers presented in this cove survey are therefore believed to be very conservative estimates of the total number in the reservoir.
Razorback suckers were by far the most abundant species observed in these spawning areas (Appendix 2). Other fish species were rare throughout the areas surveyed. Carp were observed in the spawning areas, but their numbers were low and they were usually more abundant in siItier coves not being used by razorback suckers.
4.2 Catch rates
The razorback sucker was the dominant species in the trammel net catches from most areas of Lake Mohave prior to May (Appendix 3) (Fig. 2 3-5). Razorback sucker catch rates (N fish/ 100 m net/ 24 hrs) were high in the Arizona Bay and Six Mile Coves area during winter months (Appendix 3)(Figs. 3-5). Razorback sucker catch rates were also high in 17
ARIZONA BAY COMBINED CATCH
IglRAZORBACK SUCKER \*Mn r = CHANNEL CATFISH ]? 40- W LARQEMOUTK BASS (Jlfl TROUT
35-
30 - - c/> tr • i X
1 ? tj 25- • ' u 1 1
J 20- o
1 '5-
10 -
;
j 5 -
; : ! l3 "^fTt! : 11 JAN FEB MAR APR MAY - JAN 1982 1983
Figure 3. Monthly species catch rates (expressed as // fish/lOOm' net/Z** hrs.) in the Arizona Bay area of Lake Mohave, AZ-NV during 1982-1983. 18
Tequila Cove (below Cottonwood Landing), and in Great West Cove, near Eldorado Canyon. These areas were all locations where razorback sucker ^_ spawning activity was observed during sampling. Razorback sucker catch rates were low in other areas of the reservoir netted, but still comprised a large percentage of the total catch. Carp were usually the second-most abundant fish species in the catch during winter, followed by channel catfish, largemouth bass, and rainbow trout (Appendix 3). A few cutthroat trout (Salmo clarki ), and striped bass (Morone saxatiI is) were also captured in trammel netting. Only eight bonytaiI chubs were caught during the study. These were all captured in the Six Mile Coves area during March and April, 1982.
The catch rates for razorback suckers in Arizona Bay were fairly constant during winter of 1982 and exceeded that of any other species present (Fig. 3). The area was not netted in April, but observations indicated that while spawning was still fairly intense, it had begun to decline, and carp were becoming more abundant. The catch rates for razorback suckers declined in May, and carp became the dominant species in the Arizona Bay area. The catch rates for other species also increased at this time.
The February 1982 catch rate for razorback suckers in the Six Mile Coves area appears high in comparison to other months of the spawning season during that year (Fig. 4). Catch rates were inversely proportional to net hours which largely accounts for the differences among these months (Appendix 2). Nets generally were set at dusk when razorback activity was high, yielding high catch rates when calculated on a 24 hour basis and compared to other sites. Overall, spawning densities were similar in both the Six Mile Coves area and the Arizona 19
SIX MILE AREA CATCH 1982
104.6 - > r RAZORBACK SUCKER Ml CARP CHANNEL CATFISH 40- LARGEMOUTH BASS 1TROUT h i f S BONYTAIL CHU3 Lf STRIPED BASS 35-
30 -
(T X w 25- '
tU • N£20- o 0
o 15 - tu
10 - i
' 5- 1
m lias. i3%\:\t Sof „l JAN FEB MAR APR MAY
Figure ^. Monthly species catch rates (expressed as // fish/lOOm net/24 hrs.) in the Six Mile Coves area of Lake Mohave, AZ-NV during 1982. 20
Bay area when comparing similar sample hours. March sampling was conducted specifically for bonytail chubs, and some nets were set in different areas. Re-calculated catch rates for March 1982 are presented in Table 1. These re-calculated rates exclude day sets and non-spawning area sets that were not part of routine monthly sampling. These rates are similar to those for the Arizona Bay area during the same month.
An increase in catch rates of other species was evident in April in the Six Mile Coves while those of the razor-back began to decline (Fig. 4). By May, razorbacks had fallen to third in abundance as spawning was nearly complete. Seasonal trends were similar in both areas with razorback suckers dominating the catch from January-April. Other species became more abundant in these areas during April and May.
Razorback suckers were present in the spawning areas in the Six Mile Coves area and elsewhere by November 1982 (Fig. 5). While catch rates were high due to low net hours, razorbacks were abundant in these areas. Razorback suckers dominated the catch in January and February during 1983, but by March, nearly the entire Six Mile Coves area had been vacated by razorbacks. Spawning activity and numbers of razorbacks remained high in cove areas to the north and south of the Six Mile Coves area and in Arizona Bay. Water temperatures in all spawning areas were simiIar in March.
Heavy storms during the week prior to the sampling in March appeared to be the reason for the sudden disappearance of razorback suckers from the Six Mile Coves area. Large amounts of suspended sediment, detrital and organic material were present in the water and turbidity was high. Gravel spawning areas here were silted over. The shorelines in the cove areas of the Cottonwood Basin and the Arizona Bay 21
COTTONWOOD BASIN CATCH 1983 SPAWNING SEASON 90
( P3 RAZORBACK SUCKER ij CARP = CHANNEL CATFISH 80 - g LARGEWOUTH BASS jj] TROUT
70 -
V) tr . - = 60
X s | % 50- ii: M I e ' o o 1 40- • i V) t u_ ' # 30 - p>
_
20 -
Mf 10- • 9 1
£ [ 1 \- ^ ilii NOV 82 - JAN FEB MAR MAY
SIX MILE AREA TEQUILA NINE
Figure 5. Monthly species catch rates (expressed as // fish/lOOm^ net/Z^ hrs.) in various locations of Cottonwood Basin in Lake Mohave, AZ-NV during 1983. <
Table 1. Catch per unit effort estimates for regular catch and revised catch rates excluding daytime sets and those sets occurring in non-razorback. areas (Values are total catch and # fish/24 hrs/100 m2 net in parenthesis).
Species Net Net Razorback Largemouth Rainbow Channel Striped Bonytail Hours (hr.) Area (m2) Sucker Carp Bass Trout Catfish Bass Chub
165.8* 167.5 174 18 11 7 2 4 (15.04) (1.56) (.960) (.610) (.180) (.35)
46.3 167.5 124 74 43-1 (38.37) (2.17) (1.24) (1.24) (.93) (.31)
* regular "" revised
ro ro 23
area were more protected due to their relative exposures. This helped minimize disturbances from the storms. Razorback suckers failed to
return to the Six Mile Coves area during the remainder of the 1983
spawning season but continued to spawn in the more protected shoreline
areas elsewhere in Lake Mohave. Netting was continued in Tequila and Nine Mile Coves for the remainder of 1983 where spawning activity followed the pattern similar to that in 1982.
The large mesh nets used for routine sampling were selective toward large fish. Experimental trammel and gill nets were used on two sampling dates (Nov. 1982 and Jan. 1983). There were some differences in the
sizes and numbers of fish caught in the two types of nets set in
November. The experimental nets were set for 24 hours. Trammel nets were
pulled after eight hours due to large numbers of razorbacks being taken. Only 2 species of fish, threadfin shad (Dorosoma petenense) and rainbow
trout were too small to be taken by the regular trammel nets. Both
species were taken at night in the experimental nets after trammel nets had been pulled. The rainbow trout were nearly identical in size and
appeared to be from a recent stocking. Similar nets set in January 1983 caught nearly the same types of fish. Small sunfish were not collected in sampling and were not present in the shallow water until late April
near the end of the spawning season. The species composition and
relative abundances of fish caught in the trammel nets seem to reflect those present in the sampling areas.
4.3 Length and Weight Di str ibutions, Cond ition Factors, Sex Ratios,
and Di sease
Lengths and weights of razorback suckers captured during this study are summarized in Table 2. Total lengths were similar to those reported Table 2. Length weight summary for razorback suckers captured during trammel netting in Lake Mohave. Values are the mean ± one standard error. Values in parenthesis indicate the range.
Year
Parameter Sex 1982l 19832 Combined3
Total Length (mm) M 554.4 ± 1.23 555.7 ± 2.41 554. 6 ± 1.09 (448 - 686*) (450 - 610) (448 - 686*)
Total Length (mm) F 628.2 ± 1.80 624 ± 3.48 627. 3 ± 1.61 (525 - 617) (525 - 705) (525 - 705)
Standard Length (mm) M 458.1 ± 1.18 460 ± 2.55 458 ± 1.07 (370 - 627*) (370 - 520) (370 - 627*)
Standard Length (mm) F 524.8 ± 1.72 520.6 ± 3.17 523. 7 ± 1.51 (434 - 596) (440 - 648) (434 - 648)
Weight (gm) M 2037.2 ± 15.76 2024.6 ± 33.15 2034.6 ± 14.25 (1112 - 4003*) (970 - 2950) (970 - 4003*)
Weight (gm) F 3123.2 ± 32.33 3023.7 ± 56.62 3097.4 ± 28 15 (1700 - 4453) (1840 - 4086) (1700 - 4453)
* Indicates possible female
1 Total observations = males 506; females 262. " Total observations = males 131; females 81.
3 Total observations = males 637; females 343. 25 by Minckley (1983) from Lake Mohave In 1982 and are higher than those reported from the upper basin by McAda and WydoskI (1980). Males averaged 554.4 mm (TL) in 1982 and 555.7 mm (TL) in 1983. Total lengths of females were significantly higher than males (Mann-Whitney U-test; p=.001) and averaged 628.2 mm in 1982 and 624.0 mm in 1983. Males ranged from 448-686 mm (TL) in 1982 and 450-610 mm (TL) in 1983 (Table 2). The 686 mm (TL) male caught in 1982 may have been incorrectly recorded as a female in the field. The average weight of males captured was 2037.2 gm
in 1982 and 2024.6 gm 1983. Females averaged 3123.2 gm in 1982 and 3023.7 gm in 1983 (Table 2).
The lengths and weights of bonytail chubs captured during the study are presented in Table 3. The males ranged from 508-529 mm (TL) and females from 475-535 mm (TL). The weights of male bonytail chubs ranged from 809-1105 gm and in females from 766-1040 gm.
The length frequency distribution for female razorbacks in the 1982-83 combined catch was slightly leptokurtotic and skewed toward the smaller size classes (Fig. 6). There was an abrupt decrease in the
frequency curve for females between 640 and 650 mm, and a sharp decrease at 670 mm. The length frequency distribution for males was more
symmetric (Fig. 6). There was however, a slight decrease in the frequency curve between 540 and 550 mm. The range of lengths (TL) for males was 200 mm and 180 mm for females.
There were some differences in the monthly mean lengths of male and female razorbacks in the spawning areas. Individuals present during
January were slightly larger than those present in following months (Fig. 7-8). Total lengths of males were statistically different during
the first three months of the 1982 spawning season (Kruskal-Wai I is; 26
- Table 3. Lengths and weights of bonytail chubs captured in Lake Mohave during 1982. (Those captured on 3/20/82 were released in the area. Those captured on 4/16/82 were transported to Willow Beach Hatchery for broodstock.)
Location Date Sex SL (mm) FL (mm) TL (mm) Weight (gm)
Six Mile area 3/20/82 M 435 465 508 809
" 11 F 420 451 502 873
" II F 397 426 475 766 n 3/20/82 escaped from holding pen
Six Mile area 4/16/82 F 412 446 499 985
it It M 440 472 529 1105
it " F 443 478 535 1040
" M F 425 462 512 975 ( « LENGTH FREQUENCY 20 19 18 i 5,7 t_ CN 8 7 5 I S4 £3 M 2 1 0 il 44l45l46147i48k9l5ol5l 152153S54b5 !-MflLE5 D-FEMRLES TOTAL LENGTH (cm) Figure 6. Length frequency histogram (TL) of all male and female razorback suckers collected by trammel netting during 1982 and 1983 from Lake Mohave, AZ-NV. N> I 700- » 91 650- '$? ,68 19 3'I 99 i5 V 1 9 600- X 8 h- 3 1 I 0 - . . • * 3i- * f -i 5 j 5 550- " H • 1 . 1 _J • H 500 O [• 1- 450 V MALES 400 ' 1 1 1 _j , -j "i" "• J F M AM-N- JFM-M 1982 MONTH 1983 Figure 7- Total lengths of male razorback suckers collected monthly from Lake Mohave, AZ-NV during 1982 and 1983. Bars are ± one standard error, vertical lines indicate range, and numbers are sample size. 98 700- 59 T 65 i 650- r 5 T 2- i s- i k •fr — 600- r 1 X H CD g 550- • _l 500- O h- FEMALES 450- 400 i J F M ' A M N 0 F ' M M 1982 1933 MONTH Figure 8. Total lengths of female razorback suckers collected monthly from Lake Mohave AZ-NV during 1982 and 1983. Bars are ± one standard error, vertical lines indicate range, and numbers are sample size. N) 30 p<.001). Total lengths for females were borderline (Kruskal-WalI Is p=.083). This trend in decreasing size indicates possible Immigration or movement of different fish Into spawning areas during the spawning season. Average weights of razorback suckers also varied during the spawning season (Figs. 9-10). Mean monthly condition factors (Fig. 11) varied in relation to these monthly weight changes. The condition factors of razorbacks decreased for both sexes between January and March 1982 (Fig. 11). The variability in condition factors after March 1982 was too high to describe any trends late in the spawning season. Condition factors for both sexes were high again by November 1982. The condition factors of males decreased slightly between November 1982 and February 1983, then dropped off abruptly in March, and increased again by May. The razorback suckers examined from three locations in Lake Mohave and from Senator Wash Reservoir ranged in length from 370-577 mm SL. Mean body depths were very close among the populations examined, with means ranging from 242.9 at Six Mile Coves to 250.3 at Tequila Cove (Fig. 12). Body depths of four razorback X fIannelmouth sucker (Catostomus latipinnis) hybrids from Lake Mohave ranged from 203.7 to 208.2. These low depths reflect the reduced keel typical of this hybrid combination. There was no overlap between razorback suckers and the hybrids in body depth, although some individuals from the Arizona Bay population did approach the hybrids. Least depth of the caudal peduncle was similar in all populations. Means ranged from 82.9 at Six Mile Coves to 85.7 in the upper Colorado River system. There was a broad overlap between razorback suckers, 31 4600 l» 4400 4200 96 MALES 4000 3800 - If 3600 - 3400- ^ 3200 - M £ 3000- 99 o 168 *I " J? 2800 - 19 *~ 2600 T l5 2 2400- J '- 9 ^ 2200 • }• -\3 2000 - t It T T f 1 T 1800- h 1 I 1 a. t 1600 - ] 'I f I 1400- I • 1200- *~ 1000 - . . 800 J F M AM -N - J F M • M 1982 1983 MONTH Figure 3. Total weights of male razorback suckers collected monthly from Lake Mohave, AZ-NV during 1982 and 1983. Bars are ± one standard error of the mean, vertical lines indicate range, and numbers are samples sizes. 32 c 4600i 50 13 r SO 4400- 65 4200- 59 " 4000 3800 6. ' 3600 5 9 3400- 5 t j 3200- i 1- f i i f "w 3000- / ir i -!j- 1 fl • 1 I o> 2800- ) 1 « h- 2600- J j g 2400- • s ^ 2200- •I 2000- . r • JL 1800- 1600- FEMALES 1400- 1200- 1000- J FMAM-N- J FM-M 1982 1983 MONTH Figure 10. Total weights of female razorback suckers collected monthly from Lake Mohave, AZ-NV during 1982 and 1933- Bars ± one standard error of the mean, vertical lines indicate ranges, and numbers are sample sizes. MEAN MONTHLY CONDITION FACTOR (WT/TL3'I05) IQ C o o ro r\ o 01 o 01 I — QJ -ti 3 3 3 -I (D c_ - O- Q. O 0) 33 O — Q) U) I— 3 rt CD QJ O ft> >_0 7T 3 CD rt S. • O C~> O ~< 3 O 3- (O n> 3 QJ n CO Q. < O ro in — m 3 rt rt « Q. 3 O > rt Q. 3 M — O) I O -1 -h ^ 3 D- Ol Q) c_ - 3" N X (D O —• l/> CT O TD Q) xnoi o — U1 S 3: 3 C O U3 O — 01 CD N fl> -5 O Hi (/) rt O O h-t QJ 3 O —• 3 (D — O D — rt CD VQ ft) in CO Q. Ni ^ Lip Length Head Depth Body Depth < M ro ro Ui 0"> --J co •& m cr> --J 03 0 01 oooo ooooo 0 o 0 1 I i I I I 1 1 1 t \ I 1 1 1 i I I 1 T 'i 1 ; "I• 1 1 \_ 1 Six- Mile Coves rae pJ-q ktV j^3 Tequila Coue 4 ri! ' f Arizona Bay Senator Wash Reservoir Basal Separation Gape Width Peduncle Depth of Lower Lip O* "-J 00 CO O OlUlOl OOOOOO ooooo i i i i i i i i i i I I 1 I ! 1 I 1 I I Six-Mile Coves -*- Tequila Cove Arizona Bay Senator Wash Reservoir —ife i— Upper Colorado River Figure 12. Morphological variation in some characters of razorback suckers taken from three locations in Lake Mohave, Senator Wash Reservoir, and the upper Colorado River. Horizontal lines indicate ranges, vertical lines indicate means, and shading represents ± one standard error. 35 especially at Arizona Bay, and the hybrids (range= 69.9-82.1) In peduncle depth. Population means for the width of gape showed more variability than did the other morphological characters studied. Means were higher at Tequila Cove and Arizona Bay, 68.5 nd 69.2, respectively, than at Senator Wash and the upper Colorado River system (64.0 and 63.7, repectively). Gape width was slightly lower in the hybrids (range=31.7-68.7) than in razorback suckers. Lip length appeared to be correlated with width of gape in the razorback sucker. Means for the populations examined were very close and there was a broad overlap between the razorback suckers and the hybrids. The basal separation of the lobes of the lower lip was similar in the Lake Mohave and Senator Wash populations. Means ranged from 13.9 at Arizona Bay to 14.6 at Senator Wash and were appreciably lower than the mean from the upper Colorado River population (17.1). Measurements for this character on the hybrids were 4.1, 4.4, 7.3, and 11.3. The principal dorsal fin rays in Lake Mohave and Senator Wash razorback suckers numbered from 13 to 17 (Table 3a). Means in both reservoirs were at or just above the range of means previously reported. An upward extension in the range of variation in dorsal fin rays was found in razorback suckers from both Lake Mohave and Senator Wash Reservoir. Dorsal fin ray counts in five hybrids were 13 (2), 14 (1), and 15 (2), with a mean of 14.0. This is almost one full fin ray more than Hubbs and Miller (1953) found in this hybrid combination. Arizona Bay was sampled on two occassions, and the means on both dates for al I 36 Table 3a. Principle dorsal fin ray counts of razorback suckers (Xyrauchen texanus) from Lake Mohave, compared with counts obtained from other populations. Loca 1 i ty N Mean + s. E. Lake Mohave , Ari zona Bay 178 . 14.79 + 0.07 Tequi la Cove 25 14.96 + 0.19 Si x Mi le Coves 123 14.56 + 0.07 Senator Wash Reservoir 1 4 1 5.00 ± 0.18 Upper Colorado River 1 2 14 .50 + 0.23 Lower Colorado Ri ver 8 14.63 + 0.18 Gila Ri ver System 1 7 1 4.24 + 0.18 37 razorback suckers taken were comparable at 14.75 (N= 79) and 14.82 (N= 99). N~ Males were more abundant in the spawning areas than females during this study (Figure 13). Males consistently made up over 55$ (1.2 male/ female ratio) of the razorback suckers netted in the spawning areas except for April 1982. The low percentage in April was probably a result of low sample size. The highest percentage of males during the season occurred in November and May (+ 75$ or 3.0 male/female ratio). Males captured in November were ripe more frequently than females. This may indicate an earlier spawning movement by males. Our sex ratios were lower than those reported by Minckley (1983) (3.57 male/female ratio) in 1981 for razorbacks in spawning areas. This was probably due to different sampling methods. Our nets were set perpendicular to shore, often sampling deeper water beyond spawning areas. The sex ratios in razorback suckers are biased toward females in offshore areas (Minckley 1983). Disease and injury in the Lake Mohave razorback sucker population were high during the study. Age, seasonal spawning stress, and' habitat associations seemed to contribute to the presence of these conditions. Eyes showed conditions of bulging, clouding, reductions in pupil size, and, in some instances, they were missing altogether. The percentages of razorback suckers with damaged eyes taken during 1982 and 1983 are presented in Table 4. Roselund (in Minckley 1983) identified Myxosoma sp., a protozoan, as the cause of opaque eyes, and noted that Aeromonas hydrophiI a, a bacterium, also occurred in the eyes of some affected fish. The bacterium Pseudomonas sp. was also reported from a number of razorback suckers. ' SERSONRL SEX RflTIO 100 • 90 __ 80 _ P in 70 _ ppjj ^rl i ' I?'? ' I o i | : ;• - j V* . 1 I60- PI a — • ? f. r$ ' 1 " —' 5Q 1 ; ; :--'i I _j L i ! i __ ' - j 5 40 _ , in UJ —, ™ i '•• ff 30 _ \ •i i ' 20 _ • 10 _ • | • . 0 t i JRN FEB HRR fiPR MRT NOV JflN FEB MflR MflT ,Figure 13. Monthly sex ratio of razorback suckers netted in spawning areas from Lake Mohave, AZ-NV during 1982 and 1983. Bars indicate percent males captured in total catch. 00 ( Table 4. Incidence of eye injury and eye disease in razorback suckers captured in Lake Mohave during 1982 and 1983 (Based on a total sample size of 981). Percentage of Individuals Affected Condition Males Females Both Eyes Good 63.2 53.4 One Eye Affected 28.0 .32.3 Both Eyes Affected 8.8 14.3 Incidence of gross external bacterial Infections was significantly 2 higher during the spawning months (x : males p=.02; females p=.011)(Fig. 14). The Incidence of bacterial infections in males was lowest during January, May, and November of 1982. The percentage of females infected was high during all months except May and November. The degree of bacterial infestation was noticeably less severe in both sexes during these early and late months. A decrease in external bacteria became apparent again in May after spawning intensity had decreased. Incidence of bacterial infection was significantly lower in males than 2 females throughout the spawning season (x p<.OOI). Bacteria were 2 also more prevalent in ripe females than non-ripe females (x ; p <.001). There was no relationship between ripeness and bacterial infections in males. Abrasion during spawning acts by razorback suckers observed in 1982 and 1983 appeared to be the probable cause of these infections. Minckley (1983) likewise suggested substrate and tubercular abrasion as probable causes of these infections. Greater incidence of infection in females is probably a result of them being in closer contact with the substrate during spawning. A parasitic copepod (Lernaea cyprinacea) was present on razorback suckers during this study (Fig. 15). They have formerly been reported from Lake Mohave by Minckley (1903). Lernaea were more commonly found on males than females. Statistics on Lernaea were spurious due to the frequent enumeration of scars when actual Lernaea was no longer present. The bonytaiI chubs captured in March were all infested with Lernaea and were generally in poor condition. BonytaiI chubs captured in April were in better condition and were transported to the Willow Beach Hatchery < BflCTERIRL INFECTIONS 100 LJ 6D0U -^ UJ Cf? n u oLJ 23 1C 0 JRN FEB 1 MRR RPR MflT NOV rFEMflLES Figure 1k. Monthly incidence of gross external bacterial infections found on razorback suckers during the spawning season in Lake Mohave AZ-NV, Years 1982 and T983 have been combined. c ( 5ER50NRL LERNflEfl 100 90 _ 80 _ 70 _ y 60 _ z a* 50 __ u LJ 0 40 _ M p 1 t 30 __ i 1 ;* i ^_ - 1 TOI * 20 _ • . i Jl h-a n I;! jj 10 _ j i u_ -' >( 1 ^ ^ B 1111 j II 1 I'i 1 JflN I FES IMflR i RPR I KflT I NOV I - i JBN I FEB I MflR I MflT Q-FEMfiLES Figure 15. Monthly incidence of Lernaea sp. on razorback suckers during the 1982 and 1983 spawning seasons in Lake Mohave, AZ-NV. 43 for species propagation. Other Injuries and disease were not examined In any great detail. Deformations of the vertebral column and tumors were present in the razorback sucker population, but their incidence was not very high. 4.4 SPAWNING 4.4.1 Spawning Habitat Spawning habitat for the razorback suckers in Lake Mohave consisted primarily of a gravel or gravel mix substrate located in shallow areas having a flat, shelf-1 ike morphometry. There are two principal types of shelf-like structures found in Lake Mohave. The first was located in areas of flat, low-gradient shoreline topography. The Six Mile Coves area of the Cottonwood Basin and mouths of washes such as in Arizona Bay were examples of areas where extensive use of this type of habitat occurred. The second area consisted of grave I-terraced shelves along the shoreline. The terracing is created by the lapping of waves over the gravel during sequential drawdown of the reservoir during summer. Yuma Cove and coves located across from Cottonwood Landing are examples of such areas. Spawning areas were largely free of submergent vegetation during winter when water levels were high. The areas offshore from these sites contained extensive weed beds, consisting of MyriophyI I urn sp. and Potamogeton sp. which were visible in summer when water levels were low. Gravel substrates were prevalent along shorelines of Lake Mohave. The particle size of substrates in spawning areas of Yuma Cove indicate that larger materials predominate in areas of razorback sucker redds (Mueller et al. 1982) (Table 5). Spawning activity tends to flush away finer substrate particles resulting in depressions comprised largely of ARIZONA DAY AREA ARIZONA BAY AREA COTTONWOOD BASIN AREA COTTONWOOD BASIN AREA COTTONWOOD BASIN COVES BASIN ( I METER) »--« COVE { I METER- SUBSTRATE) T r J JASON F M A M 1982 1983 MONTH Figure 16. Length of the razorback sucker spawning season during 1982 and 1983 in Lake Mohave, AZ-NV. Horizontal lines are spawning season length and double horizontal lines indicate peak spawning activity. Temperatures are those recorded at 1 m. SEflSONflL RIPENESS 82-83 100 90 _^•H 80 _ B^5 70 __ iu ~ 60 Q > 40 _J n i i 30 __ 20 _ r — 10 -^ 0 JRN I FEB 1 MfiR 1 RPR 1 MRY NOV -MRLES [3-FEMflLES Figure 17. Monthly ripeness of razorback suckers collected by trammel netting during the spawning season in Lake Mohave, AZ-NV. Years 1982 and 1983 have been combined. also decreased later in the spawning season. Water temperatures were steadily decreasing and turnover was occurring during fall when razorbacks first moved into spawning areas (Fig. 16). Photoperiod was also approaching its annual minimum at this time. Minimum water temperatures just above 10°C were reached during January and then increased steadily through the spawning season. Temperatures ranged from 10-15°C during the peak spawning period from January-March (Fig. 16). Temperatures increased to over 20°C in May when spawning was Hearing completion. Water levels in Lake Mohave fluctuate widely as a result of seasonal variations in the operation of Hoover Dam (Fig. 18). Water levels averaged about 193 m above mean sea level during November of 1981 and 1982 when razorbacks began moving into spawning areas. Water levels rose steadily during winter and spring and averaged about 195 m during peak spawning periods in January-March. Levels remained high through May, although significant daily and weekly fluctuations did occur during this period. Water levels decreased sharply after May in 1982, but remained high through 1983 due to high runoff in the Colorado River. 4.4.3 Spawning Behavoir Douglas (1952) described spawning behavoir by razorback suckers in shallow coves of Lake Havasu in March 1950, and Ulmer (1981) described spawning by razorbacks from Senator Wash Reservoir. Our observations from Yuma Cove in Lake Mohave substantiate the observations of previous investigators. Spawning predominately occurred in water less than 1 m. This was reflected in the redd surveys (Mueller et al. 1982) and by our observations. Groups of up to eight males, but more commonly four to 1971 196- 195- UJ H LU 2 194- z o 193- UJ 192 - 19! - 190 I ' I I » < ' I 1 I 1 I I I I I I I I I I • I 1 I ! I ! • I I I 1 ,' M 7 I I I I I I I I I 1 ' ' ' ' I ' ' I 1 ' ' ' I ' ' ' I ' ' I ' A' S j'F'M1 A'M'O' J'A'S' OT Nl D ' j l F ' M ' A 1981 1982 1983 WEEK Figure 18. Mean weekly water level elevations of Lake Mohave, AZ-NV during 1982-1983. .t- UD 50 five, would join and follow a female entering the spawning area from deeper water or join one already in the area. Males remained close to the female until she settled to a location on the substrate. Two of the males would position themselves on each side of the female to spawn. The group would begin to vibrate rapidly, and the eggs were deposited over and within the gravel substrate. Eggs were never plentiful in any redd surveyed by divers following a spawning act. Some spawning acts would terminate shortly after starting as if the female were not ready to spawn. The females sometimes headed into deeper water after spawning, but males usually remained in the spawning areas. It is unknown how long females vacated the area before returning. Males appeared somewhat territorial showing restricted lateral movement along the shoreline but exhibited no aggresive behavior toward each other or toward other species. Ulmer (1980) observed similar territorial behavior in razorbacks from Senator Wash Reservoir. Razorback sucker males in Lake Mohave were often seen joining other proximal spawning formations, but they stayed within deliniated areas. Spawning intensity in Lake Mohave razorback suckers appeared to be greatest during twilight periods, except during peak spawning when intensities were relatively similar throughout the entire day. 4.4.4 Spawning Success Razorback sucker eggs successfully hatched throughout the range of temperatures present during the spawning season in Lake Mohave (Table 6). Gamete viability of artificially spawned eggs was as high as 60$. Similar hatching success was found to occur at temperatures ranging from 12°C through 20°C (Table 6). There was no consistent relationship between hatching success and temperature. Hatching success was reduced Table 6. Razorback sucker hatching success and larval development rates for various temperatures as determined by artificial spawning and laboratory observations. Date Reservoir Laboratory Hatching Average Hatching Average Swim Up Average Swim Down Experiment Spawned Temperature (°C) Temperature (°C) Percentage Time (days) Time (days) Time (days) A 1/28/82 11.0 10 60.0 17.5 28.0 B 2/15/82 11.5 10 21.9 18.7 30.0 68* " 15 :>2.2 . 10.7 17.0 38* n n n 20 33.6 6.6 12.0 • 27 • C 1/27/83 11.0 . 8 0.0 ~ - " . •" " 12 36.0 - - " " " 15 39.5 - - n M ii 20 38.0 - - * D . 2/26/83 13.0 8 0 - - " " " 10 36.3' 22,1 35.0 " . " . 15 64.8 . 11.4 20.6 20 64.5 7.0 12.6 * Estimated 52 at 10°C during two of the experiments, but was never less than 50% of that occurring in the warmer incubation temperatures. A third experiment conducted at 10°C resulted In a 6Q% hatch. Hatching was not successful at 8°C. Hatching and development times were strongly affected by temperature. Hatching required 17.5-22.1 days at 10°C, 10.7-11.4 days at 15°C, and 6.6-7.0 days at 20°C (Table 6). Development, as reflected by time required to reach swim up and swim down stages, was also affected by temperatures. Variability in hatching success among experiments at similar temperatures could be a function of gamete quality. This can vary with age, condition, or stage of egg maturation in the fish. In addition, hatching success may have been influenced by experimental procedures, particularly at 10°C. The higher hatching success at 10°C in experiment A may reflect the more suitable acclimation conditions that occurred during that experiment. Acclimation occurred prior to and during the early cell cleavage stages and was complete prior to the morula stage. Toney (1974) reported unsuccessful hatching at lower ambient Lake Mohave temperatures, but likewise encountered high acclimation gradients that may have caused high mortality. Deformations in these cell divisions appeared to be more common during the less successful 10°C experiments where the ambient lake temperature and the experimental temperature were further apart. 4.4.5 Larva I Col Iection and Development Razorback sucker larvae were collected at night In the Six Mile Coves area during the spawning season of 1983 (Table 7). Most larvae Table 7. Razorback sucker larvae collected in Lake Mohavc l>y light attraction and handnetting during 1983. (CPU is number caught per 15 mill.) Larval Depth Temperature Total Larvae Sampling Larvae Total Development Stage (X) Date Location (m) Substrate (°C) Collected ' Time (Min.) CPU Length (mm) EP1 LP2 EM3 1/25/83 Six Mile • 1.2 gravel 11.0 28 (4) 15 min. 7 10.9 ± .48 57.1 42.9 0 Area 2/19/83 Six Mile 1.1 cobble/gravel 11.5 739 (5) 15 roin. 147 11.1* 13.9 21.2 14.9 Area sand • II it 2.8 gravel/sand " 211 . 0) 15 rein. 70 " 6.2 66.0 27.8 tl it 4.9 • " 23 (2) 15 min. 12 " 0 82.6 17.4 .2/19/83 North Six 1.1 cobble/gravel 11.5 106 2 15 min. 53 11.1* 29.2 63.2 7.5 Mile Area _ 3/25/83 Six Mile 1.1 gravel/sand .15.0 0 4 '15 min. - - Area _ tt ii 2.8 sand " 0 2 15 roin. - - _ If ii 4.9 _ it 0 2 15 min. 3/26/83 Tequila 1.1 gravel/sand 15.0 38 3 15 min 13 11.0* 23.7 65.8 10.5 Cove _ 11 Tequila 6.1 ii 0 . 3 15 min. ' 0 Cove 5/7/83 Nine Mile 1.1 gravel/sand - 0 3 15 rain. _ - - Area 1 early proto larval stage 3 early mesolarval stage ^Composite sample 2 late protolarval stage ui 54 were collected in February at a depth of 1 m., but many were also taken at 2.8 m. and less commonly at 4.9 m. (Table 7). This may indicate that fewer larvae are present at these greater depths or may result from the increased time needed for larvae to swim up from the substrate to the light. Fewer redds, however, were located in these deeper waters. Adult and larval razorback suckers had completely abandoned the Six Mile Coves area by March, presumably due to heavy storms and wave activity that disrupted substrates in the spawning areas. Sampling was resumed at Tequila Cove in March, but only a few larvae were collected. Larvae collected were positively identified as razorback suckers by comparing them with laboratory raised larvae and descriptions by Snyder (1981). The stellate chromataphores found on the top of the head, on the back, and along the mesoderm are diagnostic characteristics of this species. No other suckers are presently found in Lake Mohave nor are any other species of fish known to spawn in the reservoir at the times the collections were made. All larvae brought back to the laboratory for behavioral observations from the lake developed into juvenile razorback suckers. Total lengths for larvae collected averaged about 11 mm. Most larvae were in the late protolarval stage of development. Early protolarvae collected February 19 were probably spawned during mid-January as determined by rates of development in the laboratory. All larvae collected still had at least partial yolk sacs present, with none being past the early mesolarval stage of development. Larvae were never observed in the spawning areas during the daylight hours. Extensive sampling conducted with meter nets failed to 55 capture larvae both during the day and night In February. Monthly diving observations also failed to turn up any larvae. Hand netting using lights in these same areas in the evenings showed that the larval razorbacks were abundant. The larvae apparently remain in the substrate until they are large enough to migrate from the spawning area. 4.4.6 Factors Affecting Spawning Success It was evident early in the study that water level fluctuations could have a pronounced influence on spawning success of razorback suckers. Razorback suckers primarily spawned in shallow water less than 1 m in depth. Redds were indistinguishable in water less than 0.3 m. deep but frequent use of this depth for spawning was observed. Water levels in Lake Mohave can vary considerably over the period of a few days. Razorback sucker redds and eggs were therefore susceptible to dessication in the shallow waters of spawning areas. Eggs deposited early in the spawning season were particularly vulnerable because hatching times were longer at colder water temperatures. The magnitude of egg mortality caused by water level fluctuations was estimated from measurements on the depth distributions of redds (Appendix 4)(Table 8)(Mueller et al. 1982) versus daily water level fluctuations during the spawning season. Eggs were difficult to locate and enumerate in spawning areas so spawning intensity was used as a potential indicator of egg distribution. These estimates indicate that a large percentage of the spawn could have been adversely affected by decreases in water levels during February and March of both years (Figs. 19-22). Cooler water temperatures occurring in February increased hatching times, thus exposing eggs to the danger of potential dessication for longer periods. 3-1 ^ 2 to K IU •« I ' «> HI o < < 43% o 12 °o 70% < z o UJ 86% o I 5! 2 o UJ _i 96% t- s: w o 100% cc UaJ. DEC JAN FE3 1981 1982 Figure 19. Estimated percent hatch decimation of razorback sucker eggs for each day of the spawning season in Lake Mohave during December 1981-February 1982. VI in cc tu t- tu 3 UJ < § O I- SJ 70% U o 86% oUJ cc X u.< U I- 1- 96% UJ 100% Ua: Ixl MAR APR MAY ' 1982 Figure 20. Estimated percent hatch decimation of razorback sucker eggs for each day of the spawning season in Lake Mohave from March 1982- May 1982. VJl LAKE MOHAVE WATER IEVEL FLUCTUATION (METERS) W ti — 6 _ fO W c • -I * ^ * —• CD m 2 fn vr> o> in So GO o rt **• N> 3" ~ • i 3 * -n o- Q) * CD Q) rt cr~< CD ^• n Q. £ C O « Q) -h~O * n CD ' 9 •< rt T 1 3- n — CD CD * VD 3 * * OO I/) rt ^ VjO TJ * • Q) IT * i 3 rt <- • — n fc • * 33- « ' O. •* in (T> CD 0 •* • Qj _. * in 3 1 • O 03 £ Ij . 3 rt • -• o' 5 _ * 33 (o 1 * * I— O °* i 4 Qi -n £ CD n i * Q) f II 2 N j * 0 O 1 • cu cr nl < Q) CO. *• CD n ; i% O c ". 3 0 v 7T * C7 CD ~ 1 • CD n n CD CD 1 3 10 crto CD 01 1 0 O & y »« * PERCENT HATCH DECIMATION Id C LAKE MOHAVE WATER LEVEL FLUCTUATION (METERS) ro N5 4s. o< fo ^ Q) -ti 7? CD "I Ol • 3 N « O O •> =T -! Q> CT < OJ CD O * i * 7? o -ti • •n in • O C * e : * 3 O • * 7T • 2 CD Q) T * •. • O CD k 0-UD * IQ .' • — in • VD • CO -h * VjO O * 5 Table 8. Estimated percentages of spawning effort related to depth in Yuma Cove, AZ. Data on redd density are from Mueller et al. 1982. Observation data were collected by us. (*) (%) Depth (m) Observations Redd density estimate 0.6 35 43 0.9 60 70 1.7 90 86 2.5 - 96 3.4 100 100 61 Wind and wave action also seemed to have a pronounced Influence on spawning success. The storm that hit Lake Mohave In March 1983 was so severe that spawning was curtailed in the Six Mile Coves area. The fact that larvae were not collected in this area during March suggests that mortality was extremely high during the storm. The basin areas were extremely vulnerable to wave action due to the long fetch in the reservoir. Shifting sand and gravel substrates and movement of finer materials could result in suffocation and/or abrasion of eggs and larvae in the spawning areas. Shifting gravel could also have trapped larvae and prevented their emergence at swim-up, adding to the storm related mortality. The frequency, duration, and magnitude of storms and high winds seemed to be major factors influencing spawning activity and success of razorback suckers. The incidence of predation on egg and larval stages of razorback suckers was also examined in the study. Razorback sucker eggs were found in the stomachs of a few channel catfish (Table 9). No other species contained either eggs or larvae. This was surprising because with the high potential fecundity of the razorback sucker and the large numbers of spawning individuals, predation should easily be documented if it occurred to any great extent. Carp were observed feeding in redds immediately following spawning. It appeared that carp would be a major source of mortality at the egg stage during the parts of the season when they were abundant in the spawning areas. No eggs were found in any carp examined; however, one carp had a milky substance of unknown origin in its stomach. The stomach analyses were performed on fish captured during trammel netting in spawning areas. It is possible that eggs could have been regurgitated or digested while carp were in nets. The large pharyngeal teeth used for grinding food material by the carp (Scott and Table 9. Occurrence of razorback sucker eggs or larvae in stomach of potential predators in Lake Mohave. No. Stomachs Number of Stomachs Stomachs Containing Stomachs Containing Species Sampled from RZ spawning areas Eggs Larvae No. % No. % Carp 201 56 000 0 Largemouth Bass 75 43 000 0 Channel Catfish 63 33 4 12.1 0 0 Trout* 51 24 000 0 Striped Bass 4 2 0 00 0 * Rainbow and Cutthroat N> 63 Grossman 1973) may also have prevented adequate analyses of stomach contents. It also is probable that predation intensities were not as high, at least through the egg stage, as indicated by observations from the shore. The lack of eggs in any carp stomach while intense spawning activity was in progress and the large numbers of larvae collected in the Six Mile Coves area in February, 1983 support this interpretation. 4.5 PopuI at ion Size and Status Razorback suckers were abundant in Lake Mohave during the winter and spring months of 1982 and 1983. A total of 626 males and 307 females were netted and tagged or clipped during the two year study. Only seven razorbacks were recaptured during this time, and one of these was a recapture from a previous study (Table 10). The small number of recaptures indicates that a large population exists in Lake Mohave or that there was a high mortality of marked fish during this study. There was no evidence of any significant mortality in the population during our study so recapture rates are assumed to be representative. Population estimates were not attempted because the low number of recaptured fish would make the estimates meaningless, however, it is evident that the population is large. BonytaiI chubs were rare in Lake Date Location Date Location Tag No. Marked Marked Recaptured Recaptured or Clip 1/18/82 Arizona Bay 1/26/82 Arizona Bay UCF 2/27/82* Yuma Cove 2/27/82 Yuma Cove 8151 1/26/82 Arizona Bay 2/27/82 Arizona Bay 8047 1/24/82 Six Mile Area 2/24/82 Six Mile Area 5576 3/20/82* Six Mile Area 3/21/82 Six Mile Area 8416 3/20/82* Six Mile Area 3/21/82 Six Mile Area 8347 3/20/82* Six Mile Area 3/21/82 Six Mile Area 8461 2/24/82 Yuma Cove 3/17/82 Yuma Cove 8164 2/24/82 Yuma Cove 3/18/82 Yuma Cove 8164 2/27/82 Yuma Cove 3/17/82 Yuma Cove 8387 3/19/82 Six Mile Area 1/24/83 Six Mile Area LCF . * Indicates fish recaptured during same sample period. LCF Lower caudal fin clip UCF Upper caudal fin clip 65 5.0 DISCUSSION Bonytail chubs once were considered numerous In the lower Colorado River (Dill 1944) and common in Lake Mead and Lake Mohave (Moffet 1942, 1943; Jonez and Sumner 1954, LaRivers 1962). The sampling conducted by Minckley (1983) during 1974-1982 indicates that bonytails are now rare in Lake Mohave. Similarly, we captured only eight bonytails during our study, despite extensive sampling in several locations of the reservoir. These all were captured in shallow water ( <2m) over flat, shelf-like, gravel shorelines or point bars in the Six Mile Coves area of Cottonwood Basin (NV). The bonytails that we captured in March were heavily infested with Lernaea cyprinacea and bacteria and were in poor condition. Those captured in April were in much better general condition, and they appeared to be nearing ripeness. These individuals were transported to the Willow Beach Hatchery and later were used successfully as brood stock. Jonez and Sumner (1954) observed about 500 bonytails spawning on a gravel shelf near Eldorado Canyon in May, 1954. They did not observe any . young bonytails and presumed that carp ate most of the eggs. We did not observe any spawning behavior by bonytails in the reservoir. The habitat in the Six Mile Coves area is similar to that in Eldorado Canyon. It is probable that the bonytails we captured in April were preparing to spawn. However, there is no evidence to indicate that successful reproduction has occurred in recent years. Bonytail chub juveniles recently were stocked into Lake Mohave in an effort to restore the population (Johnson and Rinne 1982). Re-stocking may provide the only rro^rs of pr-2venting extirpation of the species from Lake Mohave. Razorback suckers were abundant in Lake Mohave during the 1950's 66 (Wai I Is 1951; Jonez and Sumner 1954). Jonez and Sumner (1954) reported that razorback suckers made up 5.2% of the gill net catch in Lake Mohave during 1950-1954. Razorbacks also were abundant and comprised about 12.5? of the fish caught in trammel nets in the period from 1974-1982 (Mi nek ley 1983). The cove surveys and trammel netting that we conducted show that razorbacks perhaps are more abundant than reported in previous studies. Thousands of razorback suckers were observed spawning in Lake Mohave during the winter and spring of 1982 and 1983. The largest populations occurred in the Six Mile Coves (NV) area of Cottonwood Basin and in the Arizona Bay (AZ) area (including Yuma and Gold Coves) of Little Basin. Substantial populations also were observed in Great West Cove, AZ; Tequila Cove, NV; Airport Cove, AZ; Three Mile Flats, AZ; and Nine Mile Cove, NV. Mark-recapture studies also indicate that the razorback sucker population is large in Lake Mohave. We recaptured only a few marked razorbacks, despite the fact that nearly 1000 individuals had been tagged or fin-clipped in this and previous studies. There was no evidence of significant mortality in tagged, fin-clipped or other razorback suckers during the study. Due to the low number of recaptures, no meaningful population estimate could be derived from the mark-recapture study. Nonetheless, it seems clear that razorback suckers are abundant in the reservoir. Spawning activity by razorbacks has previously been observed in Lake Mohave (Jonez and Sumner 1954, Minckley 1983) and was widespread in the reservoir during 1982 and 1983. It occurred primarily in shallow waters ( < 2 m) over gravel substrates. The gravel shorel ines in Cottonwood Basin, gravel terraces of Yuma and Gold Coves, and the mouths 67 of large grave! washes, like Arizona Bay, were utilized extensively for spawning by razorbacks. Similar habitat selection has been reported for reservoir-bound razorback populations in the lower river (Douglas 1952, Jonez and Sumner 1954, Ulmer 1981). Razorback suckers seem to prefer areas with gravel substrates for spawning. The interstitial spaces in gravel provide excellent sites for egg deposition, and also may offer considerable protection for eggs during incubation. Loudermilk (1981) believes that the lack of interstitial spaces resulting from siltation and substrate compaction is a major cause for poor reproductive success of razorbacks in most reservoirs. Lake Mohave differs from other Colorado River reservoirs in that it contains extensive areas of gravel spawning substrates. These gravel areas are maintained either by inputs of course material from flashfloods through the large washes or by internal habitat regeneration during seasonal draw-downs of the reservoir. Water levels in Lake Mohave are generally drawn down 3-5 m during the summer when the reservoir is used to re-regulate Hoover Dam discharges. Shoreline wave action during sequential draw-downs lap gravel substrates free of silt and organic material and create extensive gravel shelves and terraces in the wind-exposed, main basin areas of the reservoir. Dessication further breaks up finer materials and helps prevent substrate compaction. Summer draw-downs also prevent the extensive colonization of macrophytes in the zone that razorbacks normally use for spawning during high-water periods of winter and spring. The unique hydrodynamics and morphometry of Lake Mohave thus interact to regenerate the gravel substrates and maintain ideal spawning habitat for razorback suckers. Razorbacks began moving into these spawning areas In November. This 68 coincided with the time period when water levels were increasing and temperatures and photoperiod were decreasing. Fa I I turnover had just occurred in the reservoir. The male/female sex ratios were high during November indicating that males moved into the spawning areas before the females. The males also were ripe earlier than females suggesting that an earlier stimulus triggers their spawning migrations. However, several ripe females also were captured in the spawning areas in November 1983. Hand stripped eggs from these females were artificially fertilized, and they developed and hatched normally. Therefore, some successful spawning may have occurred as early as November. Peak razorback sucker spawning occurred during January and February. Razorbacks were by far the dominant species in littoral regions of the reservoir during winter. Spawning was so intense in Yuma, Gold and Six Mile Coves during January and February that it was difficult to distinguish individual redds. Razorback sucker catch rates were high in the principal spawning areas of Six Mile Coves and Arizona Bay (Yuma and Gold Coves). Nearly all males and females captured during these winter months were ripe. The sex ratios decreased during the winter as more females moved into the spawning areas. Condition factors increased between fall and winter. Female fecundity in razorbacks averages about 103,000 eggs/female, and gonads generally comprise about 9% of body weight prior to spawning (Inslee 1982). A drop in condition factor should occur after spawning. However, condition factors remained high throughout January and February. It appears that a cyclical replacement of ripe versus spawned-out razorbacks occurs in the spawning areas during winter. Fish that are ripe and in more robust condition seem to sequentially replace 69 those that have spawned, thus maintaining high condition factors for the spawning population. This cyclical replacement hypothesis also is supported by the mark-recapture study. We never recaptured any tagged or fin^clipped razorbacks in a particular spawning area beyond a thirty day period. On a few occassions we recaptured fish that had been tagged and released in the same area the previous day. This suggests that razorbacks remain in the spawning area for at least a couple of days but not over a month. While in the spawning areas, it appears that individual males spawn repeatedly during the course of a day. Females also seem to spawn more than once, but this is difficult to determine from observations because they frequently move back to deeper water after a spawning act. By late-March, spawning activity had started to decline. Condition factors decreased during this period, suggesting that fewer first-time spawners were entering the spawning areas. Spawning continued through April and was fairly intense in some areas. By May, only a few scattered individuals remained in the spawning areas. Sex ratios again increased as most individuals remaining were males. Catch rates for razorbacks decreased in April, and carp were the dominant species in the spawning areas by May. Our laboratory experiments showed that a large percentage (20-60?!) of artificially fertilized razorback eggs hatch at ambient reservoir temperatures ranging from 10°C-22°C, There was no apparent relationship between hatching success and temperature above 12°C. However, rates of egg and larvae development were significantly influenced by temperature. Mean hatching times ranged from 17.5 to 22.1 days at 10°C; 10.7 to 11.4 days at 15°C and 6.6 to 7.0 days at 20°C. Hatching times were most 70 variable at the colder temperatures. Temperature-induced differences in rates of development seemed to have a considerable influence on susceptibility of eggs to various sources of mortality. It took about 20 days for eggs to hatch during winter when ambient reservoir temperatures averaged about 10°C. It is not uncommon to have one or several major storms occur during this time period. Lake Mohave is well known for its long north/south fetch and high winds. The Yuma and Gold Coves and Arizona Bay are strongly influenced by northerly winds, whereas southerly winds can have a major effect on the Six Mile Coves area. High winds caused considerable disturbance and erosion of substrates in these exposed spawning areas. On a few occassions, siltation completely covered redds in the shallow waters. Smaller, more localized storms resulted in temporary abandonment of spawning areas. Razorbacks resumed spawning after the small storms, but gravel substrates were generally covered with a layer of silt. Intense spawning activity and light winds usually flushed finer particles from the substrate thus restoring the spawning habitat. Large storms completely curtailed spawning in some areas of the reservoir. Such was the case in Six Mile Coves where razorbacks completely abandoned the area after a severe storm in March 1983. It is unknown whether razorbacks move to other areas and resume spawning during these situations. Although it was not possible to quantify the effects of wind and wave action on spawning success, it was clear from repeated observations that these were major sources of egg mortality. This was especially true early in the season when egg development was retarded by lower water temperatures. The shallow water spawning preference of razorback suckers also 71 rendered the eggs susceptible to mortality from water level fluctuations. Dessication of eggs is common in reservoirs, particularly small ones, like Lake Mohave, that are subject to more extreme water level fluctuations (Kroger 1972; I I'ina et al 1972; Walburg 1972; Gustafson 1975; and Mitchell 1982). Razorback eggs were exposed to potential stranding for up to twenty days during the colder winter months due to slow rates of development. Water levels in Lake Mohave can decrease by as much as 1 m during this period. We estimated that up to 10% of the spawn may have been decimated by draw-downs during several days in February and March of 1982 and 1983. Water levels generally were more stable or increasing slightly during April and May. Hatching times were about 10 days at 15°C and 6-7 days at 20°C. Reservoir temperatures averaged about 16°C in April and 22°C in May. In 1983, some egg mortality seemed to occur from draw-downs during these months, but it did not appear to be as severe as in earlier months. Predation on eggs and larvae is believed to be the principal factor limiting recruitment in the Lake Mohave razorback population (Minckley 1983). Razorback sucker eggs previously have been reported in stomachs of carp in Senator Wash Reservoir (Ulmer, pers. comm.). Carp frequently were observed in the spawning areas of Lake Mohave and some appeared to be feeding in razorback sucker redds. 'However, there was no evidence of significant egg or larval predation from analysis of predator stomach contents. Eggs were not among the contents of 201 carp stomachs that we examined. The stomach contents in one carp was milky in appearance and this could have been derived from macerated eggs. Eggs were found in only four of 63 catfish stomachs that we analyzed. Egg predation certainly occurs, but it is doubtful that it is a 72 major source of mortality. Predators were not abundant In the principal spawning areas during winter and early spring when peak spawning occurred. Moreover, the eggs would be extremely difficult to prey on since the violent spawning behavior of razorback females appears to deposit eggs deep in the gravel substrates. Scoppetone (pens, comm.) found cui-ui sucker (Chasmistes cuj'us) eggs buried as deep as 8 cm in gravel substrates following spawning. Eggs of the largescale sucker also become buried in the substrate (McCart 1970). Despite the very intense razorback spawning activity, eggs rarely were observed on the surface of redds by divers, and only a few were collected in substrate samples (Mueller et al 1981). In the laboratory, fertilized razorback sucker eggs deposited on gravel quickly fell into interstitial spaces and became attached to the substrate. This would afford considerable protection of the eggs from predators during the incubation period. The eggs are still extremely vulnerable to suffocation by siltation, abrasion from shifting substrates and dessication from reservoir draw-downs. This combination of factors seems to exert considerable mortality on eggs early in the spawning season. Successful reproduction, nonetheless, occurs as reflected by the large number of larval razorbacks collected in the spawning areas. We captured several hundred razorback larvae in the spawning areas of Six Mile Coves and Arizona Bay during winter and early spring of 1983. Larvae were never observed during the day, despite intensive diving efforts; nor were they collected in meter nets towed over the spawning areas. Larvae were easily collected at night with artificial lights and a hand dip net. Larvae were most abundant along shorelines with gravel substrates. This apparently was the result of more extensive use of gravel for 73 spawning and the residence of larvae in the substrate during early development. Most of the larvae collected were in the protolarval and mesolarval stages of development. It appeared that the larvae either moved from the spawning areas, or did not survive after the mesolarval stage. Catostomid larvae migrate soon after the swim-up stage of development, and do so almost exclusively at night (Geen et al. 1966; Bailey 1969; Clifford 1972; Schneberger 1977; Scoppetone et al. 1981). This corresponds to the stage of development in which razorback larvae in Lake Mohave were no longer found in our collections. It is, of course, possible that larvae did not survive beyond the swim-up stage. Minckley (1983) believes that significant predation also occurs on the larval stages of razorbacks. Juvenile razorbacks were not observed or collected in our study or in recent studies (Minckley 1983). However, Jonez and Sumner (1954) observed what they believed were young razorbacks in coves near Eldorado Canyon. In the early 1950's, 6100 young-of-the-year razorback suckers also were collected near Cottonwood Landing (Minckley 1983). None has been collected since that time. Most of the recent sampling in Lake Mohave has consisted of seining or trammel netting in shallow waters during the spawning season. With a few exceptions, the trammel nets used by Minckley (1983) and by us have been large mesh and would not adequately sample juvenile or sub-adult razorbacks. Moreover, almost all the sampling has occurred during the spawning season in spawning areas where only adult fish would be expected. The smallest razorback that we captured in the trammel nets •..•->- •=. .-150 mm (T.L.), •- ....."••• Hiili V l • L . Male razorbacks from the 1981 year class that were reared at the Dexter National Hatchery ranged from 320-435 mm (T.L.) after two years of growth. Females from the same cohort ranged from 336-456 mm (T.L.) (Appendix 5). The males were sexually mature at two years of age, but females would not reach maturity for another year (Appendix 5). This difference in the rate of sexual maturation seems to explain the fact that 400-500 mm (T.L.) females were not captured in the spawning grounds, even though several males in this size range were captured during the study. Based on growth rates and reproductive development of hatchery fish, it is conceivable that the smallest razorbacks that we captured in Lake Mohave were 2-3 years of age. The age structure of razorbacks in Lake Mohave has never been established because of difficulty in reading the scales (McAda and Wydoski 1980; Minckley 1983). Minckley (1983) believes that the razorback population represents a single cohort of 30 year old fish that resulted from a successful spawn shortly after the reservoir was formed in 1950. Otol ith readings made on 17 razorbacks collected in Lake Mohave (Taubert AZGF, in Ulmer, February 1983 Newsletter) indicate that those fish were 30-32 years of age. No data were presented on the size or condition of the fish. A large percentage of the razorbacks collected in Lake Mohave are blind and diseased and appear to be old. Minckley (1983) reported that 28.8$ of the fish examined during 1975-1982 were blind in one eye and 6.8$ were blind in both eyes. We found that 30$ of the razorbacks were blind in one eye and 10$ were blind in both eyes. The incidence of external infections and parasitism on razorbacks during our study was also similar to that reported by Minckley (1983). However, the incidence of these afflictions varied seasonally and did not seem to be related to 75 age or size. Sterility occurred in 17.5$ of the female razorback sucker brood stock taken to the Dexter National Fish Hatchery from Lake Mohave in 1982 (Inslee 1982). These fish apparently all appeared old and many had atrophied ovaries. The razorback sucker undoubtedly is a long-lived fish, and many large, old-looking individuals exist in the population. Yet, it is difficult to conceive that they are all 30 years of age given the healthy appearance and size of numerous individuals in the population. Even allowing for vast differences in growth rates, it seems unlikely that a 400-500 mm (T.L.) sexually mature, healthy wild fish can be 30 years old when hatchery-reared fish achieve this size and condition in 2-3 years. The size variation in razorbacks may reflect adaptations of ''slow" and "fast" growing fish in the same cohort to different historical discharge regimes in the river (Minckley 1983). The "fast" growing, large genotype supposedly does better in wet, high discharge years; whereas, the "slow" growing, small genotype might survive better during prolonged droughts. It is difficult to evaluate this hypothesis because information on razorbacks is virtually lacking during the pre-impoundment period. However, even if such selection were operating historically, it seems it would be difficult to sustain in the regulated river system. Discharges in the river reach of present-day Lake Mohave have been regulated by Hoover Dam since 1935. Lake Mohave was not completed until April, 1949 and filling was not started until January 1950 (USGS data). During this 15 year period, discharges in the river changed considerably from those during the pre-impoundment period. Monthly discharges from 76 Hoover Dam were virtually constant at 0.5 million acre-feet during 1935-1940, when Lake Mead was filling. Discharges were increased again during the 1941-1951 period; but, except for 1941-42 when flood control releases were necessary, monthly and annual variations in discharges were minimal. The completion of Davis Dam in 1951 allowed for more operational flexibility at Hoover Dam. During the early 1950's, discharges from Hoover Dam typically were low during fall and winter and high in spring and summer. Water levels in Lake Mohave generally were high during winter and early spring and low during summer and fall, much I ike they are now. It is difficult to predict how razorbacks would have responded to such systematic changes in the hydrologic regime in the river. It seems that selection for the "slow" growing, small genotype would have been reduced in the relatively "wet", regulated river, or the "wet", stable reservoir. The genetic composition of the parental stock for the 30 (or so) year old cohort that Minckley (1983) believes comprises the Lake Mohave population obviously is uncertain, and may have little to do with size variations in the population. Size variations could simply reflect the fact that there is recruitment in the population. It need not be large to sustain, or even increase the population of a long-lived fish like the razorback sucker. If no recruitment existed in this population one would expect a progressive increase in the "old age" afflictions, like blindness and sterility and a decrease in the size variations as members of the population aged and died. No such trends are evident during the period from 1974-1983. Moreover, there is no indication that the population has decreased in abundance; in fact, razorbacks seem to be more abundant now 77 than was Indicated by Jonez and Sumner (1954) in the 1950's. The apparent lack of juvenile or subadult razorbacks has been cited as evidence that recruitment is non-existent (Mi nek ley 1983). Subadult razorbacks are known to seek refuge in dense macrophyte beds in artificial hatchery ponds (Inslee pers. comm.). Young razorbacks reared in a stream/pond at UNLV spent most of their time in dense Chara sp. beds. They were virtually impossible to view, seine or net without removing the macrophytes. Extensive macrophyte beds developed offshore from the Yuma and Gold Coves and the Six Mile Coves area during the summer of 1982. The macrophytes, mainly MyriophylI urn sp. and Potomegeton sp., grew along the "silty" bottom edges of gravel terraces and shelves when water levels were low during summer. During winter, when water levels were high, the macrophyte beds were submerged 5-10 m and were not visible from the surface. The macrophyte beds would seem to provide ideal cover for young razorbacks. An extensive program would be required to adequately sample these areas because of the dense cover provided by the macrophytes and the sedentary behavoir of razorbacks. The questions regarding recruitment, unfortunately, cannot be resolved even if young razorbacks were collected in the reservoir. Several thousand juveniles were recently stocked in Lake Mohave by the U.S. Fish and Wildlife Service. Although these fish were fin-clipped and branded, there is still a possibility that some were missed or that fins will regenerate. In addition, there was an accidental release of 354 razorbacks, some as small as 80 mm, from the Willow Beach Hatchery in 1978 (J. Johnson, pers. comm.). The occurrence of small individuals in the population may be due to such releases rather than to natural production. 78 the age structure of the population must be established In order to adequately evaluate recruitment. Major emphasis urgently needs to be placed on developing an accurate aging technique. OtolIth or opercular readings may provide the degree of resolution required to age this long-lived species. However, it is essential that a large, cross-section of the population be sampled for such analysis. Efforts should be made to collect as many fish as possible from different areas of the reservoir during different time periods. This sampling should include the population that exists in the river below Hoover Dam. Hatchery-reared razorbacks of known age should be aged to evaluate the reliability of the technique. 79 6.0 LITERATURE CITED Allan, R.C. and D.L. Roden. 1978. Fish of Lake Mead and Lake Mohave. Biological Bulletin No. 7. Nev. Dept. Wildlife, Reno, Nevada. Bailey, M.M. 1969. Age, growth, and maturity of the longnose sucker (Catostomus catostomus) of western Lake Superior. J. Fish. Res. Bd. Can., 26(5):1289-1299. Baker, J.R. and L.J. Paulson. 1980. Evaluation of possible temperature fluctuations from proposed power modifications at Hoover Dam. Lake Mead Limnological Research Center, Tech. Rept. No. 3. 23 pp. Bryant, G.L. 1977. Colorado River fish inventory. U.S. Bureau of Reclamation. Unpubl. Rept. Clifford, H.F. 1972. Downstream movements of white sucker (Catostomus commersoni) fry in a brown-water stream of Alberta. J. Fish. Res. Bd. Can., 29(7)1091-1093. Dill, W.A. 1944. The fishery of the lower Colorado River. Calif. Fish Game, 30:109-211. Douglas, P.A. 1952. Notes on the spawning of the humpback sucker, (Xyrauchen texanus, Abbott). Calif. Fish Game. 38:149-155. Geen, G.H., T.G. Northcote, G.F. Hartman and C.C. Lindsey. 1966. Life history of two^species of catostomid fishes In Sixteen Mile Lake, 80 British Columbia, with particular reference to Inlet stream spawning. J. Fish. Res. Bd. Can., 23(11):1761-1788. Gustafson, E.S. 1975a. Capture, disposition and status of adult razor- back suckers from Lake Mohave, Arizona. Quart. Rept. U.S. Fish Wildl. Sen., Contr. No. 14-16-0002-3585, Ariz. State Univ., Tempe, AZ. Gustafson, E.S. 1975b. Early development, adult sexual dimorphism, and fecundity of the razorback sucker (Xyrauchen texanus, Abbott). Final Rept. U.S. Fish Wildl. Serv., Contr. No. 14-16-0002-3585, Ariz. State Univ. Tempe, AZ. Heming, T.A., J.E. Mclnerney and D.F. Alderice. 1982. Effect of temperature on initial feeding in alevins on Chinook salmon (Oncorhynchus tshawytscha). Can. J. Fish. Aq. Sci. 39 12:1554-1562. Holden, J.P. and C.B. Stalnaker. 1975a. Distribution and abundance of mainstream fishes of the middle and upper Colorado River Basins, 1967-1973. Trans. Am. Fish. Soc., 104:217-231. Holden, J.P. and C.B. Stalnaker. 1975b. Distribution of fishes in the Dolores and Yampa River systems of the upper Colorado basin. Southwest. Nat. 19(4)-.403-412. :- .-.;, C.L. and P..R. Millar. 1933. h/br i c' zar i (_.. ;.. ..,:.,:- _.,tween fish genera Catostotnus and Xyrauchen. Mich. Acad. Sci., Arts, Lett. 38:207-233. 81 ll'ina, L.K. and N.A. Gordeyev. 1972. Water-level regime and the spawning of fish stocks In reservoirs. J. Ichthy. 12(3):373-533. Inslee, T.D. 1982. Spawning and hatching of the razorback sucker (Xyrauchen texanus). Proc. West Assoc. Johnson, J.E. and J.N. Rinne. 1982. The endangered species act and southwestern fishes. Fisheries, 7:1-10. Jonez, A. and R.C. Sumner. 1954. Lakes Mead and Mohave investigations: A comparative study of an established reservoir as related to a newly created impoundment. Final Rept., Fed. Aid Proj. F-1-R. Nev. Fish Game Comm., Reno, Nevada. Kramer, R.H. and L.L. Smith, Jr. 1962. Formation of year class strength in largemouth bass. Trans. Am. Fish. Soc. 91(U:29-41. Kroger, R.L. 1973. Biological effects of fluctuating water levels in Smoke River, Grand Teton National Park, Wyoming. Am. Midi. Nat. 89(2):487-481. Lanigan, S.H. and C.R. Berry, Jr. 1981. Distribution of fishes in the White River, Utah. Southwest. Nat. 26(4):389-393. LaRivers, I. 1962. Fishes and Fisheries of Nevada. Nev. State Fish Game Comm., State Print. Off., Carson City, Nev. Loudermilk, W.E. 1981. Aspects of razorback sucker (Xyrauchen texanus, 82 Abbott) life history which help explain their decline. Desert Fishes Council, Thirteenth Annual Symposium. McAda, C.W. and R.S. Wydoskl. 1980. The razorback sucker (Xyrauchen texanus) in the upper Colorado River basin, 1974-76. Tech. Pap. U.S. Fish WiIdl. Sen., 99:1-15. McCart, P. 1970. Spawning habits of the largescale sucker (Catostomus macrocheiI us), at Slave Lake, British Columbia. J. Fish. Res. Bd. Can., 527(6)-.1154-1158. McSwain, L.E. and P.M. Gennings. 1972. Spawning behavior of the spotted sucker (Minytrema melanops, Rafinesque). Trans. Am. Fish. Soc. 191(4):738-740. Miller, R.R. 1961. Man and the changing fish fauna of the American Southwest. Pap. Mich. Acad. Sci., Arts, Lett. 46:365-404. Minckley, W.L. 1973. Fishes of Arizona, Ariz. Game Fish Dept., Phoenix, AZ. Minckley, W.L. 1983. Status of the razorback suckers (Xyrauchen texanus, Abbott), in the lower Colorado River basin. Southwest. Nat. 28(2) :165-187. Minckley, W.L. and J.E. Deacon. 1968. Southwestern fishes and the enigma of "endangered species." Science. 159:1424-1432. 83 Mitchell, O.F. 1982. Effects of water level fluctuation on reproduction of largemouth bass (Micropterus salmoides), at Millerton Lake, California in 1973. Calif. Fish. Game, 68:68-77. Moffet, J.W. 1942. A fishery survey of the Colorado River below Boulder Dam. Calif. Fish Game, 28:76-86. Moffet, J.W. 1943. A preliminary report on the fishery of Lake Mead. Trans. Eight. N. Amer, Wildl. Conf. 179-186. Mueller, G., B. Rinne, and T.Burke. 1982. Spawning behavior and habitat selection of razorback suckers (Xyrauchen texanus) observed in Arizona Bay, Lake Mohave, Arizona, U.S. Dept. Int. Bur. Rec. Agency Report. Nie, N.H., C.H. Hull. J.G. Jenkins, K. Steinbrener, and D.H. Bent. 1975. Statistical package for the social sciences. 2nd. ed. McGraw-Hill, Inc., New York. Paulson, L.J., T.G. Miller, and J.R. Baker. 1980a. Influence of dredging and high discharge on the ecology of Black Canyon. Lake Mead Limnological Research Center. Tech. Rept. No. 2. 59 pp. Paulson, L.J., J.R. Baker and J.E. Deacon. 1980b. The limnoloigcal status of Lake Mead and Lake Mohave under present and future powerplant operations of Hoover Dam. Lake Mead Limnological Research Center. Tech. Rept. No. 1. 229 pp. 84 Seethaler, K.H., C.W. McAda and R.S. Wydoski. 1979. Endangered and threatened fish in the Yampa and Green Rivers of Dinosaur National Monument. Pages 605-612 In: R.M. Linn, ed. Proceedings of the first conference on scientific research in national parks. U.S. Dept. Int., Nat I. Park Serv. Trans, and Proc. No. 5. Schneburger, E. 1977. The white sucker. It's life history, ecology, and management. Wise. Dept. Nat. Resources Pub. 13-3600(77). Madison, Wisconsin. Scoppettone, G.G., M. Coleman, H. Burge, and G. Wedemeyer. 1981. Cui-ui life history: river phase. Ann. Rept. U.S. Fish Wildl. Sen., Nat. Fish. Res. Ctr., Seattle, Washington. Scott, W.B. and E.J. Grossman. 1973. Freshwater fishes of Canada. Bull. Fish. Res. Bd. Can. 966 pp. Sigler, W.F. and R.R. Miller. 1963. Fishes of Utah. Utah State Dept. Game and Fish, Salt Lake City, Utah. 203 pp. Snyder, D.E. 1981. Contributions to a guide to the cypriniform fish larvae of the upper Colorado River system in Colorado. Bur. Land Mgmt. Bio. Sci. Series No. 3., Denver, Colorado. Suttkus, R.D. and G.H. Clemmer. 1979. Fishes of the Colorado River in Crand Canyon National park. Natl. Park Sen., Trans. Proc. Ser. 5:599-604. 85 Toney, D.P. 1974. Observations on the propagation and rearing of two endangered fish species in a hatchery environment. Proc. West. Assoc. State Game Comm., 54:252-259. Tyus, H.M. 1982. Green River fishery investigations: 1979-1981. J_n: Colorado River Fishery Project, Part 2, 1982. U.S. Fish Wildl. Sen., Salt Lake City, Utah. Ulmer, L. 1981. Preliminary results of studies on movement and reproduction of the razorback sucker inhabiting Senator Wash Reservoir, California. Calif. Fish and Game, Unpublish. Rept. Ulmer, L. 1983. Endangered Colorado River fishes newsletter. 1:12 pp. Valdez, R. 1982. Tributary report: fishery investigations of the Gunnison and Dolores Rivers. In; Colorado River Fishery Project, Part 2, 1982. U.S. Fish Wildl. Sen., Salt Lake City, Utah. Vanicek, C.D., R.H. Kramer and D.R. Franklin. 1970. Distribution of Green River fishes in Utah and Colorado following closure of Flaming Gorge Dam. Southwest. Nat., 14:297-315. Walburg, C.H. 1972. Some factors associated with fluctuation in year class strength of sauger, Lewis and Clark lake, South Dakota. Trans. Am. Fish. Soc. 101(2):311-316. Wai I is, O.L. 1951. The status of the fish fauna of the Lake Mead Recreation Area, Arizona-Nevada. Trans. Am. Fish. Soc. 80:84-92. 86 APPENDICES 87 APPENDIX 1 SPECIES DATA CODE SHEET Columns Data Parameter Codes 1-6 Month-day-year As presented 8-9 Location 01 Arizona Bay, AZ 02 Yuma Cove, AZ 03 Gold Cove, AZ 04 Six Mile Coves Area, NV 05 Tequila Cove, NV 06 Great West Cove, AZ 07 Placer Cove, NV 08 Tyee Cove, NV 09 Sidewinder Cove, AZ 10 Stop Sign Cove, AZ 11 Car Cove, AZ 11-12 Fish species 01 Razorback sucker 02 Carp 03 Largemouth Bass 04 Channel Catfish 05 Rainbow Trout 06 Cutthroat Trout 07 Bonytail Chub 08 Striped Bass 09 Threadfin Shad 10 Razorback sucker - flannel' mouth sucker hybrid 11 Bluegill Sunfish 14 Sex 0 Unknown 1 Male 2 Fema1e 16-18 Standard length (mm) 20-22 Forked length (mm) 24-26 Total length (mm) 28-31 Weight (gm) 33-36 Tag 0001 Lower caudal fin clip 0002 Upper caudal fin clip Other numbers as indicated on Carli n tags APPENDIX 1 (continued) Columns Data Parameter Codes 38 Ripeness 0 Not ripe 1 Ripe 2 Unknown kO Left eye condition 0 Good condition k2 Right eye condition 1 Cloudy-bulge 2 Reduced pupil 3 Blind k Missing eye kk Gross external bacteria 0 Absent 1 Present 46 Lernaea spp. 0 Absent 1 Present 48 Other Not specified en oo C3 o 1-1 o CD o I-H o cr -»-i .j cr o o »-4 CD c: CDOO 1*5 CM if> CD CD CD o o CD CD i~( o CP co N- '.C -t r*r> PJ o c: CMo o in u\s CM \ IT! 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O CD CD ^-t r-- APPENDIX 2 COVE SURVEY COVE SURVEY-LITTLE BASIN February 21, 1982 0900-1230 Hrs, Cove Substrate Vegetation Fi sh Rel. Abundance (AZ) Arizona Bay East st, some b tk several carp no RZ West tk hundreds of RZ at middle of wash Yuma Cove g, sd none over 100 RZ along entire terraci ng shoreline several carp Gold Cove sd , smal 1 g none 25-30 RZ Jeff Davis Cove sd, fa- tk carp very abundant 4 RZ in long arm of cove Castle Cove st, b several carp no RZ Mile 26 b, some g sparse tk some carp, no RZ Owl Cove sd-g-c mix beaches; some tk 1 RZ over gravel st surface substrate Twin Coves sd, g beaches some carp st surface substrate Owl Point cove sd, g beaches tk a few carp 1 RZ COVE SURVEY-LITTLE BASIN (Continued) Cove Substrate Vegetation Fi sh Rel. Abundance (NV) Opel Cove sd with st exposed g tk several carp 7 unidentified fish - believed to be RZ turbid water Basalt Cove c, sd possible 6 RZ Nevada Bay sd, g, c, dense tk scattered carp b mix Several small coves sd, g, c, dense tk scattered carp (near mile 26) b mi x Two Dollar Cove dense tk few carp Copper Mtn. Cove sd, st, c sub carp abundant Legend b = boulders c = cobble g = gravel sd = sand st = silt tk = tamarisk oo COVE SURVEY-LOWER COTTONWOOD BASIN £ KATHERINE LANDING February 26, 1982 0830-1230 Hrs. Cove Substrate Vegetation Fi sh Rel. Abundance (NV) Empi re Cove sd with fast dropoff sub shoreline none (at mile 52) to st veg Rattlesnake Cove varied Ig c deeper none c, sd, st Area between Roman c, b none & Turtle Coves heavy periphyton (at mile 56) growth Telephone Cove bk none (steeper coves at sides) (At base of Wash) sd, some g shoreline several carp 50 RZ (AZ) Katherine Landing Ig sd area none Cove Island by Katherine sd, c none Landi ng Telephone Cove c & g 60 RZ spawning on S. South pockets shore none on East shore North sd, c none u> COVE SURVEY-LOWER COTTONWOOD BASIN & KATHERINE LANDING (continued) Cove Substrate Vegetation Fish Rel. Abundance (AZ) Shoshone Cove c, sd point - 35-^0 RZ Redlight Cove sd - 2 RZ (South of Redlight) Point North of Red- b, sd interspersed - 180 RZ 1ight Cove (at 6 mile 1ight) Chili Pepper Cove g, c, sd - 50 RZ (in South area) (1st cove North of Redlight) Sidewinder Cove Varied - Several carp Topography of coves between Sidewinder and the Cottonwood Basin is canyon-like and was not surveyed. Glory Hole Area c sub none South North c, periphyton sub 10-15 RZ scattered 3 Mile Flat Area sd, c, g sub veg (trees) none South Middle g, c, sd, trees kO RZ (within trees) g, c none 150-200 RZ fo o COVE SURVEY-LOWER COTTONWOOD BASIN & KATHERINE LANDING (continued) Cove Substrate Vegetation Fi sh Rel. Abundance (AZ) Area is extensive and believed to have many more spawning RZ all along shore of this flat area. Cobble-'gravel-sand substrate are present. Some areas more populated than others but fish interspersed in both inundated vegetation and open areas. Vegetated areas not silted in. Note: Area not well surveyed due to dense tree concentration in water. Waterwheel Cove Varied Heavy cone trees None Carp Cove Difficult to survey due to wind and waves. COVE SURVEY-OPPOSITE COTTONWOOD COVE February 25, 1982 9099-1100 Hrs. Cove Substrate Vegetat ion Fish Rel. Abundance (AZ) 2 small coves across c areas none from Cottonwood Cove South of 2 coves sd wi th c none Levi Cove g terracing 1000-1500 RZ (very heavy cone) (limited spawning area) Next Cove South sd, c- area none smal1 c sect ion 150 RZ Large Shallow Cove c 1 RZ (South) Airport Cove g 30 RZ South of Airport Cove c, some g none (shallow coves) Carp Cove North shore sd, g several RZ Back of Cove g island 30 RZ South shore sd sub veg 10 RZ NJ fo COVE SURVEY-COTTONWOOD BASIN February 2k, 1982 1330-1500 Hrs. Cove Substrate Vegetation Fish Rel. Abundance (NV) Tequila Cove (at terraced g 300 RZ (mile 43) c interspersed South shore g, sd 80 RZ extending point c, g 30 RZ Small bay at extended c, g 100-120 RZ peninsula (between (redds between c on g) Sandy Point & Cot- tonwood Island Cove Cottonwood Island st organic several carp g, sd 20 RZ Mesquite Cove sd, g none Windmi11 Cove sd, g trees 60 RZ Six Mile Cove varied - 350-400 RZ g predominant 500-1000 RZ Stopped counting, to be surveyed later, Thousands of RZ suckers. Area so torn up by spawning that redds are indis- tinguishable. Note: Appears to be target Lake Mohave population. Hog Farm Cove g, sd 200-300 RZ Last small coves just sd, g + 600 RZ North of big open CO flat area COVE SURVEY-COTTONWOOD BASIN (continued) Cove Substrate Vegetation Fish Rel. Abundance (NV) Shore!ine of Bas in g, c + 250 RZ to 9 Hile Cove area Spawning occurring along shelf approximately 1 meter deep before dropping off. It extends approximately 6 meters from shore. Fish appear to be equal in density along this entire area. 2 coves South of c - 2 RZ Big Basin Nine Mile Cove st - c few carp Coves between 9 heavy sd-g 20 RZ Mile & Nell is 125 APPENDIX 3 Trammel Netting Catch Data 1982-1983 Appendix Table 3. Summary of trammel netting catch data in Lake Mohave during 1982 and 1983 (Catch expressed as total numberscaptured and catch pen unit effort, // fish/24 hr./lOO m7 net, in parenthesis). Net Area RZ RZ RZ Location Date Time Period Net Hours Carp LMB RT CC SB BT (m2) M F T Sidewinder Jan 28-29 1400 167.5 4 2 6 1. ' 1 5 0 0 0 Cove , Az . 1982 1100 58 (.99) (.49) (1.48) (.25) (.25) (1.24) Arizona Bay Jan 26 0800-2315 30.4 167.5 43 22 65 0 0 0 0 0 0 1 1982 (20.27) (10.37) 30.64 Arizona Bay Jan 26 0830-2230 28 167.5 15 13 28 0 2 - 0 0 0 0 2 (Yuma) 1982 ( 7.68)( 6.65) (14.32) (1.02) Arizona Bay Jan 26 1000-2000 10 167.5 35 17 52 0 0 2 3 0 0 3 (Cold) 1982 (50.15) (24.36) (83.10) (2.86) (4.29) Arizona Bay Jan 1982 Combined 68.4 167.5 93 52 145 0 2 2 3 0 0 Combined - 19.48 (10.89) (30.37) (.42) < .42) ( .63) Placer Cove Feb 1-2 1400-0830 54.5 167.5 1 1 2 10 2 0 1 0 0 NV 1982 ( .26)( .26) ( .53)(2.63) ( .53) ( .26) Tyee Cove Feb 2 1145-1450 10.3 167.5 0 0 0 0 0 0 0 0 0 NV 1982 Great West Feb 2-3 1430-0930 46 lf.7.5 23 13 36 0 0 0 0 0 0 Cove, Az. 1982 ( 7.16)( 4.05) (11.21) Arizona Bay Feb 15 1600-2030 4.5 167.5 27 16 43 0 1 1 0 0 0 3 (Gold) 1982 (85.97) (50.95) (136.9) (3.18) (3 .18) Cottonwood Feb 24 1400-2130 17 167.5 . 76 48 124 2 1 3 5 1 0 Basin 1982 (64.05) (40.46) (104.51) (1.69) ( -84.) (.253H4.21) (.84) Arizona Bay Feb 27 0845-2320 14.5 167.5 6 9 15 0 1 0 0 0 0 1 1982 ( 5.93)( 8.87)( 14.83) ( .99) ro Appendix able 3. (Cent.] Net Area RZ RZ RZ Location Date Time Period Net Hours Carp LMB RT CC SB BT (m2)'. M F T Arizona 1 .;y Feb 27 0910-1130 13.5 167.5 35 22 57 3 1 0 0 0 0 2 (Yuma) 1982 (37.15) (23.35) (60.49) (3.18) (1.06) Arizona s; iy Feb 27 0900-2115 12.25 167.5 16 12 28 0 . 2 0 3 0 0 3 (Gold) 1982 18.71 (14.04) (32.75) (2.34) (3.50) Arizona ray Feb 27 Combined 40.25 167.5 57 43 100 3 4 -0 3 0 0 Combined 1982 (20.29) (15 .31) (35.60) (1.07) (1.42) (1.06) Tequila Cove Mar 7 1530-2030 10.3 167.5 18 8 26 0 4 0 0 P 0 NV 1982 (25.04) (11.13) (36.17) (5.56) Arizona -.ay Mar 17-18 1100-1430 5.5 167.5 2 1 3 0 0 0 0 0 0 1 . 1982 ( 5.21) ( 2.61) ( 7.82) Arizona '5ay Mar 17-18 1030-1345 13.0 167.5 42 18 60 1 Q 1 1 0 0 2 (Yuma) 1982 0815-1700 (46.29) (19.84) (66.13) (1.10) (1.10) (1.10) Arizona Bay Mar 17-18 1045-1415 12.25 167.5 13 7 20 0 3 0 0 0 0 3 (Gold) 1982 0815-1700 (15.21) ( 7.84) (23.39) (3.52) Arizona Bay Mar 1982 Combined 30.75 167.5 57 26 83 1 3 1 1 0 0 Combined (26.56) (12.11) (38.67) (1.47) (1.40) ( .47) ( .47) Cottonw1:, id Mar 19-20 1405-1515 L17.8 167.5 96 30 126 15 10 3 2 0 1 Basin, ' 1982 (11.68) ( 3.65) (15.33) (1.82) (1.22) ( .37)( .24) ( .12) Cottonw i Mar 20-21 1735-0135 48 167.5 48 3 1 4 1 0 3 Basin, i,.' 1982 14.32 .90 .30 1.19 .30 ( .90) Cottonwood Mar 1982 Combined 165.8 167.5 174 18 11 7 2 4 Basin, Kv 15.04 1.46 .95 ( .61)( .17) ( .35) Combined Appendix Table 3. (Cont.) Net Area RZ RZ RZ Location Date Time Period Net Hours (m2) M F T Carp LMB RT CC SB BT Tequila Cove April 13-14 1715-2345 56.5 167.5 2 5 7 15 9 0 5 1 0 NV ** 1982 (.507) (1.27) (1.78) (3.86) (2.28) (1.27) .254 Scop S April 15-16 1705-07.5 82.25 167.5 0 0 0 5 8 8 0 0 0 Cove, Az. 1982 ( .87)(1.39) (1.39) Cottonwood Arpil 17 1645-2430 37 167.5 26 13 4 8 10 0 4 Basin, NV 1982 (10'.07) (5.04) (1.55) (3.10) (3.87) (1.55) Cottonwood May 17 1315-2245 .52.75 167.5 10 46 10 0 13 1 0 Basin, NV 1982 ( 2.72) (13.47) (2.72) (3.53) (.272) Cottonwood Mav 19 1215-2300 12.95 167.5 1 6 0 0 3 0 0 Basin, NV 1982 .( 1.106) ( 6.64) (3.32) Cottonwood May 1982 Combined 65.7 167.5 11 52 10 0 16 1 0 Basin, NV ( 2.40) (11.34) (2.18) (3.49) (.218) Combined Arizona Bay May 18-19 1705-2410 7.1 167.5 5 2 1 3 5 0 0 1 1982 (10.09) ( 4.04)(2.02) (6.05) (10.09) Arizona Bay May 18-19 1700-2430 21.95 167.5 7 14 1 2 2 0 0 2 (Yuma) 1982 1645-2200 ( 4.57) ( 9.14)( .65) (1.31) (1.31) 1640-0100 Arizona Bay May 18-19 1630-2300 6.5 167.5 13 16 1 0 2 0 0 3 (Gold) (28.66) ( 3.5) (2.20) (4.41) Arizona Bay May 1982 Combined 35.6 167.5 0 0 25 32 3 5 9 0 0 Combined (10.07) (12.90) (1.21) (2.015) (3.63) N> oo 'Appendix Table 3. (Cont.) Net Area RZ RZ RZ Location Date Time Period Net Hours (m2) M F T Carp LMB RT CC SB BT Carp Cove May 19 1800-2320 15.5 167.5 0 0 0 11 0 0 3 0 0 AZ 1982 (10.17) (2.77) Cottonwood Nov 27 1600-2000 8 167.5 38 10 48 2 0 0 0 0 0 Basin, NV 1982 (68.06) (17.9) (85.97) ( 3.58) . , Cottonwood Nov 27 0200-1200 24 37.16 5 2 8 0 2 2 Basin, NV 1982 13.45 5.38 21.53 5.38 61.89 Cottonwood Jan 25 1540-1930 19.68 167.5 53 33 86 3 0 0 1 0 0 Basin, NV 1983 1550-2530 (38.78) (24.15) (62.93) (2.19) ( .73) 1605-2410 Arizona Bay Jan 24 1425-2210 15.8 167.5 37 25 62 0 0 0 0 0 0 Combined 1983 1440-2240 (33.66) (22.74) 56.40 Arizona Bay Jan 24 1450-2305 8.25 55.7 5 2 .7 0 1 1 2 0 0 2 (Yuma) 1983 (26.11) (10.45) (36.56) (5.22) (5.22) (10.45) Cottonwood Feb 18 0900-1300 8 167.5 15 14 29 0 0 0 0 0 0 Basin, Nv 1983 (26.87) (25.07) (51.94) Tequila Mar 26 1230-2030 20 167.5 9 6 15 3 3 0 0 0 0 Cove, NV 1983 (6.45) (4.299) (10.75) (2.15) 2.15 Nine Mile May 6 0800-1200 17 167.5 8 3 11 7 5 2 1 0 0 Cove, NV 1983 1100-1830 . (6.74) (2.53) (9.27) (5.90) (4.21) (1.69) ( .843) 1320-1900 * Excludes deep set (20 carp 2 LMB 2 CC) ** All nets set at surface N> 130 APPENDIX A Razorback Sucker Redd Size, Depth, and Particle Size Distributions (from Mueller et al 1982). 131 APPENDIX k Scuba Observations Taken of Razorback Redds Redd Depth Substrate Redd No. (meters) Size (cm) Size Eggs 1 1.00 10.2-f 0.5m2 2 0.85 10.2-f 0.5m2 3 0.94 16.2-f 0.5m2 4 0.94 10.2-f 0.5m2 5 1.00 10.2-5.1-f 0.5m2 6 1.13 0.5m2 7 0.85 10.2-5.1-g 1.0m2 8 1.07 5.1-2.5-g 1.0m2 9 1 .00 b-7.6 0.5m2 10 1.07 b-7.6-2.5 0.5m2 X 11 1.00 15.2-7.6-f 1.0m2 X 12 1 .10 0.5m2 13 1 .00 7.6-2.5 0.5m2 }k 0.82 20.3-10.2-f 0.5m2 X 15 1 .00 15.2-5.1-g 0.25m2 16 0.98 I5.2-g_-f 1 .00m2 17 0.85 10.2-5.1-f 1.00m2 18 0.98 7.6-g 0.5m2 19 0.76 b-5.1 0.5m2 20 0.91 b-15. 2-5.1 0.5m2 X 21 0.82 b-7.6 0.5m2 22 0.58 7.6-1 0.5m2 23 0.88 15.2-7.6-g 4.0m2 X 2k 0.94 10.2-5.1 1.0m2 25 1 .00 7.6-f 0.5m2 26 0.79 7.6-g 0.5m2 27 0.82 b-5.1-2.5 0.5m2 28 0.94 7.6 0.5m2 X 29 0.82 b-7.6-2.5 0.5m2 30 0.94 b-7.6-_f 0 . 5m2 31 0.70 5.1-£-f 0.5m2 32 0.88 b-5.1-2.5 0.5m2 33 0.98 7.6-f 1 .Om2 132 APPENDIX 4 (continued) Redd Depth Substrate Redd No. (meters) Size (cm) Size Eggs 3* 0.64 9 0.5m2 35 0.91 7.6-g 0.25m2 36 1 .00 7.6-2.5-g 1.00m2 37 0.67 5.1-f 0.25m2 38 0.88 5.1-f 2.00m2 39 0.82 2.5-f 0.5m2 40 1.49 15.2-7.6-g 0.25m2 41 1.55 b^-10.2 0.5m2 x 42 1.31 £-f 0.25m2 43 1.43 15. 2 -5.1_-f 0.5m2 44 1.62 10.2-g 0.25m2 45 1.46 7.6-f 0.25m2 46 1.68 5.1-g 0.5m2 47 1.43 10.2-5.1 0.5m2 48 1.62 b-10. 2-5.1 0.5m 49 1.55 b-10.2 1.0m2 50 1 .49 b-10. 2 2.0m2 51 1.55 7.6-f 0.5m2 52 1.52 7.6-f 0.5m2 53 2.04 7.6-g 1.0m2 54 2.47 b-7.6-g 4.0m2 55 2.56 5.1-g 4 . Om2 56 2.49 15.2-2.5 4.0m2 57 2.38 15.2-5.1 1 .Om2 58 2.38 2.5-1 0.5m2 59 2.35 5.1-f 0.25m2 60 2.23 5.1-f 1 ,0m2 61* 3.26 - - 62 2.96 2.5-f 0.25m2 63 2.80 5.1-f 0.25m2 - 64 >v 3.08 - 65 3.44 10.2-f 0.5m2 66 * 3.90 - - 67* 3.84 - - 133 APPENDIX A (continued) Redd Depth Substrate Redd No. (meters) Size (cm) Size Eggs 68* 3.76 69- 6.7^ 70* 8.02 " Represents depths -- no redd f = fines g = gravel b = boulder Underlined size is major substrate component. Analysis does not include shoreline lap zone. APPENDIX 5 Dexter National. Fish Hatchery Two Year Old Razorback Sucker Lengths and Weights UNITED STATES GOVERNMENT U.S. FISH ft WILDLIFE SERVICE T0 . Regional Director, Region 2 DATS: April 20, 1983 Albuquerque, MM (SE) • Hatchery Manager, Dexter NFH Dexter, New Mexico SUBJECT: Status of two-year-old razorback sucker broodstock He just moved our two-year-old RBS brood from Pond 5C to Pond 10B and provide the following data for your perusal. We have a total of 318 fish; 190 females and 128 males. As reported to you earlier, nearly all of the males were sexually mature and running milt; micro- scopic examination showed the milt to contain viable spermatozoa. Examination of female ovaries showed developing eggs. We are confident they will spawn in 1984 at three years of age. We measured and weighed 51 individuals; the males averaged 392 mm and 706 grams and the females 404 mm and 859 cms. Individual measurements are listed on the attached page. These fish are beautiful, really beautiful. If some of the photos turn out we'll send you a copy (slides). Buddy Lee 'Jensen Attachment cc: George Divine W.O. Minckley Jim Brooks Paul Marsh /•""Jim Deacon Files 136 Table 1. Lengths (TL) and weights of SI two-year-old razorback suckers at Dexter NFH. Lengths in mm and weights in grams. Males (15) Females (36 Lt. Wt. Lt_._ 422 652 413 907 320 454 440 1162 323 369 396 765 334 510 395 794 415 794 419 822 415 794 381 595 422 822 392 794 327 425 367 652 364 624 430 936 410 709 378 737 418 822 384 851 435 936 411 851 432 992 456 1106 416 851 385 737 420 822 415 907 405 851 Mean 391.9 705.5 395 822 (15.7 inches) 8.9.oz) 420 879 438 1021 405 907 440 992 450 1134 392 879 405 851 430 964 420 964 373 595 336 482 366 765 409 879 438 992 393 822 375 794 358 624 420 879 404 737 Mean 403.8 859 (16.2 inches) (l lb, 13.8 oz) UNIVERSITY OF NEVADA, LAS VEGAS 4505 Maryland Parkway . Las Vegas, Nevada 89154 Lake Mead Limnoiogical Research Center (702) 739-3269 (702) 739-3399 March 13, 1984 Dr. W.L. Minckley Department of Zoology Arizona State University Tempe, Arizona 85281 Dear Mink: I have enclosed a copy of our report on the Lake Mohave study. Please let me know if you have any comments on it. Sincerely yours, Larry J. Paulson Director LJP/lhv encl. IN MCPIYftCfCf t TO: UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE SE POST orncc BOX \yx> ALaMUtMUC, MM MEXICO 87109 March 14, 1984 Dr. Larry J. Paulson Director Lake Mead Limnological Research Center University of Nevada, Las Vegas 4505 Maryland Parkway Las Vegas, Nevada 89154 Dear Larry: I have received, read and accepted your latest draft on razorback suckers on Lake Mohave. I find the discussion substantiated by the data you gathered. The biological conclusions make sense. I have approved payment of the remainder of the funds. Thank you for your efforts on this study. Sincerely yours, James E/i Johnson Chief./'Endangered Species IN MPtY KCrCft TO: UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE SE far orrice BQK oat AtMOMMUC. NCW MEXICO 09109 March 5, 1984 Dr. Larry J. Paulson Director Lake Mead Limnological Research Center University of Nevada, Las Vegas 4505 Maryland Parkway Las Vegas, Nevada 89154 Dear Larry: I am requesting your final report on Contract Number 81-251 for razorback sucker studies on Lake Mohave. That report was due on September 30, 1982. The draft report we received dated October 19, 1983, was returned to you on November 4, 1983, with comments and suggestions. Please finalize your rewriting and try to get the report in by April 16, 1984. Sincerely yours, ES E. Johnson ;Chlfef, Endangered Species UNIVERSITY OF NEVADA, LAS VEGAS 4505 Maryland Parkway Las Vegas, Nevada 89154 Lake Mead Limnological Research Center (702) 739-3269 (702) 739-3399 5 March 1984 Dr. James E. Johnson U. S. Fish & Wildlife Service P. 0. Box 1306 Albuquerque, N. M. 87103 Dear Jim: I have enclosed a copy of our final report on razorbacks and bonytail.6 in Lake Mohave. I essentially rewrote the discussion and made numerous other revisions as a result of your comments on the draft. im and I are both satisfied with it. I hope it also meets with your approval . Thank you for your help and patience. Sincerely yours, Dr. Larry J. Paulson LJP/dia Enclosure UNIVERSITY OF NEVADA, LAS VEGAS 4505 Maryland Parkway .LasVegas,Nevada 89154 Lake Mead Limnologicol Research Center (702) 739-3269 (702) 739-3399 17 October 1983 Dr. James E. Johnson U. S. Fish and Wildlife Service P. 0. Box 1306 Albuquerque, N. M. 87103 Daar Jim: I have, enclosed a draft copy of our report for the Lake Mohave study. Thanks for your patience and I look forward to receiving your comments. Sincerely yours, Larry J. Paulson, Ph.D. Director LJP/dia Enclosure IN REPLY REFER TO: UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE SE POST orricc BQK 1300 ALAKUEROJE. NEW MEXICO 87109 November 4, 1983 Dr. Larry J. Paulson Director Lake Mead Limnological Research Center University of Nevada, Las Vegas 4505 Maryland Parkway Las Vegas, Nevada 89154 Dear Larry: I reviewed your draft final report on razorback suckers (RBS) in Lake Mohave and have a few thoughts/questions. 1. On page 2 you state, "Razorback suckers are presently extremely abundant in Lake Mohave." I would not have used that adjective to describe their abundance. 2. The discussion on abundance (pages 18-22) does make RBS seem extremely abundant. The point should be more strongly made that you were netting in the spawning congregation of RBS and the results are not indictative of the ichthyofauna of Lake Mohave. 3. On page 38 your N=79 and N=99 appears to refer to hybrids sampled. 4. On page 54 you state, "...eggs were deposited over the substrate." and yet you later state it was difficult to find eggs in the wild. Did you actually see eggs deposited or find them there after observing the behavior? If so, state it. 5. Page 65. How about some literature citation on problems with finding eggs and larvae in predatory fish guts? 6. Page 68. Very little discussion about this data—Why? For instance, why no population estimate after tag and recapture? Comments about lack of observed movement? Why only maximum of 54 days between mark and recapture? 7. Page 69. I find the statement incredulous that "...there is no evidence to indicate that successful reproduction has or has not occurred in the Lake Mohave bonytail chub population in recent years." -2- 8. Page 71. "Catostomid fishes are known to successfully reproduce in both lentic and lotic habitats." It seems strange that it was necessary to use a Canadian reference for this statement. It is extremely obvious that there are no catostonid build-ups in any mainstream reservoirs in in the Southwest. The only time I am aware of catostonid build-ups in reservoirs concerns small, higher elevation reservoirs with inflowing creeks in which the fish spawn. 9. Page 74. "Catch frequency ...was_...." Also "years"? 10. Page 75. RBS common in Lake Mohave. How does this relate to extremely abundant on page 2? 11. Page 83. You mention 100 percent egg mortality. I thought eggs were difficult to find in the wild. How many did you observe? 12. Page 86. Your personal communication with Don Toney is different from what he has frequently told me. At my request, Jerry Burton just called Don and he^Biannot remember for sure if he ever took any young RBS out of Lake Mohave. He often told me that he believed he could find young RBS, but when pressed to do so, he was unable to demonstrate their presence. Be careful about how much weight you place on Don's statement. 13. Your discussion on pages 86 and 87 about the size distribution appears highly speculative and biased toward the presence of young RBS. I can make a better case that there are no fish under 400 mm, and no growth in the senile population, especially if you discount Toney's statements. 14. Page 88. Be careful about comparing growth of hatchery fish with wild fish. 15. Page 89. This whole discussion about immature females is speculation or word of mouth. Where is the hard data? 16. The whole paragraph on page 90 about L-F is "double speak" (i.e., symmetrical curves, stable age distribution). A nongrowing, nonreproducing population would show the same results, but nowhere do you mention that. If you are going to speculate, at least give both sides of the interpretation. 17. Page 94. Why obviously genetic? My overall impression of the draft report was lukewarm. Some portions of the report are based on facts and will be truly valuable (possible drawdown mortalities, larval captures), but I get the strong feeling that in many areas real data was lacking and in those areas you have strongly biased -3- the discussion without presenting other possible viewpoints. I am disappointed that the final answer as to RBS reproduction in Lake Mohave is still unclear, but feel it is better to leave the question open than to speculate. I await your final version. Sincerely yours, James E. 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