In the Lower Colorado River

BIOLOGY OF THE STRIPED BASS, MORONE SAXATILIS (WALBAUM), IN THE LOWER COLORADO RIVER

(ARIZONA-CALIFORNIA-NEVADA) by

Gary Bruce Edwards

A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science

ARIZONA STATE UNIVERSITY May 1974 ABSTRACT

Between 1962 and 1969 a program was conducted by the Arizona and California Game and Fish Departments to create a striped bass fishery in the Colorado River (Arizona-California-Nevada) between Davis and Parker Dams. Ninty-three thousand striped bass fingerlings and yearlings were introduced during this period. Beginning in 1966, data on various aspects of the biology of this newly-introduced population was accumulated. There was evidence of limited reproduction occurring between 1966 and 1969. The first spawn capable of producing a "dominant year class" did not occur until 1970. Spawning apparently occurs in that portion of the river from Davis Dam approximately 16 km downstream in April through mid-June. Many of the females mature and spawn in age-group 3 and almost all spawn in age- group 4. The feeding habits of these fish are similar to those reported throughout its range. It is a total carnivore, but fairly generalized in the choice of prey. In the Colorado River, threadfin shad are the primary food item of adult bass, both in frequency of occurrence and total volume. Striped bass in the Colorado River exhibit an accelerated life history as compared to other populations of the species. By the sixth year of life they have grown approximately 20% more in length than the average of various other native and introduced populations. At present, the outcome of these introductions appears to be one of success. The striped bass has become the major trophy fish sought by anglers fishing the Colorado River between Davis and Parker dams.

111 "How like fish we are: ready, nay eager, to seize upon whatever new thing some wind of circumstance shakes down upon the river of time! And how we rue our haste, finding the gilded morsel to contain a hook".

Aldo Leopold

A Sand County Almanac 1948

iv ACKNOWLEDGMENTS

As one reflects back over the many individuals who were instrumental with their help and support, it becomes a difficult task to know where to begin with saying thank-you. To the Arizona Game and Fish Department who sanctioned the study, and my returning to school, and the men of Region III, particularly my supervisor Wes Martin, whose confidence in me and the project were instrumental in having it approved, and to Wildlife Managers George Welsh and Jack King who not only helped with the gathering of data, but with their constant verbal support, I say thank-you. To the two Game and Fish secretaries, Brenda Whisman and Suzanne Osborn who typed the paper and endured me and my writing, I say thank-you. To my neighbor Mr. Brown who accompanied me on many of my trips and operated the boat while I was diving, and to the many anglers in the Bullhead City area who took the time to notify me when they caught a "striper" so that I could gather data from it, I say thank-you. To persons at Arizona State University, Dr. W. L. Minckley who accepted me as a student and provided much of the knowledge I needed to complete the study and to Jerry Landye, John Rinne and Donna Portz, fellow students and friends who were constantly involved, I say thank-you. And to my wife Jan, who drew the figures and never stops believing in me, I say I love you. TABLE OF CONTENTS

PAGE

LIST OF TABLES . • VII

LIST OF FIGURES ...... VIII

INTRODUCTION 1

DESCRIPTION OF STUDY AREA ...... 5

METHODS AND MATERIALS ...... • . . • • • • • • • • 7

REPRODUCTION . • ...... • • • • • • • • • 11

FOOD 16

GROWTH . . 25

ANGLING SUCCESS ...... 37

SUMMARY AND CONCLUSIONS ...... 41

REFERENCES. . • • • ...... • • . • • . . 43

VI LIST OF TABLES

Table Page

1. Striped bass introductions into the Colorado River, 1962-64 3

2. Comparison of striped bass having full stomachs to method of collecting ...... 18

3. Food items and their numbers and frequency of occurrence in adult striped bass stomachs ...... 19

4. Reconstructed values and weights of food items in 55 adult striped bass ...... • • • • • • • • 21

5. Indication of feeding trends from percentage of empty stomachs of striped bass, March through September 1969 ...... 22

6. Striped bass predation on tagged trout following stocking ...... 24

7. Comparison of scales from one large and one small striped bass, for variability of individual scales ...... 27

8. Calculated mean fork length plus standard error at each annulus for striped bass collected from the Colorado River ...... 29

9. Comparison of growth rate from various studies of the striped bass ...... 34

10. Mean fork length and weight of striped bass entered in the Bullhead City Striped Bass Derby 1968-1972 ...... 39

vii LIST OF FIGURES

Figure Page

1. Correlation of volume to the five terminal vertebrae of threadfin shad ...... 9

2. Body length-scale length relationship of striped bass collected during 1969-1972 ...... 28

3. Comparison of annular growth rate of introduced and spawned striped bass populations ...... 30

4. Annular growth of male and female striped bass from the Colorado River ...... 32

5. Comparison of growth rate from various studies of the striped bass ...... 35

6. Mean number, length and weight of striped bass entered in the Bullhead City Striped Bass Derby, 1968-1972 . . . . 40

viii INTRODUCTION

The striped bass, Morone saxatilis (Walbaum), is a bony fish, order Perciformes (Percomorphi; Acanthopterygii), belonging to the temperate bass family Percichthyidae (Gosline 1966). It has three relatives in fresh-and brackish-waters of North America: the white bass Morone chrysops (Rafinesque); white perch Morone americanus (Gmelin); and the yellow bass Morone mississippiensis Jordan and Eigenmann. Use of the generic name Morone rather than Roccus follows Whitehead and Wheeler (1966). This species is native to the Atlantic coast of North America, ranging from the St. Lawrence River, Canada, to the St. Johns River, north- eastern Florida, and to tributaries of the Gulf of Mexico from western Florida, and to Lake Pontchartrain, Louisiana. Its center of abundance lies from Cape Cod to northern North Carolina, including Chesapeake Bay (Raney 1952). Along the Atlantic Coast, the striped bass has differentiated into a number of races. Based on meristic studies, Raney (1957) concluded that three sub- populations are present within tributaries of Chesapeake Bay alone. In North Carolina an endemic stock of striped bass has differentiated into an upstream form which may not go to sea and a downstream form that does so (Raney 1955). Striped bass in the Gulf of Mexico, isolated from the Southeastern Atlantic by the Florida peninsula, have formed a very distinctive race (Raney 1955). Striped bass from New Jersey were planted in the Sacramento River, Cali- fornia, in 1879 and 1881, and since have dispersed to inhabit waters from San Diego, California, to the Columbia River, Oregon. Relative abundance of striped bass varies temporally and geographi- cally within its native range. Once common between North Carolina and Canada, its history has been one of decline, broken occasionally by short periods of sporadic production of strong year classes (Raney 1952). Popula- tions of striped bass, in the San Francisco Bay area, appeared to remain constant through 1948 (Calhoun 1950); with a slight downward trend occurring in the early and mid-fifties (Emig 1970). Beginning in 1957 angling success for striped bass started increasing, reaching a twenty-five year high in 1960. 2

There has been a nearly-steady decline in fishing success in all parts of the California fishery after 1963 and an increase in average age of fish taken provides evidence that recruitment and population size have declined since 1963 (McKechnie and Miller 1971). In 1954, establishment of a landlocked population of striped bass in Santee-Cooper Reservoir in South Carolina and Kerr Reservoir in North Carolina and Virginia was reported (Scruggs and Fuller 1954). These landlocked populations created widespread interest in establishing the species in reservoirs throughout the United States. Although problems encountered in hatchery production of fry have generally been overcome (Anderson 1966), attempts to establish populations from hatchery fish have often met with failure (Bonn 1968). On 13 August 1962, through a cooperative effort by the Arizona and California Fish and Game Departments, an initial attempt was made to establish striped bass in the Colorado River (Arizona-California-Nevada), between Davis and Parker dams. Seventeen thousand fingerlings (average fork length 51 millimeters [inn-) ), and 200 yearlings (average fork length 158 mm) were stocked between Blankenship Bend and Topock, Arizona. Additional plants, bringing the total stocked to 82,935 fingerlings and 896 yearlings, were made in 1962, 1963 and 1964 (Table I). Fish for these introductions were collected from the Tracey Fish Screens at Tracey, Cali- fornia, and planted by use of truck, boat and airplane. No additional plants were made until October 1969 when the Arizona Game and Fish Department planted 9,175 (9, 000, 102 mm fork length; 175, 178 mm) at Bullhead City. These fish were received as sac fry from the Minck's Corner State Fish Hatchery (South Carolina), and reared at the Arizona Department's warm- water hatchery at Page Springs. This brought the total introduction to 93 , 000. At present, the outcome of the program appears to be one of success. Striped bass have become the major trophy fish sought by anglers fishing the Colorado River between Davis and Parker dams. Each year more people come to the area intent on catching a "striper", and as new techniques have been developed the angling success rate has increased. Since 1968, the Arizona state record has been broken annually. The largest bass (by weight) 3

TABLE 1. Striped Bass Introductions into the Colorado River Between Davis and Parker Dams, 1962-64.

— Fork length Date Number (mm) Location

August 13, 1962 17,000 50 Topock area

August 13, 1962 200 177 Topock area

August 14, 1962 21,025 50 Davis Dam to Needles

August 29, 1963 3,080 50 Blankenship Bend, Topock Gorge

August 29, 1962 125 76-177 Blankenship Bend, Topock Gorge

August 20, 1964 14,950 50 1.6 km south of Devils Elbow, Topock Gorge

August 20, 1964 150 177 1. 6 km south of Devils Elbow, Topock Gorge

August 21, 1964 9,000 50 0. 8 km north of Topock Bridge

August 21, 1964 421 177 0.8 km north of Topock Bridge 4 taken to date was a 21.3 kilograms (kg) female caught in May 1972. In addition to creating outstanding sport fishing and exhibiting excellent growth, the striped bass apparently have succeeded in spawning in the river. Whether it will be able to maintain populations without additional introductions cannot be determined at this time. This paper provides limited observations on reproduction and movement; and details growth rate, food habits, and population dynamics of the recently established stock of striped bass in the Colorado River between Davis and Parker dams. DESCRIPTION OF STUDY AREA

The Colorado River after emerging from beneath Davis Dam flows alternately through alluvial valleys and mountain canyons on its course to the Gulf of California, approximately 450 km to the south. The river trav- erses several mountain ranges, which are largely granitic and/or metamorphic in nature, often overlain or interspersed with tuffs, lavas and other extrusive volcanics. Their bases are bordered with metamorphosed sedimentaries. Five valleys traversed by the river are the Mohave Valley (Davis Dam to Topock), Parker, Palo Verde and Cibola Valleys (Parker to Imperial Dam) and the Yuma Valley (Imperial Dam to International Boundary). Soils of the valley floors are heterogeneous deposits of river sediments laid upon the floodplain prior to channelization and control by upstream dams. In some instances, old meander patterns evidence that the river has moved across the valley and back many times. The climate is hot, dry and sunny. Annual precipitation averages between 7.5 and 12.6 centimeters (cm) with most of the rainfall occurring in the form of summer thunderstorms. Maximum air temperatures range be- 0 0 tween 46 and 52 celsius (c). Summers are long while winters are short and mild with an almost-complete absence of freezing temperatures. Spring winds are common, often accompanied by sand or dust storms (Dunbier 1968). The lower Colorado River basin lies in the Lower Sonoran Life Zone (Lowe 1967). The low-lying mountains are sparsely vegetated by scattered low shrubs and cacti except in their lower washes, where palo verde (Cercidium), ironwood (Olyneya), and smoke tree (Dalea) appear and gradu- ally become more abundant near the alluvial plains. Mesquite (Prosopis) is partially replaced by cottonwoods (Populus), willows (Salix), saltcedar (Tamarix), and arrowweed (Pluchea) as the water table becomes higher on the lower reaches of the plain. Cattail (Typha), bulrush (Scirpus), and other semi-aquatic plants fill the shallow waters and low-lying lands of the sloughs, back bays and seepage areas. Davis Dam, completed in 1953, was constructed by the Bureau of 6

Reclamation in Pyramid Canyon 107.9 km downstream from Hoover Dam. The dam is an earth and rockfill embankment with a concrete spillway, intake structure and powerplant. It was constructed to satisfy in part the Mexican Water Treaty of 1944 which requires an annual delivery of 238,095 hectare meters (ha-m) of water to Mexico. Davis Dam regulates stream flow under a daily, weekly and annual cycle. The daily cycle is power oriented, the weekly cycle for down-river demands, and the annual cycle satisfies seasonal demands to all users below Davis Dam. Maximum flow is approximately 64,516 cubic meters per minute (m3/min) and the minimum daily flow (24- 3 hour average) is 2,547 m /min. Below Davis Dam the river flows uninterrupted for 81.0 km before entering the headwaters of Lake Havasu impounded by Parker Dam. Completed in 1938, Parker Dam provides a forebay and desilting basin for the Colorado River Aquaduct which delivers water to the Los Angeles and San Diego coastal areas of California. This dam assists Hoover and Davis dams in regulating stream flow for downstream irrigation. Lake Havasu, with a total capacity of 98,370 ha-m covers 8,090 ha and extends 72.5 km upstream from Parker Darn. Within this 81.0 km portion of flowing river striped bass were collected and observed for this study. METHODS AND MATERIALS

Beginning in 1967, data on striped bass were collected in conjunction with Arizona Game and Fish Department Dingell-Johnson F-7-R projects. Although striped bass are common throughout the river and in Lake Havasu during various times of the year, the majority of bass collected and observed during this study were located from Davis Dam downstream 25.7 km to Fort Mohave. The majority of food habit and growth data were collected in 1969, with additional information obtained in 1971 and 1972. Scales and stomach samples came from bass taken by angling and/or spear gun using SCUBA equipment. Beginning in 1968, data on catch-rate were recorded from entries in the annual, Bullhead City Striped Bass Derby. Data on reproduction was limited to gonadal examination made in conjunction with stomach sampling, and from the presence of one- and two year old bass occurring in the 1971-72 catch. Stomach analysis reflects only the food habits of adult bass, within the period February through November. Upon removal, all ovaries were weighed to the nearest 0.1 gram, labeled and preserved in 10 percent (%) formalin. At a later date they were drained and a one-gram sample of ova was removed from the anterior, central and posterior sections Of each ovary. Mature ova in each sample were counted with the aid of a compound microscope. A single mean value, based on the three samples, was computed for each ovary. The mean number of mature ova per gram was multiplied by the total fresh weight of both ovaries, This technique followed that outlined by Lewis and Bonner (1966), with the exception that their calculations were based upon the gross weight of preserved ovaries. To determine state of maturity, a minimum of 100 ova were selected randomly from each sample, as described by Lewis (1962) and Chadwick (1965) and were measured under a compound microscope using an ocular micrometer. Maturity was then estimated by diameters of ova present as was established by Lewis (1962). Between February and November 1969, 100 stomachs were collected and analyzed. Stomachs were placed in cheesecloth bags, labeled and pre- 8 served in 10% formalin prior to analysis in the laboratory. Determination of volume of food items was accomplished by the reconstructed-weights method (Lagler 1952). This procedure was selected over volumetric measurements due to reliability of determining weights of threadfin shad, the dominant food, the limited number of food items present and the presence of food items of exceptionally great weight. Reconstruction of shad weights was based upon a high correlation of volume to the combined length of the five caudal vertebrae preceding the hypural plate (Figure 1). These data correspond closely to those obtained by McConnell and Gerdes (1969). Remaining food items were weighed and an estimate of their original weight was made based upon their percentage of digestion. Four of the 21 trout taken from bass stomachs were represented by the presence of tags that had been placed on them prior to stocking. These trout were assigned a weight based on their known average size at time of stocking. Scale analysis was used for age determination. Back calculation of growth at each annulus was determined by the direct proportion formula:

DA FLx = FL ( x) DM where FLx is the calculated fork length at the "x" annulus, FL the fork length at time of collection, DAx the distance from focus to annulus "x" and DM the distance from the focus to the scale margin (Lagler 1952). Scale samples from 200 striped bass were collected between 1966 and 1972. Several scales were taken from the left side of each fish above the lateral line between the dorsal fins. At least two scales from each fish were mounted and impressed on a 0.15 x 2.5 x 7.6 centimeters (cm) cellulose acetate slide. This circumvented problems created by marked thickening of the scales of large fish, which has hampered some previous studies of this species (see latter). An Eberbach scale-reading scope with a maximum magnification of 80X was used to examine scales and 15X was used for measurement. Scales were aged by counting the annular rings, according to method- ology of several authors (summarized by Raney 1952). All scales were aged independently by two readers. Those scales upon which no agreement could be made were submitted to a third reader, and a decision was made after Figure 1. Correlation of volume to the combined length of the five caudal vertebrae preceding the hypural plate of threadfin shad. 9

Y=1:8G -3GX _ N=48

•

1 a 3 Lf LENGTH OF FIVE CAUDAL VERTEBRAE IN MM 10 consultation among the personnel. Age groups employed in this paper refer to scales with a specific number of rings if taken after annulus formation. Thus a three-ring fish was from age group III. Fish taken just prior or during annulus formation were advanced one year. Striped bass appear to grow year around in the Colorado River as to be discussed later, and time of annulus formation seemingly did not relate to what might be considered "winter," and was quite variable. Therefore, in order to determine if a current annulus had been formed, each scale was evaluated on the basis of sex and age of fish, amount of growth occurring since the previous annulus, and time and location of catch; some revision may be necessary in age and growth estimates when tagging studies now in progress (1973) are completed. Measurements were taken along the enlarged anterior field of the impression of each scale from the focus to each annulus and to scale edge to determine body-scale length relationship and for use in back calculating growth. Fork length in mm was used as the standard for measurement. A conversion factor of 0.95 converted total length to fork length when the latter was not available. REPRODUCTION

The striped bass is coastal in habit, seldom found more than a few km from shore. It is highly euryhaline, but spawning occurs in fresh water, where a rather extensive spawning ground is required. Ova drift with the current until hatching, and the young remain in fresh water for a time before typically moving to sea. Water temperature appears to be the principal factor controlling time of spawning. The minimum temperature at which 14.50 spawning is known to occur is about C. Peak reproductive activity 60 0 occurs between 15. and 18.4 C, and declines rapidly thereafter (Albercht 1964). The dates at which spawning activity reaches a peak vary with lati- tude. In Coos Bay, Oregon, the spawning season extended from about mid- May to the end of June. The greatest percentage of ripe females was present in the week ending June 10 (Morgan and Gerlach 1950). In California, the spawning season extended from late March to early July with a peak in May (Calhoun 1950). In Santee-Cooper Reservoir, eggs were collected from April 8 to June 2, with a peak between April 21 and May 5 (Scruggs 1954). In tributaries to Chesapeake Bay the period was from May to July (Raney 1952). No exact method of externally distinguishing between the sexes in striped bass has been found, escept when they are "running ripe". A method of gonadal biopsy (95.7% accurate) was described by Ritchie (1965), but was not used by me. On both the Atlantic and Pacific coasts, male striped bass were disproportionately abundant on the spawning grounds (Raney 1952; Morgan and Gerlach 1950). Merriman (1941) mentioned that 10 to 50 small males may accompany a single female weighing from 1.8 to 22.7 kg. Pearsen (1938) reported many more mature males than females on the spawning grounds. In May 1938, Merriman (1941) found only six females among 127 striped bass at Welden, North Carolina. Ritualized or complex courtship prior to actual spawning does not appear to take place. An account of spawning activity was provided by Morgan and Gerlach (1950). During the spawning act, a group of three or four fish or many times that number, milled in a circle and splashed for about a minute, 12 throwing water as high as 1.5 m into the air. Shortly afterward they abruptly submerged. Ovum diameters have been used by several authors to determine sexual maturity in female striped bass (Lewis 1962; Chadwick 1965). Lewis classified ova of striped bass from the Albemarle Sound-Roanoke River System, North Carolina, into three groups; (i) translucent ova between 0.03 and 0.23 mm in diameter in which the yolk had not begun to form; (ii) translucent ova between 0.16 and 0.30 mm in which yolk had begun to form; and (iii) opaque ova 0.33 to approximately 1.00 mm in diameter. Females were separable into three categories--those with type 1 eggs only, those with type 1 and 2 eggs only and those with type 1, 2 and 3 eggs. He concluded that fish with types 2 and/or 3 ova were maturing, and would spawn in the next spawning season. These criteria were not satisfactory for defining the state of maturity of female striped bass from San Francisco Bay (Chadwick 1965). Eggs of the striped bass in California appear to develop a year or more before ripening, and many ova were intermediate in size between Lewis's three categories. Color of the ovaries of striped bass also has been used as a criterion of sexual maturity (Woodhull 1947). Immature ovaries are usually reported as "cream" colored, whereas ovaries ready to be spawned are typically greenish in coloration. Female striped bass from the Colorado River examined during March and April had ovaries which were cream colored and contained ova having a mean diameter of 0.77 mm. These ova compare to the type-3 ova described by Lewis (1962), would continue to increase in size and be spawned during the current year. These females also contained ova ranging in size between 0.10 mm and 0.20 mm with a mean of 0.14 mm. At first these were believed to be type-1 ova from mature females, as described by Lewis (1962). However, due to the lack of transparency and the larger mean sizes they are classified as type-2 ova and type-1 ova were subsequently found to have been overlooked. This oversight was corrected when discovered, specifically when working with younger, immature fish. Ovaries examined from females taken during May and the first part of June were green in color and contained type-3 ova ranging in size up to 1.35 mm, with a mean of 1.02 mm. These ova had neared maximum growth and were ready to be spawned. 13

The majority of mature females examined after June 15 had spawned. Their ovaries were flaccid and had thickened walls. The majority had at least a few unspawned, type-3 ova. These last observations correspond well with those of Chadwick (1965) for bass from California. All females from age class IV, due to the green color of the ovaries and the presence of type-3 ova, were classified as mature and should have spawned during the current year. This high percentage of mature, age-class- IV females is far greater than reported from other regions. Merriman (1941) found that only 25% of female striped bass taken in Connecticut spawned at four years. In California, 35% of the females matured and spawned first in their fourth year (Scofield 1931), and in Coos Bay, Oregon, Morgan and Gerlach (1950) noted 67.9% of the year-class-IV females were mature. Fifty percent of the year-class-III females collected from the Colorado River had greenish colored ovaries containing type-3 ova, having a mean diameter of 1.04 mm, and were classified as mature fish that should have spawned during the current year. The remaining year-class-III females contained type 1 and 2 ova as described by Lewis (1962) and were classified as immature; the ovaries were cream colored. The high percentage of mature females from this age class also differs from that found in other populations. The only other authors reporting mature ova in age-class-III females are Morgan and Gerlach (1950), who found 18.2% of year-class-III females from Coos Bay, Oregon, were mature. The use of ovum diameters for determining sexual maturity of female striped bass is believed to be valid for females from the Colorado River populations and the presence of intermediate type-3 ova as described by Chadwick (1965) did not occur. Although the sample size of females from age class III and IV was small (15 fish), it indicated that a large percentage of females from the Colorado River population were mature and would have spawned in their third year, and almost all were mature by the fourth year. Estimates of the number of mature ova released by spawning striped bass have been published by several authors (Lewis and Bonner 1966; Morgan and Gerlach 1950). Jackson and Teller (1952) found that the number of ova per female increased with age and size. In comparing the relation of number of eggs to body weight, fork length and age, Lewis and Bonner (1966) 14 determined that weight had the least amount of variability and was the most accurate for estimating the number of mature ova that a spawning population can produce. Morgan and Gerlach (1950) found that striped bass in Oregon produced approximately 220,000 ova per kg of body weight. Striped bass from the Roanoke River, North Carolina yielded a value of approximately 176,000 per kg (Lewis and Bonner 1966). The number of mature ova present in female striped bass from the Colorado River ranged from 149,142 per kg of body weight to 308,086 per kg with a mean of 231,643, considerably greater than that recorded from other areas. The apparent ability of the Colorado River striped bass to produce more eggs is also evident when the ratio between ovary weight and body weight are compared. In the Colorado River population, this ratio was 1:7.6, whereas, Vbardykov and Wallace (1952) reported a ratio of 1:12 in Chesapeake Bay. This difference in reproductive potential most likely is due to food availability, which appears as one of the reasons the Colorado River striped bass population exhibits greater overall growth than what has been reported in other areas, as will be discussed later. In the present study, observations on spawning migration, time of spawning, successful spawning, and sex ratio at spawning, were made in conjunction with the food and growth study. An annual upstream spawning migration to the tailwaters of Davis Dam was made as early as 1964. There was no evidence of a substantial spawn until possibly in 1969, but a strong year class was produced in 1970. This is reflected in the 1971-72 age- growth data to be given later, which included one- and two-year old bass that could have resulted from the last introduction in 1969, and/or a successful 1969 spawn, followed by another in 1970. Examination of gonads, suggested spawning occurred from May through mid-June, with a few ripe individuals still present in July. This coincides with data from the San Francisco region of California (Calhoun and Woodhull 1948; Calhoun, et al. 1950). The presence of large concentrations of sexually mature bass in the area from Davis Dam to approximately 16 km downstream suggests this sector of the river to be the primary spawning grounds. Eighty-four percent of the bass sampled in 1969 were taken in this reach and 53% of these were females. 15

As noted before, males usually greatly outnumber females in spawning aggregations, and the reason for the essentially 1:1 sex ratio in the Colorado River is unknown. Large numbers of bass with spent gonads were taken in this same portion of river during July and August. This area offers strong and turbulent currents similar to that preferred by striped bass in California (Calhoun and Woodhull 1948). FOOD HABITS

The striped bass is a voracious carnivore. Amounts and kinds of foods taken appear a function of availability more than any other factor (Raney 1952; Johnson and Calhoun 1952; Stevens 1958; Merriman 1962; Morgan and Gerlach 1950). One exception reported in Chesapeake Bay by Hollis (1952) listed anchovies occurring at a higher frequency in stomachs despite silversides being more numerous. Ontogenetic change in foods and feeding is pronounced. Young feed largely near the bottom on crustaceans and other small invertebrates. During the second summer they begin to school and commence to feed on other species of schooling fish (Raney 1952; Herbach, et al. 1963). The earliest food analysis of striped bass from the east coast was reported by Hildebrand and Schroeder (1928). Bass stomachs examined from the Chesapeake Bay region contained fishes, crustaceans, annelid worms and insects. Larger individuals fed principally on fishes, whereas juveniles ate mainly crustaceans. Hollis (1952) analyzed 1,736 stomachs from Chesapeake Bay in the period June 1936 to April 1938. The results of his study was as follows: (i) 969 (55%) contained some food; (ii) by weight, fishes comprised 95.5% of the food; (iii) 26 different fishes were identified; (iv) food of individual bass sometimes consisted of only a single species; and (v) crustaceans varied from 0.0 to 46.3% in occurrence, but made up less than 2% of the total weight. Johnson and Calhoun (1952) analyzed 387 striped bass stomachs taken by anglers in the course of one year's fishing in and around San Francisco Bay, California. The stomachs were divided into a summer and winter sample and results given by season, respectively, were as follows: (i) foods of some sort were found in 72% of the samples in summer, larger shrimps were present in 35% of the fish, and made up 53% of all identifiable organisms by volume, anchovies were next to shrimp in importance, other foods included isopods, small shrimp, and miscellaneous fishes; (ii) 42% were empty in winter, remains of small fish comprised 64% of the total volume, small 17 shrimp were present in more stomachs than any other food, making up 20% of the volume. Stomachs from striped bass from Coos Bay, Oregon, during 1948-59 were examined by Morgan and Gerlach (1950). Fishes that moved in large schools predominated in stomachs during most of the year. Numerous salmonid fry and fingerlings were eaten in the months of April, May and June. Between February and November 1969, 100 stomachs from Colorado River striped bass were collected and analyzed. Forty-five percent of the stomachs I examined were empty, a value comparable to findings of others (Morgan and Gerlach 1950; Merriman 1941; Stevens 1957). Merriman (1941) attributed the large number of empty stomachs, in part, to interrelation- ship of angling success and feeding period (i.e. bass that had been feeding were not as readily taken and vice versa). Furthermore, he felt that bass caught by hook and line often regurgitated their food as they were retrieved. Comparison of fish taken by spear gun versus angling revealed that 71.4% of the bass taken by the former method had full stomachs as opposed to 50.6% taken by angling (Table 2). My findings therefore substantiate those of Merriman's that anglers tend to catch those fish just beginning to feed, when based on the premise that those taken by spear represented a more random method of selection. However, my observations on fish taken by angling do not substantiate a high percentage of regurgitation. Of more than 175 fish I personally watched being taken by hook and line, only one showed evidence of regurgitation. Regurgitation was noticed on only three occasions in specimens taken by spear. Threadfin shad (Dorosoma petenense) was the most common food item in stomachs, occurring in 37 (67.3%) of the 55 which contained foods. It also ranked first in total number (1,439 of 1,476) of identifiable, natural food items (Table 3). The largest number of shad found in a single stomach was 257, removed from a 7.6-kg female. A primary source of shad is Lake Mohave, situated on the upstream side of Davis Dam. Underwater observations below Davis Dam revealed large numbers of shad having been drawn through the penstocks and expelled into the tailrace in a moribund or dead condition. Next to shad, rainbow trout (Salmo gairdneri) and crayfish 18

TABLE 2. Comparison of Striped Bass Having Full Stomachs to Method of Collecting. Any Stomach Not Entirely Empty was Classified as Full.

Number of Percentage of Method Number full stomachs full stomachs

Angling ------79 40 50.6

Speargun ------21 15 71.4 TABLE 3. Food Items and Their Numbers and Frequency of Occurrence in 55 Adult Striped Bass Stomachs from the Colorado River; March through September 1969.

Number of Frequency of Percentage of Food item food item occurrence occurrence

Threadfin shad ------1.439 37 67.3 Dorosoma pentenense Rainbow trout ------21 13 23.6 Salmo gairdneri Crayfish ------10 7 12.7 Procambarus clarki Centrachids ------4 4 7.3 Micropterus salmoides Lepomis cyanellus European carp ------1 1 1.8 Cyprinus carpio Tiger salamanders ------2 2 3.6 Ambystoma tigrinum Unidentified items ------6 6 10.9 20

(Procambarus clarki) were the most important food items in both frequency- of-occurrence and total number (Table 3). The remainder of the natural food items consisted mostly of centrarchids , three largemouth bass (Micropterus salmoides), and one green sunfish (Lepomis cyanellus). A single European carp (Cyprinus carpio) was tallied. Six food items were not identifiable by me. In addition to natural food items, two salamanders (Ambystoma) were found, attributable to anglers using them for bait, since ambystomidis do not maintain a reproducing population in the reach of the Colorado River under study. Reconstructed food items totaling 4.98 kg were obtained from the 55 stomachs (Table 4). Threadfin shad accounted for 2.46 kg (49.4%) of the total. Trout were second, comprising 27.6% of the total volume. Although centrarchids made up 12.8% of the volume, this resulted from the presence of one largemouth bass weighing 0.58 kg which was found in a 10.8-kg female striped bass. Data on percentages of striped bass taken and the relative number of empty stomachs in a given period (bass taken by anglers) may indicate feeding trends (liable 5). Nearly 58 percent of the bass were taken between 1800 hours and midnight in the period March through September 1969 while only 8.0% were caught between noon to 1800 hours. Hollis (1952) stated that bass feed avidly in evening hours just after dark as well as just before dawn, similar to my data from the Colorado River. It should be considered, however, that periods of highest catch are also probably the most convenient hours for anglers, and no data were obtained on man-hours spent during the different time periods by Hollis (1952) or by me. When compared with landlocked striped bass from the Santee-Cooper Reservoir, the Colorado River fish exhibited similar food habits only in their consumption of clupeid fish and general lack of predation on game fishes. Stevens (1957) summarized examination of 1,093 stomachs from Santee- Cooper Reservoir as follows: (i) clupeid fishes supported the striped bass population most of the year; (ii) in April, May and June, mayfly nymphs became the dominant food items; and (iii) game fishes and rough fish were taken in insignificant numbers. Predation on game fish is of particular interest in the Colorado River TABLE 4. Reconstructed Values of Weights of Food Items in 55 Adult Striped Bass from the Colorado River; March through September 1969.

Frequency of Reconstructed Percentage of Food item occurrence values (g) reconstructed value

Threadfin shad ------37 2,458 49.7

Rainbow trout ------13 1,375 27.8

Crayfish ------7 319 6.5

Centrachids ------4 639 12.9

European carp ------1 56 1.2

Salamanders ------2 92 1.9

Total reconstructed values - 4,939 100 TABLE 5. Indication of Feeding Trends from Percentage of Empty Stomachs of 80 Striped Bass from the Colorado River; March through September 1969.

Number stomachs Percentage total Number stomachs Percentage Hours examined stomachs examined containing food containing fooc

2400-0600 - - - - - 18 22.5 14 77.8

0600-1200 - - - - - 15 18.7 9 60.0

1200-1800 - - - - - 8 1.0 0 0.0

1800-2400 - - - - - 39 57.8 11 22.2 23 system due to the catchable-trout stocking program presently conducted there. From the initial stomachs examined, rainbow trout were the second most important food item in number and volume (Tables 3 and 4). As given before, Morgan and Gerlach (1950) reported numerous trout and salmon (fry and fingerlings) in striped bass stomachs in April, May and June taken from Coos Bay, Oregon. Salmonids formed almost 7% of the items present during this period nearly equaling other fishes and shrimp in importance. This period coincides with the presence of large schools of striped bass and heavy downstream migrations of young salmon and trout. Although trout were second in total numbers of food items in specimens examined, they were far less abundant in bass stomachs than were threadfin shad (1,439 shad to 21 trout). As a precautionary measure, it was recommended to the Willow Beach National Fish Hatchery (Arizona, Dingell-Johnson Project F -7-R-11, WP 3, JR-3) that trout stocking directly below Davis Dam be curtailed during the months of striped bass concentration. In a later study (Arizona, Dingell-Johnson Project F-7-R-14, WP 3, JS-3), stomachs from 20 bass (not subsequently included in the present report) were examined over a 13-day period following stocking of tagged trout (Table 6). Eighteen of the 24 trout in stomachs were tagged. From this analysis, it appeared that predation on trout might be higher and more significant than originally thought; although high predation may occur only for a short period of time following stocking. Stocked trout are presumably adjusting to their new environment during this period, and might, therefore, be highly susceptible to predation. Once they have adjusted, the high predation must drop. If this were not true, then the percentage of trout occurring in stomachs examined in 1969 should have been considerably higher. Also, catch-rate (fish-per-hour) of rainbow trout in the river below Davis Dam was extremely high before striped bass were present, and remains so (Arizona, Dingell- Johnson Project F-7-R-14, WP 3, JR-3). Information on food of young-of-the-year striped bass and on adult bass occurring further downstream and in Lake Havasu is not available at this time. TABLE 6. Striped Bass Predation of Tagged Trout Following Stocking, as Indicated by Stomach Analysis.

Number of days Number stomachs Total number of Number of Percentage of following stocking examined trout found trout with tags tagged trout

1 ------1 0 0 0

2 ------4 8 6 75

3 ------5 8 4 50

7 ------8 8 8 100

13 ------1 0 0 0

Total - - - - - 19 24 18 75 GROWTH

Young striped bass generally grow most rapidly in late spring and early summer. By comparison, adult bass lengthen rapidly in summer and early fall, but growth rates diminish greatly in late autumn and winter. Male and female bass grew at about the same rate for the first three years of life in Chesapeake Bay (Mansueti 1961). Females grew at a significantly faster rate after age four and a consistent difference in size and was maintained between sexes. Mansueti also found the greatest annual increment for both sexes occurred in the second year of life, 162.6 mm for males and 167.6 mm for females. Stevens (1957) suggested the greatest growth increment occurred in the first year of life in the Santee-Cooper Reservoir. He attributed this to the extended spawning season of that population, but the reasons for such a relationship seem obscure. Growth rates in California waters are similar to those in most other striped bass populations. Robinson (1960) reported a significant increase in growth rate of California striped bass in the last 30 years (10% increase in length and 25% in weight). This occurred after the second year of life and was again greater in females than males. Striped bass taken by sport and commercial fishermen on the Atlantic Coast most commonly weigh from 0.45 to 4.5 kg although, fish of 11.3 to 13.5 kg are not rare. Larger fish, caught on occasion in abundance according to old records, weighed 22.5 kg or more, but are uncommon at present. Fish of more than 27.2 kg were exceedingly rare (Merriman 1941). The two largest striped bass ever recorded were taken at Edenton, North Carolina, in commercial nets, and weighed 56.3 kg each (Raney 1952). Calhoun (1948) reported that no striped bass weighing more than 45 kg was known to have been caught on the Pacific Coast. However, there was a record of 35.1 kg fish taken in 1910, and in 1911, a 27.9 kg specimen was taken near Napa, California. Scale analysis has generally been validated as the best method for age determination of striped bass. Scofield (1931) reported formation of 25 to 150 circuli during the course of a year's growth and that circuli ceased to 26 form in early winter. Younger bass formed more circuli than older individuals as a result of their faster growth rate. Some difficulties have been encountered in scale analysis, especially in scales from large bass (more than eight years of age) which are more difficult to read than those from small individuals (Merriman 1941). Merriman also recorded false annuli, perhaps resulting from marked thickening of the scales in older (larger) fish. As given in methods, this problem was circumvented by use of cellulose- acetate impression of the scale surface; rather than the scales themselves. Scale samples from 200 Colorado River striped bass were collected between 1966 and 1972, for study of age-and-growth of the newly-established population. To determine variability among scales from individual striped bass, samples from one large (4 annuli) and one small (2 annuli) fish were measured along both the anterior and lateral fields from the focus to each annulus and to the scale edge. Data derived from scales from each bass were similar in the relative position of annuli and over-all morphalogy (Table 7). The anterior field was measured for general convenience since little or no substantial difference was present between data from either measurement. A linear relationship between scale size and body length of striped bass has been verified by several authors (Scofield 1931; Merriman 1941; and Robinson 1960) and was obvious for striped bass in the Colorado River (Figure 2). Specimens were grouped according to year of capture. Year classes in which the sex of the bass was recorded were divided accordingly. Mean lengths and standard error for each age group within a given year class were calculated (Table 8). Analyses of calculated growth histories of the various year classes indicate a difference between growth rates of introduced bass and those produced by successful spawning in the river (Figure 3). Striped bass initially introduced from California (year-classes 1961 through 1965) grew an average of 152 mm during their first year. River-spawned individuals (year classes 1966, 68, 70, 71) grew to a mean of 207 mm in their first year of life. Several factors might be responsible for this difference. First, the initial effects of a new environment and food source might in itself have a physiological effect on transplanted fish. On the other hand, those fish that were able to TABLE 7. Comparison of Scales From One Large (4 Annuli) and One Small (2 Annuli) Striped Bass, for Variability of Individual Scales.

LARGE BASS SMALL BASS Sample Statistics Anterior field Lateral field Anterior field Lateral field Annulus I II M W Edge I II M IV Edge I 11 Edge I 11 Edge

Number of scales measured ------32 32 32 32 32 32 32 32 32 32 31 31 31 31 31 91 Mean value in mm - - - 99.4 127.5 149.6 161.7 175.7 79.4 96.2 107.0 115.4 125.5 100.2 136.6 148.9 72.9 88.8 95.0 Sample variance - - - - 103.8 83.6 90.7 107.5 91.7 71.2 67.0 83.3 85.7 112.7 48.7 78.5 95.1 79.0 92.3 89.6 STD deviation ------10.2 9.1 9.5 10.4 9.6 8.4 8.1 9.1 9.3 10.8 6.9 8.9 9.8 8.9 9.6 9.5 STD error ------1.8 1.6 1.7 1.9 1.6 1.5 1.5 1.6 1.7 1.8 1.3 1.6 1.8 1.6 1.8 1.7

Mean conf. limits 75 ------97-102 126-129 148-152 160-164 174-178 78-81 95- 98 105-109 114-117 124-128 99-102 135-139 147-151 71-75 87-91 98-87 95 ------96-103 124-131 146-153 152-166 173-179 76-83 93-100 103-110 112-119 121-129 98-103 134-140 145-153 70-76 85-92 92-99 99 ------94-104 123-132 145-154 157-167 171-180 75-84 92-100 103-112 111-120 121-130 97-104 132-141 144-154 68-77 84-94 90-100 Figure 2. Body length-scale length relationship of striped bass from the Colorado, 1969-1972. 28

1000 950

900 • • • 850 Y-5.05 +.24X 800 N=85 750 700 650 • 600 550 FORK LENGTH 500 450 400 350 0 30 0 50 100 150 200 250 ENLARGED SCALE RADIUS MEASUREMENT IN MM TABLE 8. Calculated Mean Fork Lengths Plus Standard Error (In Parentheses) at Each Annulus for Striped Bass Collected from the Colorado River.

Calculated lengths at des gnated annuli Number of Year of Age Fork length fish capture Sex Group at capture (mm) 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972

7 - - - - 1966 Combined IV 720.0 155(11) 419(14) 589 (10) 689 (9) 2 - - - - 1966 Combined III 669.6 153 (2) 447 (11) 621 (24) 1 - - - - 1969 Males VII 812.8 177 (0) 433 (0) 557 (0) 652 (0) 710 (0) 751 (0) 788 (0) 22 - - - - 1969 Males VI 836.4 162 (8) 433 (7) 569 (9) 665 (9) 738 (11) 795 (13) 14 - - - - 1969 Males V 791.2 149(6) 455(11) 599(13) 686(14) 746(14) 10 - - - - 1969 Males IV 717.8 148 (15) 441 (12) 568 (8) 659 (9) 4 - - - - 1969 Males III 664.9 210 (32) 509 (25) 621 (14) 2 - - - - 1969 Males II 518.0 211 (38) 504 (38) 2 - - - - 1969 Females VII 925.9 134 (17) 375 (18) 553 (2) 685 (26) 781 (18) 846 (28) 905 (37) 25 - - - - 1969 Females VI 923.3 177 (8) 435 (12) 606 (12) 718 (13) 816 (13) 873 (14) 14 - - - - 1969 Females V 848.1 155 (8) 437 (15) 605 (14) 727 (13) 803 (11) 11 - - - - 1969 Females IV 783.9 156 (13) 439 (10) 604 (9) 719 (9) 7 - - - - 1969 Females III 689.1 204(22) 488(11) 639(15) 1 - - - - 1971 Combined III 723.9 179 (0) 456 (0) 612 (0) 515 (11) 2 - - - - 1971 Combined II 547.8 239(5) 14 - - - - 1971 Combined I 364.1 208 (7) 1 - - - - 1972 Combined V 683.3 124 (0) 284(0) 510(4) 597 (0) 650 (0) 587 (5) 6 - - - - 1972 Combined IV 616.4 172 (18) 346 (38) 472 (14) 583 (22) 9 - - - - 1972 Combined III 607.8 235 (11) 447 (23) 19 - - - - 1972 Combined II 467.5 209 (5) 435 (10) 261 (0) 1 - - - - 1972 Combined I 304.8 ANGLING SUCCESS

The striped bass is an outstanding anadromous sport fish along the Atlantic Coast. In that region it supported extensive commercial and recreational fisheries, yielding nearly 2.3 x 10 4 kg annually to market fishermen, and about the same to sport fishermen (Raney 1952). Commercial fishing for striped bass was abolished in California in 1935. Since then, numbers taken by sport fishermen have exceeded even the highest catches ever recorded for market (Curtis 1949). It is estimated that 1.6 x 1O 4 anglers in 1948 or about 16% of all fishermen in that state, successfully fished, at one time or another, for striped bass, and captured more than 1.65 x 10 4 individual fish. in the San Francisco Bay area, both trolling and surf-casting are successful fishing methods, but the greatest proportion of the catch is by still-fishing with sardines. In 1953, California biologists valued the State's striped bass sport fishery at (1.8 x 106 based on an average expenditure of $9.00 per angler-day (Raney 1958). Commencing in 1966, anglers began successfully fishing the Colorado River for the introduced striped bass. Each year an increasing number fished, intent upon catching a "striper". Not until 1972, with the advent of bait fisherman using cut anchovies, could the "average" angler expect to catch a striped bass. Prior to this, a majority used artificial lures which required a boat and heavy tackle. This last method discouraged many persons, particularly the retired local residents. Use of anchovies has increased not only the number of anglers, but also apparently has increased fishing success. Although a scheduled creel census of striped bass anglers on the river had not yet been conducted, fishing effort (beginning in 1968) may be indicated from the number of entries in the Annual Bullhead City Striped Bass Derby. These data must be qualified prior to utilization. First, the majority of bass entered were caught between Davis Dam, Arizona-Nevada, and Needles, California, and therefore the information does not reflect angling success and population structure in the river downstream from Needles and in Lake Havasu. Secondly, data only covers the period in which the contest 36 season and in the great variability in time of annulus formation previously mentioned. Another factor that must be considered is that the population (s) I studied was newly established, and therefore was enjoying a surge in growth rate, and other factors of its biology. In Santee-Cooper Reservoir, the striped bass population expanded rapidly until about 1960, then declined, at least in population size (South Carolina, Dingell-Johnson Project F1R-10). The presumably-low density of striped bass in my study area could have contributed to almost all the usual features that were found, e.g., rapid growth, high fertility and early age of maturation. 35

900 -

800 MORGON AND GERLACH PRESENT STUDY /(1950)OREGON 700 COLORADO RIVER 600 STEVENS 0960 SOUTH CAROLINA 500

Li00 ROBINSON (958) 300 CALIFORNIA

ZOO MANSUETI (1958) CALCULATED FORK LENGTH IN MM MARYLAND 100

1 2 3 4 5 6 7 YEAR CLASS Figure 5. Comparison of growth rate from various studies of the striped bass. TABLE 9. Comparison of Growth Rate From Various Studies of the Striped Bass. All Data Converted to Fork Length in Millimeters if Not Originally Reported as Such.

Present study 1969 year class Mansueti Robinson Stevens Morgan & Gerlach Age collected in 1963 (1961) (1960) (1957) (1950) group Colorado River Maryland California Sexes combined Sexes combined South Carolina Oregon Males Females Males Females Males Females

I - - - - 162.1 177.4 134.6 124.5 104.1 111.8 203.2 II - - - - 433.3 439.9 297.2 292.1 248.9 248.9 375.9 373.4 DI - - - 568.7 605.8 381.0 388.6 386.1 388.6 472.4 482.6 IV - - - - 664.8 717.6 467.4 494.7 500.4 579.1 535.9 579.1 V 738.3 815.9 500.4 556.3 566.4 599.4 614.7 635.0 VI - - - 795.0 873.4 594.4 645.2 622.3 683.3 688.3 690.9 33 sample sizes and lower standard errors of the means. Combined mean lengths of both males and females were employed in the figure because not all other studies used for comparison separated their fish by sex (Table 9). Although Table 9 and Figure 5 both suggest that the Colorado River striped bass grew slower as yearlings than reported by Stevens (1957) in South Carolina, it must be remembered that individuals hatched in the river grew approximately 208 mm as yearlings (Figure 3) as opposed to a mean of ca. 170 mm for the 1969 stock. The Colorado River striped bass reached fork lengths as great as 832 mm at age-group six, or 17% longer than recorded from any other region. When compared with several areas, this growth exceeds the combined mean six-year growth of other striped bass populations by 20%. The six year growth reached by Colorado River bass is equivalent to that of striped bass between 9 and 11 years of age obtained elsewhere (Scofield 1931; Mansueti 1961). The largest specimen collected in my study was approximately 1.02 m in fork length and had completed seven years of growth. Several verified catches of striped bass from the Colorado River greater than 1.09 m have, however, been recorded. In summer 1972, a female taken by angling measured 1.2 m. This fish, if a yearling from the original introduction in 1962, would be (at the maximum) only 11 years old. This rate of growth becomes even more impressive in light of the statement by Merriman (1941) suggesting that striped bass nearing 1.3 m in length were 20 to 25 years old. Upon introduction into the Colorado River, striped bass became the primary predator on an almost-unutilized food chain. However, availability of food may not be the only reason for such growth. In the majority of its range, striped bass do not appear to grow the year around. Merriman (1941) stated that growth of striped bass practically ceased in winter and Scofield (1931) reported the period of growth extending from April until October (7 months) in California, whereas during the period from November through March there was little indication of linear growth. In the Colorado River, 0 90 due to warm-water conditions (average 18 C. annually--minimum C, in January) it is believed that striped bass grow to a greater or lesser degree all year. This year-around growth was manifested in the long spawning 32

MALES --- mo FEMALE5

700

z 600 A- RATE OF GROWTH

qe°

3 °° B-ANNUAL GROWTH INCREMENTS

L) ') ZOO

'110.

YEAR CLASS Figure 4. Annular growth of male and female striped bass from the Colorado River, 31 survive the transplant might well have represented the larger and healthier individuals of the original population, and therefore strong intraspecific competition might have occurred. This difference in first-year growth was true for all year classes except in 1969 (Figure 3). Fish sampled from the 1970 year-class probably represent the final introduction, consisting of hatchery-reared fingerlings. These fish attained mean lengths of 237 mm during the first year, thus exceeding lengths reached by river-spawned individuals. Such growth may be attributed to the steady, high-protein diet received at the hatchery prior to stocking, or might have been a result of the fresh-water origin of brood stock producing those fish stocked in 1969, as opposed to saline-adjusted breeders which spawned stocks imported from California in 1962 through 1964. Perhaps the intermediate growth rate of fish spawned in the Colorado River reflects acclimation of the California stocks to a landlocked situation. Introduced bass that were small as yearlings showed a decidedly greater growth rate during their second year, nearly reaching the length of the remainder of the population. This compensatory growth must have resulted from rapid adjustment to their new environment. Except in the extent of annular growth, the Colorado River striped bass population exhibited growth characteristics similar to those reported from other regions (Morgan and Gerlach 1950; Stevens 1957; Robinson 1960; Mansueti 1961). Male and female striped bass grew at approximately the same rate for the first three years of life. Females began to grow faster during their fourth year, and this rate, consistently greater, than that of males, was maintained. Females from the 1963 year-class in the Colorado River population were 78 mm longer than males at the end of six years (Figure 4). Greatest annual growth for both sexes occurred in the second year of life (mean increments being 271 mm for males and 262 mm for females). After the second year, increments of growth decreased annually (Figure 4). A further comparison with other studies shows annual growth to be far greater for Colorado River striped bass than for other populations. To determine growth difference it was necessary, due to the variability in yearling growth (Figure 3) to use average growth figures calculated from a single year class. The 1963 year class was selected because of its larger 30

700 3 YEARS 3 YEARS 600 3 YEARS

2YEARS

INTRODUCED SPAWNED INTRODUCED POPULATION POPULATION POPULATION (96Z— 64) (1966-(o8;70;71) (c169) Figure 3. Comparison of annular growth of introduced and spawned striped bass population in the Colorado River. 38 was conducted, generally between the latter part of May and early September, Thirdly, many of the bass taken, especially the smaller ones, were not entered. Information obtained however, does indicate an annual increase in the number of striped bass taken by anglers (Table 10, Figure 6), which was the result of several factors: an increase in fishing effort; the employment of more effective fishing techniques; and an increase in number of bass available due to successful spawning and/or the 1969 introduction. A noticeable decrease (225 mm) in the average length of bass taken occurred in 1971 (Table 10, Figure 6). This decrease was the result of 66% of the 1971 catch being composed of age-II bass. These two-year old bass were from a successful spawn in 1969 or were part of the 9,000 that were planted that same year, or a combination of both. Mean length of fish in the 1972 catch increased 38 mm over that of 1971. This increase reflected continued presence of the 1969 year class. In addition, 29% of the 1972 catch was composed of two-year old fish, representing a successful 1970 spawn. I suggest that the 1972 catch does not accurately show the impact of this 1970 spawn, since I observed many of the smaller fish being released after landing or simply being taken and not entered in the contest. Angling success for striped bass (like most fisheries) varies with the angler. Similar to other fisheries "10% of the fishermen catch 90% of the fish." During the 1971 Bullhead City Derby, in which approximately 500 fishermen participated, six anglers accounted for 30% of all bass entered. Detailed fish-per-hour data are not available for the striped bass in the Colorado River. On one occasion, however, I observed an angler fishing directly in the tailrace of Davis Dam catch and release 15 bass (the majority of which were two-year-olds) in an hour and 19 minutes. That success rate is not suggested as a norm, but many anglers successfully catch their present legal limit (three bass having minimum length of 16 inches) in fewer than six hours. Economic importance of the striped bass to the communities along the river appears to be increasing. Currently there are three striped bass derbies held annually, with prizes as high as $1,000 to the person catching the largest fish. 39

TABLE 10. Mean Fork Length and Weight of Striped Bass Entered in the Bullhead City Striped Bass Derby 1968-1972.

Year Month Number of fish Mean fork length Mean weight (mm) (g)

1968--- Total period 54 753 5890

1969--- February 2 694 6400 March 5 887 8460 April 7 789 5690 May 23 818 6360 June 14 813 6190 July 21 803 6470 August 19 803 6480 September 3 811 6070

1970--- May 15 845 7393 June 36 813 5811 July 23 840 6017 August 13 847 6808 September 3 805 6800

1971--- April 24 598 2230 May 53 584 2580 June 82 549 1830 July 59 576 2640 August 3 849 8130 September 10 635 3730

1972--- April 47 607 3700 May 102 674 3100 June 124 600 2420 July 41 679 3600 August 15 749 4840 Figure 6. Mean number, length and weight of striped bass entered in the Bullhead City Striped Bass Derby, 1968-1972. NUMBER OF FISH MEAN LENGTHS MEAN WEIGHTS (MM) ( 9 ) ) i r § -r A WOUVg.g SUMMARY AND CONCLUSIONS

Data were gathered on striped bass between Davis and Parker Dam on the Colorado River, 1966 through 1972 and several conclusions may be drawn concerning various aspects of their life history. Evidence of limited reproduction from 1966 through 1969 was recorded. However, the first spawn perhaps producing a "dominant year class", did not occur until 1970. Spawning apparently is in that portion of river from Davis Dam to approximately 16 km downstream in April through mid-June. Many females mature and spawn in age-group 3, and almost all spawn in age-group 4. Threadfin shad is the primary food item of adult bass in the river, both in frequency of occurrence and in total volume. Although rainbow trout were found in some stomachs, I feel they are not preyed upon in sufficient numbers to adversely affect the catchable trout fisheries which is maintained below Davis Dam. Striped bass in the Colorado River exhibit an accelerated life history as compared to other populations. By the sixth year of life, they have grown approximately 20% more in length than the average of various other native and introduced populations. The importance of striped bass as a sport fish seems ever-increasing. Each year, progressively more anglers fish the Colorado River solely for this species, thus adding to the general economy of the area. If the fish is to become the primary component of the sport fisheries in the Colorado River between Davis and Parker dams, three tightly-interrelated factors must be determined. Firstly, how long can the threadfin shad population in the river support an ever-increasing striped bass population? Secondly, to what extent are the striped bass reproducing, and will it be successful enough to maintain the population without additional introductions, or, as the third question, how many striped bass can inhabit the area without effecting a decline in the food supply and thereby suffer pangs of over- population. This study in attempting to answer questions concerning the biology of this newly introduced striped bass population has succeeded mostly in 42 leading to unanswered ones. Many additional data, particularly concerning reproduction, food habits of young-of-the-year, and availability of threadfin shad must be obtained before the introduction can be considered a success. REFERENCES

Albrecht, Arnold B. 1964. Some observations on factors associated with with survival of striped bass eggs and larvae. Calif. Fish and Game 50 (2) : 100-113. Anderson, James C. 1966. Production of striped bass fingerlings. PROGR. Fish-Cult., 28 (3) 162-164. Bonn, E. W. 1968. Striped bass introduction. Texas Parks and Wildlife Dept., D - J Fed. Aid PROJ. No. F-8-R-14, Job No. 9. Calhoun, Alex J. 1948. Striped bass estimates for 1946. Rept. Bur. of Fish Cons., Calif. Dept. of Fish and Game, Inland Fish. Br., No. 49-2 : 36-50. . 1950. California angling catch records from postal card surveys: 1936-1948. Calif. Fish and Game, 36 (3) : 178-234. Calhoun, A. J. and C. A. Woodhull 1948. Progress report on studies of striped bass reproduction in relation to the Central Valley Project. Calif. Fish and Game, 34 (4) : 171-188. Calhoun, A. J. and C. A. Woodhull, and W. C. Johnson. 1950. Striped bass reproduction in the Sacramento River System in 1948. Calif. Fish and Game, 36 (3) 135-145. Chadwick, Harold K. 1965. Determination of sexual maturity in female striped bass Roccus saxatilis Calif. Fish and Game, 51 (3) : 202-206. Curtis, Brian. 1949. Striped Bass. (In) The warm-water game fishes of California. Calif. Fish and Game, 35 (4) : 266-268. DUNBIER, R. 1968. The Sonoran Desert. Univ. Ariz. Press, Tucson, Arizona. 426 P. Emig, John W. 1971. California inland angling survey for 1969, with corrections for the 1964 survey. Calif. Fish and Game, 57 (2) : 99-106. GOSLINE, W. A. 1966. The limits of the fish family SERRANIDAE, with notes on other lower periods. PROC. California Acad. SCI. , 33 : 91-112. HERBACH, W., R. J. Toth and A. M. McCready. 1963. Food of young of the year striped bass Roccus saxatilis in the Sacramento-San Joaquin River System. Calif. Fish and Game, 49 (4) 224-239. Hildebrand, Samuel F. and William C. Schroeder. 1928. Fishes of Chesapeake Bay. Bull. U. S. Bur. Fish. (1927). 43 : 247-250. Hollis, E. H. 1952. Variation in the feeding habits of the striped bass, Roccus saxatilis (WALBAUM), in Chesapeake Bay. Bingham Oceanographic Coll. Bull., 14 (1) : 111-131. 44

Jackson, H. W. and R. E. Tiller. 1952. Preliminary observations on spawning potential in the striped bass Roccus saxatilis (WALBAUM). Md. Dept. Res. and Ed., No. 93 : 1-16. Johnson, W. C. and A. J. Calhoun. 1952. Food habits of California striped bass. Calif. Fish and Game, 38 (4) : 531-534. LagLER, K. F. 1952. Freshwater Fishery Biology. W. C. Brown Co., Dubuque, Iowa : 421 P. Lewis, Robert MINTURN. 1962. Sexual maturity as determined from ovum diameters in striped bass from North Carolina. Trans. Amer. Fish. Soc., 91 : 279-282. Lewis, Robert M. and Rupert R. Bonner, Jr. 1966. Fecundity of the Striped Bass, Roccus saxatilis (WALBAUM). Trans. Amer. Fish. Soc. , 95 : 328-331. Lowe, C. H. 1967. The Vertebrates of Arizona. Univ. Ariz. Press, Tucson, Arizona. 426 P. MANSUETI, Romeo. 1961. Age, growth and movements of the striped bass, Roccus saxatilis, taken in size selective fishing gear in Maryland. Ches. SCI., 2 (1-2) : 9-36. McConnell, W. J. and J. H. Gerdes. 1964. THREADFIN shad, Dorosoma petenense, as food of yearling CENTRARCH DS. California Fish and Game, 50 (3) : 170-175. MCKECHNIE, Robert J. and Lew W. Miller. 1971. The striped bass party boat fishery: 1960-1968. Calif. Fish and Game, 57 (1) : 4-16. Merriman, Daniel. 1941. Studies of the striped bass Roccus saxatilis of the Atlantic Coast, U. S. Fish and Wildlife SERV. , Fish. Bull. 50 (35) : 1-77. Morgan, Alfred R. and Arthur R. Gerlach. 1950. Striped bass studies on Coos Bay, Oregon in 1949 and 1950. Oregon Fish Comm., Contr. No. 14, 31 P. Pearson, J. C. 1938. The life history of the striped bass, or rockfish, Roccus saxatilis (WALBAUM). Bull. U. S. Bur. Fish., 49 (28) : 825-851. Raney, E. C. 1952. The life history of the striped bass Roccus saxatilis (WALBAUM). Bull. Bingham OCEANOGR. Coil., 14 (1) : 5-97. . 1955 and William S. WOOLCOTT. 1955. Races of the striped bass, Roccus saxatilis (WALBAUM), in southeastern United States. PROC. 9TH Ann. CONF. , S. E. Assn. of Game and Fish Comm., 60-64. . 1957. SUBPOPULATION of the striped bass Roccus saxatilis (WALBAUM) in tributaries of Chesapeake Bay. U. S. Fish and Wildlife SERV. Spec. SCI. Rept. , Fish. No. 208 : 85-107. . 1958. The Atlantic states cooperative striped bass program, 1952-1957. 16TH annual report of the Atlantic States Marine Fish. Comm., 70-73. 45

Ritchie, Douglas E. 1965. Sex determination of live striped bass Roccus saxatilis (Walbaum) by biopsy technique Ches. Sc., 6 (3) : 141-145. Robinson, John B. 1960. The age and growth of striped bass Roccus saxatilis in California. Calif. Fish and Game, 46 (3) : 279-290. Scofield, Eugene C. 1931. The general ecology of Roccus lineatus. M. S. thesis, Stanford University. Printed as: The striped bass of California Roccus lineatus. Sacramento, Calif. Div. Fish and Game. Fish Bull. No. 29, 84 p. Scruggs, G. D., Jr. 1954. The rock fish puzzle - are they landlocked? So. Car. Wildlife, Summer 1954, 1 (3) : 4-5. . 1957. Reproduction of resident striped bass in Santee-Cooper reservoir, South Carolina. Trans. Amer. Fish Soc. , 85 : 144-159. Scruggs, G. D., Jr. and J. C. Fuller, Jr. 1954. Indications of a freshwater population of striped bass, Roccus saxatilis (Walbaum) in Santee- Cooper reservoirs. Proc. 8th Ann. Conf. , S. E. Assn. of Game and Fish Comm., 253-264. Stevens, Robert E. 1957. The striped bass of the Santee-Cooper reservoir. Proc. 11th Ann. Conf. , S. E. Assn. of Game and Fish Comm., 253-264. Vladykov, V. D. and D. H. Wallace. 1952. Studies of the striped bass, Roccus saxatilis (Walbaum), with special reference to the Chesapeake Bay region during 1936-38. Bull. Bingham oceanogr. Coil., 14 (1) : 132-177. Whitehead, P. J. and A. C. Wheeler. 1966. The generic names used for the sea basses of Europe and North America (Pices: Serranidae) Ann. Mus, Civ. Stor. Nat. Genova, 74 : 23-41. Woodhull, Chester. 1947, Spawning habits of the striped bass Roccus saxatilis in California waters. Calif. Fish and Game, 33 (2) : 97-102. BIOGRAPHICAL SKETCH

Gary B. Edwards was born in Portsmouth, Ohio on November 15, 1942. Shortly thereafter he moved to Glendale, Arizona where he completed his elementary education at Glendale Grammar School. He then moved to El Cerrito, California, where he attended and graduated from El Cerrito High School, in 1960. His college education was received from Whitworth College; Spokane, Washington, Humboldt State College; Arcadia, California and University of Arizona, Tucson. He graduated from the latter in 1966 with a Bachelor of Science in Wildlife Management. He was then employed by the Arizona Game and Fish Department as a Wildlife Manager at Bullhead City. In 1969 a leave of absence was taken to enter the Graduate School of Zoology at Arizona State University. He presently has returned to work for the Game and Fish Department and holds the position of Fishery Specialist in Region V, Phoenix, Arizona.