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Dissertations, Theses, and Masters Projects Theses, Dissertations, & Master Projects
1980
The life history of the cownose ray, Rhinoptera bonasus (Mitchill 1815), in lower Chesapeake Bay, with notes on the management of the species
Joseph W. Smith College of William and Mary - Virginia Institute of Marine Science
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Recommended Citation Smith, Joseph W., "The life history of the cownose ray, Rhinoptera bonasus (Mitchill 1815), in lower Chesapeake Bay, with notes on the management of the species" (1980). Dissertations, Theses, and Masters Projects. Paper 1539617505. https://dx.doi.org/doi:10.25773/v5-jb3f-zk42
This Thesis is brought to you for free and open access by the Theses, Dissertations, & Master Projects at W&M ScholarWorks. It has been accepted for inclusion in Dissertations, Theses, and Masters Projects by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. THE LIFE HISTORY OF THE COWNOSE RAY,
RHINOPTERA BONASUS (MITCHILL 1815), IN LOWER CHESAPEAKE BAY,
WITH NOTES ON THE MANAGEMENT OF THE SPECIES
A Thesis
Presented to
The Faculty of the School of Marine Science
The College of William and Mary
In Partial Fulfillment
Of the Requirements for the Degree of
Master of Arts
by
Joseph William Smith
1980 ProQuest Number: 10626291
All rights reserved
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In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest,
ProQuest 10626291
Published by ProQuest LLC (2017). Copyright of the Dissertation is held by the Author.
All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC.
ProQ uest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 APPROVAL SHEET
This thesis is submitted in partial fulfillment of
the requirements for the degree of
Master of Arts
n Joseph W. Smith
Approved, August 1980
JohnAV. Merriner
J 5 ^ T : Mus'/ck
Joseph (G. Loesch DfH Robert J. Orth yV
ii To
Frank Smith, an old Delaware Bay fisherman.
iii TABLE OF CONTENTS
Page
DEDICATION...... iii
ACKNOWLEDGEMENTS ...... vi
LIST OF TABLES...... viii
LIST OF F I G U R E S ...... x
ABSTRACT...... xiii
INTRODUCTION ...... 2
METHODS AND MATERIALS ...... 4
Distribution and Collections ...... 4
Food Habits ...... 8
Reproduction ...... 9
Age and G r o w t h ...... 10
RESULTS ...... 15
Distribution ...... 15
Food H a b i t s ...... 38
Feeding Behavior ...... 45
Reproduction...... 48
Age and G r o w t h ...... 65
DISCUSSION...... 76
Distribution ...... 76
Food H a b i t s ...... 91
iv Table of Contents (concluded)
Page
Feeding Behavior ...... 98
Reproduction ...... 104
Age and G r o w t h ...... 117
MANAGEMENT OF THE COWNOSE RAY IN THE CHESAPEAKE B A Y ...... 122
APPENDICES...... 127
LITERATURE CITED ...... 131
V I T A ...... 151
v ACKNOWLEDGEMENTS
My sincere appreciation is extended to Dr. John V. Merriner for his continual advice and assistance during the course of this study
and for his critical review of the various drafts of the manuscript.
Dr. John A. Musick and Dr. Joseph G. Loesch provided helpful comments during various phases of this investigation and gave valuable editorial assistance. The contributions of my other committee members, Dr. Robert J. Orth and Dr. Robert Byrne are gratefully acknowledged.
Numerous graduate students and Virginia Institute of Marine
Science staff members assisted during various phases of this study and their contributions are gratefully acknowledged. J. Gourley, R.
Lambert, R. K. Dias and Dr. Robert J. Orth capably assisted in the
field work and offered numerous helpful suggestions. R. K. Dias and
F. J. Wojick assisted in data programming and processing. C. E.
Richards acquired several eastern shore ray collections. I am
indebted to fellow graduate students, E, F. Lawler, G. Sedberry, J.
Govoni, J. Colvocoresses, J. Ross and J. Travelstead for allowing me to converse with them on countless occasions about the cownose ray.
Special thanks to W. Raschi for curatorial assistance.
Captains Buddy Ponton of Corolla, North Carolina, Alton Hudgins of Hampton, Virginia, George Ross of Lynnhaven, Virginia, Herman Green
vi and Benny Belvin of the Perrin River, Virginia, Morris Owens of
Wicomico, Virginia, F. T. Jett of Ophelia, Virginia, C. Conklin of Mt
Holly, Virginia, J. Dryden of Poquoson, Virginia, and E. Grinnell of
Horn Harbor, Virginia kindly supplied R. bonasus specimens and
generously shared their watermans' knowledge of the "stinger's"
habits.
I am grateful for the contributions of the following persons: F
M. Douglas provided information on the West Coast bat ray; R. Minkler
(NMFS, Pascagoula, Mississippi) and M. Wolff (North Carolina Division
of Marine Fisheries, Morehead City) provided data on wintertime
catches of R. bonasus; Dr. W. S. Otwell (University of Florida,
Gainesville) unselfishly provided information on the movement and habits of R_. bonasus in North Carolina waters; Dexter Haven (VIMS)
shared his extensive knowledge of Virginia's oyster industry; Dr.
Frank Schwartz (University of North Carolina, Morehead City) shared
information on R. bonasus biology in upper Chesapeake Bay; Sam White
piloted the VIMS pound net survey flights; D. Taphorn (University of
Zulia, Maricaibo, Venezuela) provided information on cownose ray movements in Venezuelan waters; VIMS Art Room personnel prepared many
of the figures in this work; Ruth Edwards of the VIMS Report Center prepared the final draft of the manuscript.
My sincere thanks to my wife, Rosemarie, for typing initial drafts of this work.
The perseverance of all is gratefully appreciated. LIST OF TABLES
ge
Initial sightings of Rhinoptera bonasus during Spring 1977 slong the Virginia-North Carolina coast ...... 26
Initial sighting of Rhinoptera bonasus during Spring 1978 along the Virginia-North Carolina coast ...... 27
Late fall and wintertime records of Rhinoptera bonasus from the South Atlantic Bight ...... 39
List of representative food items from Rhinoptera bonasus stomachs ...... 42
List of shell fragments from Rhinoptera bonasus spiral v a l v e s ...... -...... 42
Percent frequency occurrence (F), percent number (N) , percent volume (V), and index of relative importance (iRl) of food items in 40 Rhinoptera bonasus stomachs from the York, Poquoson, and Ware rivers in Virginia ...... 43
Food items in Rhinoptera bonasus stomachs from areas other than York River and vicinity ...... 44
A list, frequency of occurrence, and number of accidental and questionable ingestions from Rhinoptera bonasus stomachs ...... 46
Data for Rhinoptera bonasus young, as collected chronologically, summers of 1976, 1977 and 1978 . . . 51
Data for Rhinoptera bonasus young supplied by W. S. Otwell from a haul seine set in Core Sound, N.C., April 24, 1978 ...... 56
Mean back-calculated disc widths (mm) of male Rhinoptera bonasus ...... 68
Mean back-calculated disc widths (mm) of female Rhinoptera bonasus ...... 69
viii List of Tables (concluded)
Table Page
13. Analysis of covariance for male and female Rhinoptera bonasus disc width-weight regressions ...... 74
14. Estimated speed (nautical miles/day) of Rhinoptera bonasus between Cape Hatteras, N.C. and northern Florida during the fall migration south (data from Schwartz 1 9 65)...... 88
15. Estimated speeds (nautical miles/day) of Rhinoptera bonasus between several points along the U.S. East Coast during the spring migration north ...... 88
ix LIST OF FIGURES
Figure Page
1. Major collection sites for Rhinoptera bonasus and locales noted in text in the lower Chesapeake Bay and and vicinity...... 5
2. Collecting sites for Rhinoptera bonasus in the lower York River and v i c i n i t y ...... 6
3. Overall disc width frequencies of male and female Rhinoptera bonasus collected in 1976, 1977 and 1978 . . . 16
4. Average weekly water temperature of York River at Gloucester Point April through November 1976, 1977 and 1978 (Source: VIMS Department of Physical Oceanography) ...... 17
5. Dates of first appearance of Rhinoptera bonasus in various parts of lower Chesapeake Bay, as per response from commercial fishermens’ questionnaire .... 18
6. Dates of peak abundance of Rhinoptera bonasus in various parts of lower Chesapeake Bay, as per response from commercial fishermens' questionnaire .... 19
7. Dates of last capture of Rhinoptera bonasus in various parts of lower Chesapeake Bay, as per response from commercial fishermens' questionnaire ...... 20
8. Massive school of Rhinoptera bonasus photographed near Barden Inlet, Core Sound, N.C. on April 24, 1978 ...... 23
9. Initial sightings or collections of Rhinoptera bonasus along the East Coast of the U.S. during Spring 1977 . . . 24
10. Initial sightings of Rhinoptera bonasus along the Virginia-North Carolina coast during Spring 1978 ...... 25
11. Disc width frequency of Rhinoptera bonasus by sex collected at Sandbridge, Va. and Corolla, N.C. by haul seines, May 6, 1976, May 2-5, 1977 and May 16-17, 1978 ...... 29
x List of Figures (continued)
F igure Page
12. Disc width frequencies of Rhinoptera bonasus by sex collected in the lower and upper York River by pound nets, gill nets and seines. A. May 1977. B. June 14-29, 1976 and 1977. C. July 1-14, 1976. D. July 15-26, 1976. E. July 28, 1976 and July 21, 1978. F. August 3-6, 1976 and August 4, 1977. G. August 16-29, 1976 and August 25-31, 1977. H. September 8, 1976, September 21, 1977 and September 7, 1978 .... 32
13. Disc width frequencies of Rhinoptera bonasus by sex collected in the Potomac River by pound net. A. July 30, 1976 and August 13-26, 1976. B. September 13, 1976 . 34
14. Disc width frequencies of Rhinoptera bonasus by sex collected near Lynnhaven Inlet by pound net and otter trawl. A. July 7 and 26, 1976 and July 15, 1977. B. September 10 and 23, 1976, October 7, 1976, September 23, 1977 and October 12, 1977 35
15. Disc width frequencies of Rhinoptera bonasus by sex collected near Kiptopeke by pound net July 20-29, 1976 and August 17 , 1976 ...... 37
16. Late Fall and Winter records of Rhinoptera bonasus from the South Atlantic B i g h t ...... 40
17. Clasper length (mm)-disc width (mm) relationship for 188 male Rhinoptera b o n a s u s ...... 49
18. Rhinoptera bonasus embryo disc width (mm) versus date of capture, 1976, 1977 and 1978 57
19. Three-quarter term Rhinoptera bonasus young collected May 19, 1977 (DW = 275 mm). Nearly full term Rhinoptera bonasus young collected June 14, 1977 (DW = 380 nun) ! T ...... 59
20. Series of second brood Rhinoptera bonasus embryos ranging in size from 18-140 mm D W ...... 60
21. Yolk sac volume (ml) of second brood Rhinoptera bonasus embryos-disc width relationship ...... 63
22. Disc width (mm)-vertebral radius (ocular micrometer units) relationships for 69 male and 60 female Rhinoptera bonasus (1 o.m.u. = 0.131 mm) ...... 66
xi List of Figures (concluded)
Figure Page
23. Walford lines for male and female Rhinoptera bonasus with estimates of asymptotic disc widths Tdw^ ...... 71
24. Empirical lengths and Von Bertalanffy calculated lengths-at-age for male and female Rhinoptera b o n a s u s ...... 72
25. Disc width (mm)-weight (g) relationships for male and female Rhinoptera bonasus ...... 75
26. Spring 1978 sea surface isotherms along the Virginia-North Carolina coast ...... 82
27. Maryland and Virginia Mya arenaria landings, 1953- 1976 ...... 95
28. Disc width frequency of small Rhinoptera bonasus (< 630 mm DW, sexes combined) collected along the Virginia-North Carolina coast in early May 1976, 1977 and 1978 ...... 119
xii ABSTRACT
Study objectives included investigation of the distribution, feeding habits, reproductive biology, age and growth and population structure of the cownose ray, Rhinoptera bonasus, in lower Chesapeake Bay and suggestions for possible management options for protecting the Bay's commercially important shellfish beds from cownose ray depredations. Over 900 cownose rays were obtained primarily from commercial fishing gears (pound nets and haul seines) in lower Chesapeake Bay and vicinity from May through October for three consecutive years (1976, 1977 and 1978). Massive schools of rays arrived off the North Carolina coast in April and entered Chesapeake Bay by early May. The spring migrants schooled by size. Cownose rays were abundant in the river systems of the Bay throughout the summer and were collected in salinities as low as 8.1 °/oo and at water temperatures between 15-29°C. Size segregation continued during the summer and the adults schooled by sex. Most rays left the Bay by early October. Possible overwintering habitats are discussed.
Stomach content analyses indicated the soft-shelled clam, Mya arenaria, was the preferred food item of the cownose ray in the river systems of western shore. Young-of-the-year and juvenile rays fed on shallow- or non-burrowing bivalves. Feeding schools of R. bonasus invaded shoal waters during high tide. Several hypotheses concerning the feeding dynamics of _R. bonasus are proposed.
Adult males averaged 89 cm across the disc and 11.8 kg. Sexual maturity was reached when about 80-84 cm wide. Adult females were somewhat larger, averaging 96 cm and 15.5 kg in weight. They matured at about 90 cm across the disc. The young, usually one per gravid female, were born in late June and early July and averaged 40 cm wide. Gravid females left the Bay in the fall with a second brood of young. Aspects of embryonic development and nutrition are discussed.
Age and growth analysis as determined by vertebral sectioning indicated R. bonasus are moderately long lived. Yearling and age II rays measured about 51 and 59 cm wide, respectively. Ages at maturity were 5-6 years for the males and 7-8 years for females. Both sexes matured after attaining about 80% of their maximum size. Derived growth coefficients were K = 0.215 and 0.149 for males and females, respectively.
Fences composed of large mesh netting represent the best short-term method of protecting shallow (< 2 m), commercial shellfish beds from cownose ray predation. Creation of a directed fishery for rays (i.e., ray wing meat) is suggested as the best long-term solution to the cownose ray predation problem.
xiii THE LIFE HISTORY OF THE COWNOSE RAY,
RHINOPTERA BONASUS (MITCHILL 1815), IN LOWER CHESAPEAKE BAY
WITH NOTES ON THE MANAGEMENT OF THE SPECIES INTRODUCTION
The cownose ray, Rhinoptera bonasus (Mitchill 1815), is a moderately large ray and is one of the most abundant elasmobranch
fishes in the Chesapeake Bay during the summer months. Bigelow and
Schroeder (1953) listed its range as the elbow of Cape Cod to the Gulf coasts of Florida and Louisiana. They also recognized a second species of cownose ray, R. brasiliensis, in Brazilian waters. Tooth
formulae were suggested as the separating character, although considerable intergradation was observed between the two species.
Subsequent workers recommended synonymizing both species under R. bonasus (Springer and Woodburn 1960; Schwartz 1965).
Early investigators contributed information to the life history of JR. bonasus (Smith 1907; Gudger 1910, 1912; Radcliffe 1914; Coles
1915). In 1953 Bigelow and Schroeder reviewed and summarized existing information on the species.
Several authors provided notes on the occurrence and abundance of
R. bonasus in Chesapeake Bay (Hildebrand and Schroeder 1928; Bayliff
1951; Joseph 1961; Hoese 1962; Musick 1972).
The most comprehensive work to date on the life history of
R. bonasus has been reported by Schwartz (1965, 1967) in the form of two brief author's abstracts. Aspects of the ray's migratory habits,
2 3 gestation cycle, feeding behavior and systematics were briefly d iscussed.
Notes on the cownose ray diet which is primarily restricted to bivalve molluscs have been reported by Bayliff (1951) , Wallace et al.
(1965), Orth (1975) and Otwell and Crow (1977). Also ancillary dietary notes have appeared in larger faunal surveys (Mitchill 1815;
Smith 1907; Radcliffe 1914; Darnell 1958; Aquirre 1965; Wang and Raney
1971). Aspects of R. bonasus reproductive biology are primarily limited to notes on the occurrence of embryos (Gudger 1910; Bigelow and Schroeder 1953; Joseph 1961; Hoese 1962; Bearden 1965; Orth 1975).
Information concerning growth rates are unavailable, however, Mitchill
(1815) and Smith (1907) claimed that maximum sizes of 100 pounds and
7 feet are attained, respectively. Schwartz (1959) and Joseph (1961) have published notes on albinism in R. bonasus, whereas Gregory (1935) addressed aspects of the ray's swimming ability.
During the summers of 1972-1975 several Rappahannock River oyster growers reported substantial losses to seed and harvestable oyster beds due to cownose ray predation. Concurrently, feeding schools of
R. bonasus were reported to have a detrimental impact on eelgrass
(Zostera marina) beds (Orth 1975). The objectives of this investigation were to study the life history of the cownose ray in lower Chesapeake Bay, including distribution patterns, food and feeding habits, reproductive biology, age and growth and population structure, and to suggest management options for controlling cownose ray predation on commercially important shellfish beds in the lower
Chesapeake Bay. METHODS AND MATERIALS
Distribution and Collections
Commercial fishermen in Chesapeake Bay and vicinity were the primary sources of study materials from May through October, 1976,
1977, and 1978. Variability in fishermen's habits, adverse weather conditions and sporatic commercial catches of R. bonasus resulted in a highly variable sampling regime. Since rays are generally considered nuisances and culled overboard, I contacted fishermen one day prior to a sampling date and requested that they land the next day's catch of
R* bonasus. Occasionally, I accompanied a fisherman to his nets. A few collections were frozen until pick-up was convenient. Major collection sites are noted on Figures 1 and 2.
Pound nets located in the lower York River (east of Gloucester
Point; Fig. 2) yielded a majority of the collections during 1976.
Pound nets are normally fished once every 24-48 hours; thus multiple schools of R. bonasus may have entered the gear during the fishing period. Fishermens' comments and personal observations confirmed that a majority of the rays do not enter the head of the net, rather they stay in the wings or bays.
During August and September 1976 and throughout the summer of
1977 efforts were made to collect actively feeding schools of rays in
4 5
Figure 1. Major collection sites (*) for R_. bonasus (circled in
figure) and locales noted in text in the lower Chesapeake
Bay and vicinity.
*1. Corolla, N.C. 16. Stingray Point
*2. Sandbridge, Va.- 17. Windmill Point False Cape State Park
3. Rudee Inlet 18. Towles Point
4. The "Lumps” 19. Jones Point
5. Cape Henry 20. Bowler's Wharf
*6. Lynnhaven Inlet 21. Warsaw, Va.
7. Ocean View, Va. 22. Fleet Point
8. Surry Vepco Plant *23. Smith Point
9. Hampton Roads 24. Yeocomico River
10. Back River 25. Kingcopsico Point
*11. Poquoson River 26. Solomons, Md.
*12. Goodwin Inlands 27. Tangier Island
*13. Lower York River 28. Wachapreague, Va.
14. Claybank, Va. 29. Cape Charles City Inlet
15. New Point Comfort *30. Kiptopeke, Va.
31. Wreck Island 26
25
38 00
20 22 -f-
28
30 6
Figure 2 Collecting sites for Rhinoptera bonasus in the lower York River and vicinity. o o < 9
GLOUCESTER PT. k VIMS
YORKTOWN 7 shallow water by pursing a 91 m monofilament gill net or a 91 m shark gill net.
Commercial haul seine operators at Sandbridge, Virginia and
Corolla, North Carolina provided samples in May 1976, 1977 and 1978.
Other gears used for summer collections were rod and reel, haul seine, otter trawl, and longline.
York River water temperature data collected at Gloucester Point
(VIMS) for 1976-1978 were used to relate ray movements and estuarine water temperature. Water temperatures along the Virginia-North
Carolina coastline in Spring 1978 were obtained from the National
Weather Service's sea surface temperature analysis charts (Rockville,
Md . ) .
Two airplane flights (20-IV-76, 14-IV-77) were made along the
Virginia-North Carolina coastline to observe migrating schools of rays. I accompanied the VIMS pound net survey flight on 6 occasions to chart the distribution of R. bonasus in Chesapeake Bay and its major tributaries. Commercial haul seiners and N.C. Division of
Marine Fisheries personnel (Morehead City) provided information concerning the initial spring appearances of R_. bonasus along the
North Carolina coast. A questionnaire was mailed to all licensed pound net and haul seine fishermen in Virginia which requested the time of initial and final catches, period of peak abundance, and trends in abundance of R. bonasus (Appendix Fig. 1). 8
Food Habits
Stomachs containing food items were separated from the esophagus and spiral valve and the contents poured into a 600 ml Twirl Pak®.
The stomach was then cut open longitudinally and preserved with the contents in 10% buffered formalin. Later, stomach and contents were stored in 40% isopropyl alcohol after soaking in water. The spiral valve was also cut longitudinally and examined for shell fragments.
Identification of molluscs was based on valve descriptions in
Turgeon (1968) and a reference collection of bivalves obtained in the field. Crustaceans and annelids were identified using Gosner (1971).
An index of relative importance (Pinkas et al. 1971) was used to evaluate the contributions of various food items found in the stomachs of R. bonasus. Tabulation of the index (iRl) incorporates measurements of numbers, volume and frequency of occurrence of food items. Food item numbers when recorded separately may be biased for numerous small items and volume may be biased for large bulky items.
The index of relative importance, IRI, was calculated as follows:
(N + V) F = IRI where
N = Numerical percentage
V = Volumetric percentage
F = Frequency of occurrence percentage
IRI = Index of relative importance 9
Molluscan bodies were often, partially digested. Numerical abundance in these cases was based on the occurrence of body parts most resistant to digestion; each foot (Tagelus, Ensis, Mytilus, and
Mercenaria), siphon (Mya) or hinge (Mulinia) was counted as one item.
Volume displacement was measured by placing food items in a graduated cylinder and filling it to a calibration mark with a self-leveling buret (McEachran 1973).
Reproduction
The attainment of sexual maturity in male R. bonasus was based on several criteria: 1) length and development of the claspers; the length of the right clasper was measured as the distance from junction of the clasper and pelvic fin to distal end of the clasper, and 2) condition of the testes in fresh specimens (color, size, presence of sperm upon incision) and presence of sperm in the seminal vesicles and clasper grooves.
The following criteria were used to determine the state of sexual maturity in female R. bonasus (modified from Smith 1975):
1) immature - ovaries thin and flaccid; uterus thin and
elongate, lining appears rugous.
2) maturing - ovary showing some development, yellowish eggs
visible, < 1 cm diameter; uterus showing some dilation,
trophonemata small, generally < 0.5 cm.
3) mature - large yellowish eggs in ovary, > 1 cm diameter;
uterus well-developed and rich in trophonemata, generally
> 1 cm long. 10
Uteri and oviducts were cut open and inspected for the presence of eggs or embryos. Condition of both uteri was noted. Gravid females usually aborted their young shortly after capture; hence, with large catches, it was often impossible to identify each embryo with its parent. Young were preserved in 10% buffered formalin. Weight, disc width, sex and yolk sac volume were determined in the laboratory.
The latter was determined by volume displacement in a graduated cylinder. The disc width and weights of all three-quarter and full term young were measured in the field prior to preservation.
Age and Growth
Age analysis of elasmobranch fishes by conventional techniques employed for teleosts is not possible. Several investigators have used other methods such as tagging (Holden 1972a), size frequencies
(Olsen 1954; Aasen 1963; Sage et al. 1972) and rate of tooth replacement (Moss 1967, 1972). The spines of dogfish (Holden and
Meadows 1962; Ketchen 1975) and vertebrae have also proven useful for aging chondrichthians.
Increase in the number of concentric vertebral rings with increase in specimen size has been observed in sharks (Haskell 1949;
Parker and Stott 1965; Stevens 1975) and skates (ishiyama 1951; Daiber
1960; Richards et al. 1963; Taylor and Holden 1964). Holden and Vince
(1973) validated vetebral rings as annuli in Raja clavata, using tetracycline injections and recapture of these tagged specimens. They demonstrated that one opaque and one hyaline zone were deposited annually. Several techniques involving treatments such as 11 dehydration, impregnation with paraffin, decalcification and staining have been recommended to improve the "readibility" of the vertebral rings (Haskell 1949; Ishiyama 1951; Daiber 1960; Richards et al. 1963;
Taylor and Holden 1964; La Marca 1966; Stevens 1975).
Vertebrae were chosen as the aging materials for R. bonasus.
Most rays were obtained from commercial fishing gears in lower
Chesapeake Bay and vicinity between May and October 1976, 1977 and
1978. Specimen disc width (distance between the tips of the pectoral fins, DW hereafter) was measured to the nearest mm on a measuring board with the ray's ventral surface down. Weights were recorded to the nearest quarter pound (later converted to grams) on a top-loading scale (Berkel, Inc. La Porte, Ind., Model no. M - 6 4 ) . The vertebral column with the first to approximately the thirtieth vertebrae was excised through the body cavity after removal of the vicera. Excess flesh and connective tissue was stripped away in the field. The column was stored in 70% ethanol in a Twirl-Pak® labeled with station and specimen number. The twentieth to the twenty-fifth vertebrae were used as aging materials, because a radiograph of a small ray indicated these vertebrae were the largest of the column. The selected vertebra was removed with a scalpel. Connective tissue and lateral portions of the neural arch were removed. When viewed macroscopically, the innermost concentric rings on the cleaned, vertebral centrum appeared as alternating regions of wide (light) and narrow (dark) circuli.
Small ridges, coincident with the light zones were detected on the face of the centrum. Individual rings and ridges toward the periphery of the centrum were difficult to detect. 12
Attempts were made to stain whole vertebrae by the methods of La
Marca (1966) and Stevens (1975). Unsatisfactory results were obtained with the former. The light zones of small vertebrae stained well with the silver nitrate technique of Stevens, however individual rings on the outer margins of large vertebrae were indistinguishable. Sagittal sections through the center of vertebrae were also made and stained using Stevens method. Again, the outermost rings (bands on the face of the cut surface) were indistinct.
The final method adopted was similar to that of Lawler (1976), the vertebrae were sectioned in half. Instead of a stain, heat was used to accentuate the rings. Cleaned vertebrae were air dried for
2-3 hr, then mounted with paraffin on a small cardboard tag (3 cm x
5 cm) bearing station and specimen number. Orientation of the mount was such that the horizontal longitudinal axis (3-9 o'clock) of the vertebra was in the 12-6 o'clock position with respect to the longitudinal axis of the tag. An Isomet 11-1180 low speed saw
(Buehler Ltd., Evanston, 111.) with a circular diamond watering blade
(Norton Co., Worchester, Mass.) was used to perform the cut. A chock on the arm of the saw held the tag in position as the cut was made through the center of the centrum. The cut was made through
3-9 o'clock axis because the vertebrae appeared compressed dorsoventrally. The result was an hourglass-shaped face which was then polished with honing compound on a glass plate. Vertebrae were then heated in an electric oven at about 200°C for 2.5-3 minutes.
Heat-treated vertebrae had alternating wide, light bands and narrow, dark bands. These were identical to the light and dark bands of 13 unheated vertebrae, but much more distinct. Reasons for this enhancement on teleost hard parts have been discussed by others
(McEachran 1968; Blacker 1974). After 18 months of dry storage, the banding pattern remained highly conspicuous.
Regarding the light and dark zones of teleost otoliths, the descriptive terms hyaline and opaque have often been inverted due to differences in the method of illumination (Blacker 1974). Herein, opaque zones are those which appear light when R. bonasus vertebrae are viewed with reflected light against a black background. Hyaline zones are those which appear dark when viewed with reflected light against a dark background.
Vertebrae were immersed in 70% ethanol to improve the clarity of the rings and viewed on a dark field with reflected light under a dissecting microscope. The dark hyaline zones were considered annuli.
The distance from the center of the centrum (notochord aperture), along the edge ~f the hourglass, to the lateral margin of the vertebra was considered the vertebral radius. Measurements from the center of the centrum along the edge of the hourglass to each annulus were made for back-calculation purposes. All measurements were made using an ocular micrometer (1 ocular micrometer unit = 0.131 mm). Vertebrae were read on two occasions several weeks apart. Those difficult to read were examined a third time. Approximately 25% were discarded due to a cloudy cut surface, oblique sectioning or indistinct annuli. A total of 91 vertebrae were utilized for my age analysis. 14
Back calculations of DW at age for each sex were performed as described in Everhart et al. (1975). Von Bertalanffy growth equations were derived according to Ricker (1975). Equations describing the disc width-weight relationships for R. bonasus were calculated according to Bagenal and Tesch (1978). Computations were performed on an IBM 370-115 computer with programs from the computer library of the
Ichthyology Department of the Virginia Institute of Marine Science. RESULTS
Distribution
A total of 899 R. bonasus were obtained at 12 collecting sites
(Figs. 1 and 2): pound net (606), haul seine (212), shark gill net
(37), gill net (26), otter trawl (9), rod and reel (8) and longline
(1). Disc width frequency distributions (5 cm increments) for 445 males and 428 females are presented (Fig. 3). The overall sex ratio did not differ significantly from 1:1 (xA= 0.331). Number of specimens by sex for all collections are shown in Appendix Table 1.
Average weekly surface temperatures in the York River (Gloucester
Point, VIMS) ranged from 8.6 to 29.5°C for April through November
1976, 1977 and 1978 (Fig. 4). Cownose rays were taken at temperatures from 15-29°C and in salinities from 8.1 to 30 °/oo during the study.
The intitial appearance, period of peak abundance, and date of final capture of R. bonasus in lower Chesapeake Bay as reported by fishermens' questionnaire respondants (n = 40, 21% response) are summarized in Figures 5, 6 and 7.
Turbidity of Chesapeake Bay waters during the summer precluded aerial observation where water depth exceeded 2-3 m, unless R. bonasus were swimming near the surface. From an altitude of 100 m the blunt snout of R. bonasus was apparent, the dorsal surface appeared light
15 16
Figure 3 Overall disc width frequencies of male and female Rhinoptera bonasus collected in 1976, 1977 and 1978. PERCENT OF TOTAL CATCH 12 12 10 13 13- 10- II • 8 7- 6 9 6 5 2 7- 2 4 3- 8- 4- 3- 9- 5------
0 400 50 0 70 0 900 1000 0 0 9 800 700 600 500 0 0 4 300 5 40 5 60 5 80 5 1050 950 850 750 650 550 450 350 DISC IT (mm) WIDTH N=445 N=428 6 9 100 — II5C > 17
Figure 4 Average weekly water temperature of York River at Gloucester Point April through November 1976, 1977 and 1978 (Source: VIMS Department of Physical Oceanography). CD MONTH
o CD co c\j 00 ro CM < \J CM OJ
(Do) 3UniVU3dlN3± 18
Figure 5 Dates of first appearance of Rhinoptera bonasus in various parts of lower Chesapeake Bay, as per response from commercial fishermens' questionnaire. NONE c a u g h t L aug a • SEP AUG JULY
• MAY
NONE v. CAUGHT
NONE CAUGHT
AUG & SEP. MAY
END OF MAY MID MAY
• JUNE .JUNE 1 END OF MAY
JUN MAY# • MAY t END#/ OF f MAY !? • MAY I ^UEND OF MAY
37' 00 MAY I
NAUTICAL MILES 20 25
20 3040 50 KILOMETERS 19
Figure 6 Dates of peak abundance of Rhinoptera bonasus in various parts of lower Chesapeake Bay, as per response from commercial fishermens’ questionnaire. MID SEP AUG a SEP
38c JUN. a JUL 00
SEP 10-30 '•JU L a AUG
,»MID JUN -SEP • JUN a JUL
^ -A JULY -I JUNE
AUG •JUN-SEP JUN- • JUL a AUG -AUG
• AUG
37
00 JULY
NAUTICAL MILES 20 25
20 30 40 50 KILOMETERS
7 7 00 76°, 00' 20
Figure 7 Dates of last capture of Rhirtoptera bonasus in various parts of lower Chesapeake Bay, as per response from commercial fishermens' questionnaire. NOV
OCT
OCT 15 SEP
OCT
SEP
CT
\SEP EN •OCT SEP
OCT 1-15 .SEP 1 5 -3 0
37< 00
NAUTICAL MILES 20 25
20 30 40 50 KILOMETERS
77 00 21 brown, and the tips of the pectoral fins were visibly angular. Most
R. bonasus were observed in shoal or nearshore waters in schools of several individuals to several hundred rays. Stray individuals were infrequently observed. The distance between members of a school appeared no greater than 1-2 m. Most schools formed solid configurations and a broad triangular formation was commonly observed.
On occasions, a diamond-shaped school or an oblique line of 5-10 individuals was seen. All members were oriented in the same direct ion.
Seasonality by Aerial Survey (Locales noted on Figs. 1 and 2).
1976 - No rays were observed in the lower Bay (Poquoson
River to Cape Henry) during the flight of April 20. However, two schools of approximately 15-20 R. bonasus and a third school of adult cownose rays apparently engaged in feeding activity were seen near
Corolla, N.C. Several schools were sighted near Tangier Island on
May 18 (R. J. Orth, pers. comm.). On June 8 schools of rays were observed near Stingray and Windmill Points (Rappahannock River), Smith
Point (Potomac River), New Point Comfort (Mobjack Bay) and along bayside eastern shore. Solar glare hampered ray sightings on the
June 23 and July 16 flights. During the flight of September 29 no rays were seen in the York, Poquoson and Back Rivers or in the Hampton
Roads area. However, a large concentration of R. bonasus schools was observed along bayside eastern shore from Kiptopeke south to Cape
Charles, while two schools were sighted near Ocean View, Virginia.
The latter rays were visibly smaller than the adult R. bonasus 22 observed throughout the summer of 1976 and were obviously young-of- the-year and/or juvenile R. bonasus.
1977 - Only 2 R. bonasus were seen on April 14 during the beach transect flight from Cape Henry to Cape Hatteras. On June 2, schools of rays were sighted along bayside eastern shore from
Kiptopeke to Cape Charles City inlet and at Towles Point (Rappahannock
River). Additionally, large concentrations of ray schools were observed in nearshore shoal waters from the Yeocomico River to Smith
Point in the Potomac River and from Smith Point south to Fleet Point.
1978 - On April 24 North Carolina Division of Marine
Fisheries personnel (Morehead City) sighted a vast school of
R. bonasus in Core Sound, N.C., estimated to contain several thousand rays (Fig. 8). Otwell and Lanier (1978) described the commercial harvest of a portion of this school. No rays were observed in lower Chesapeake Bay or its tributaries on the VIMS' pound net survey flights ~f June 1 and 2. Many schools of rays were sighted in the lower Rappahannock and lower Potomac Rivers on June 19
(J. Travelstead, pers. comm.).
Spring - Virginia-North Carolina Coast
Sightings of _R. bonasus along the Virginia-North Carolina coast during the spring of 1977 and 1978 are summarized in Figures 9 and 10 and Tables 1 and 2. The northern Florida sightings in mid-March, 1977 were made by U.S. Coast Guard aircraft and duly noted on the Airborne
Radiation Thermometer Program surface isotherm chart of March 15-25. 23
Figure 8 Massive school of Rhinoptera bonasus photographed near Barden Inlet, Core Sound, N.C. on April 24, 1978.
24
Figure 9. Initial sightings or collections of Rhinoptera bonasus along the East Coast of the U.S. during Spring 1977. 80 ° 75 ° 40 40
2 JUN 77 28 MAY ' 7 7 ^
II MAY 77 6 MAY 77
26 APR 77
35 35 8 APR'77
800 USCG, 15 MAR 77
302000, 30 USCG, 18 MAR *77 20
25 25
80 ° 25
Figure 10. Initial sightings of Rhinoptera bonasus along the Virginia-North Carolina coast during Spring 1978. 19 JUN 78
10 JUN'78
35 # II MAY '78
CAPE HATTERAS
19 APR 78
35 80
75 ° INITIAL SIGHTINGS OF RHINOPTERA BONASUS DURING SPRING 1977 ALONG THE M O 1-4 pq Pi i—i H od O Pi O hJ M d: CO E-* 1
• r4 • r4 • r4 • r4 jq O C o H Pi .-4 Q 4-1 4—1 44 44 QJ o d cO CD 5 o d o a CO CO O 5 U QJ CO > CO QJ e QJ B cfl5 QJ 5 C d m m m m l
o z N I OQJ OO o o l -4 i- —i i— CN o T3 CN o O O 1— o o i <44 TO O T jq jq jq 5 5 H — 44 co O d co d QJ QJ CO o o o O CO o O 5 CO CO TO ) 0 5 d 5 co d 5 O 1 m m m n m rt . 1 1 o * • l—I • H • “ r*H OO <44 q j — i—4 i—4 3 T r^- r-- UN CN f— H jq o o < P - i— 1-4 pq CO o c 1-4 1-4 n E 44 44 4 4 44 co CO QJ rH O >4 QJ QJ cO QJ QJ X cO 54 60 QJ 54 CO 5 OJ CO QJ > O 5 B QJ 6 OJ PL, O n 1 I 1 m m n m t4 • —1 r— 44 a QJ CO C > QJ 5 O m n l -4 QJ 4 i- oo T—I TO <44 rd qq4 > CO P w CO CO £\ P 4 OJ 14 0 6 • o d a O „C o d cn O 5 J *“ J • H * 6D QJ > O J QJ 4-1 5-1 QJ CO x QJ e co co s-4 o co O 4 - 54 i i I m m M '<4-4 '<4-4 TO O P d O o u - m j
Pi o co co
* H d 4 r • CD r4 • CO r4 • v40 r^- » r 1—I joaj <4-4 CN jc o > pq o .-4 W p - T ' TO 4-1 H - J 4 44 QJ QJ o CO O TO CO CO QJCJ CO d o O CO CO d o QJ O 5 o co d COCO ^ JO 54 d d PC i—4 d I =H m h w JO 54 CO
• rH 1— 4
E CO QJ co 54 a. 5 CO QJ (0 d QJ h m m
• r4 • H • 5m H • <4-4 O J - r O T O T O J r- j: 4*5 Pi\ 1-4 > P >4 t-j p 44 44 4-1 4-1 a o cO co CO CO m O 5m JrfC 60 • o d QJ CJ CO o d 3 o d d QJ j c o c CO co l C C d C J QJ QJ 54 O 54 d I I m l , >4 U i
o • r4 • H • r4 • r4 • H O O <44 4-J O T jq <4-1 00 N C - ^ r i—4 c , JC jq > O C O pi J C P - 44 4-1 CO CO o o o O o CO CO 5 QJ o d 60 i—I 4 0 6 54 QJ 0) CO co O CO J2 J d - d CO q p d O I I m m n
*-) CO O T jq CO CO 0>4 IS CO CO d QJ 5 O O n m
2-VI-77 Aerial Rappahannock Schools of R. bonasus sighted from R. and Yeocomico R. to Smith Pt. in Potomac and Towles Potomac R. Pt. in Rappahannock R. 26 INITIAL SIGHTINGS OF RHINOPTERA BONASU.S DURING SPRING 1978 ALONG THE <3 O O <3 > I—tPi O M 23 i—i <3 JS Pi H E <3 O CO X 0 Pi O i-di—ISS H 1
• rH • - CO r-1 • Jd -l M 1 Q O -d l- O C H Pi 4—1 4-J J 4 4-J O 3 3 a CO 3 3 > 3 3 3 3 o O 3 U 3 O 3 3 3 o 3 O 3 3 B 3 B 3 3 CO D C o oo • H • r( o> * rH " r as • r *—H - r 3 3d 1 jd M-l JO Odd d CO > 13 Jd Pi O d P I— l I — 3 4 3 3 3 3 3 CT CT 3 O CO CD 3 3 0 6 CO 0 6 O O) CO CO CO 0 6 C 3 3 1 -J —1 3 3 • ' • rH r rH rH M-J i p— Jd CO 3 1 Sc o PQ — - -l M 4-J 3 4-1 3 3 3 cd CD cd O CD 3 3 3 o 3 3 3 CO o CO o c 3 3 s 3 \ — n »> < • ■• <■ - •, — o o P P s i 3 3 3 3 O CD o CD 3 JrH CJ CD E E D C 3 • • • • •S • • o *
The USCG makes no distinction between species of rays, aside from that
of "manta" versus "rays" and schools versus individuals. However, due
to the aerial conspicuousness of R. bonasus schools, it is assumed
that the rays were probably tl. bonasus.
Early to mid-May haul seine collections from Sandbridge and
Corolla were composed primarily of juvenile R. bonasus (< 800 mm disc
width, DW) (Fig. 11). The sex ratio of the largest single beach
collection of juveniles (CR77-4; 49C?, 35$) did not differ
significantly from 1:1 (P > 0.1). At Corolla (5-V-77) one haul seine
catch (specimens not measured) contained approximately 150 juveniles.
Catches of rays at this site throughout May, 1977 consisted primarily
of juveniles (E. Lawler, pers. comm.).
Only 11 adults (> 800 mm DW) were obtained in the beach
collections (Fig. 11). Haul seine fishermen at Corolla, North
Carolina stated that their initial spring catches were predominantly
adult-sized R. bonasus. A similar pattern, i.e., adult cownose rays
preceeding juveniles north along the coastline in the spring, was
noted by commercial fishermen on Edisto Island, South Carolina in
April 1979 (Dave Otten, pers. comm.).
Dr. W. Stephen Otwell (formerly of the N.C. State University
Dept. Food Science Lab., Morehead City) provided data concerning a massive school of adult R. bonasus (Fig. 8) caught by haul seiners in
Core Sound, N.C. on April 24, 1978. The school was estimated to Figure 11 Disc width frequency of Rhinoptera bonasus by sex collected at Sandbridge, Va. and Corolla, N.C. by haul seines, May 6, 1976, May 2-5, 1977 and May 16-17, 1978. FREQUENCY (%) 30- 0 2 0 2 30-i 10- 0- - - i 350 ______i_ 5 50 5 70 850 750 650 550 450 0 600 700 0 0 6 500 IC IT (mm) WIDTH DISC N=87 N= 60 9 I I 90 00 1100300 1000 900 0 0 8 400 i ---- - i — 1- 5 15 ne 1050 950 t=J -- f j ---- 1 ---- 1 30 contain 2,000 rays. A sample of 57 rays (36C^ 21£>) showed marginally significant deviation from a 1:1 sex ratio (0.05 > P > 0.025).
York River Seasonality
May - A total of 18 juveniles ( I0(f} and 36 adults (19C^
17 £) were obtained in 3 pound net collections in the lower York during
May 1977 (Fig. 12A). One adult female was also taken at Claybank by rod and reel. Gravid females (n = 3) carried three-quarter term young. Schools were first observed on the flats at Claybank on May 28 and June 10 in 1977 and 1978, respectively.
June - Eight pound net collections and 1 gill net collection from the lower York between June 14-28, 1976 and 1977 yielded 79
R_. bonasus (Fig. 12B) . Adult males (n = 61) dominated these catches.
Outstanding were collections CR76-7, 9, 10 and 11 (Appendix Table 1) which accounted for 81.9% (n = 50) of the adult males taken in June.
Only 1 juvenile was taken. The first free-swimming young-of-the-year cownose ray (4Aq mm DW) was taken on June 25, 1976. Gravid females bore full term young in late June.
July - During the first half of July 1976, 32 R. bonasus were collected in 5 pound net samples (Fig. 12C). As in the latter half of June, the catch was dominated by adult males (n = 32, 75%) and five young-of-the-year were taken.
Young-of-the-year cownose rays (22(5*, 15^) dominated five pound net collections from the lower York between July 15-26, 1976
(Fig. 12D) . A total of 16 adults ( 7(5*, 9$) and one juvenile were also acquired. 31
A school of approximately 100 feeding R. bonasus were caught by haul seine at Green Point on July 21, 1978 (Fig. 12E). A sample of 22 rays consisted of 19 adult females, 2 adult males and 1 juvenile.
Females aborted shell encapsulated ova upon capture. A similar haul seine set was witnessed in the Poquoson River (Bay Tree Point) on
July 28, 1976. There was only 1 male among approximately 125 adult
R. bonasus brailed from the net.
August - Adult females dominated pound net collections from the lower York and 1 gill net collection at Green Point during the first week of August 1976 and 1977 (Fig. 12F). Additional gill net sets (3) at Green Point during this period yielded 10 adult females
(no disc widths or weights recorded). All females bore small yolk-sac embryos.
During the latter half of August 1976 and 1977, 7 gill net and 2 rod and reel collections were made at Goodwin Island, Ware River,
Gloucester Point, Mumfort Islands, Pages Rock and Claybank. These accounted for 32 adult female R. bonasus which carried yolk sac young and 1 adult male (22 were weighed and measured; Fig. 12G).
September - A gill net collection at Gloucester Point, rod and reel and haul seine captures at Claybank during September 1976,
1977 and 1978 yielded 27 R. bonasus (23 weighed and measured;
Fig. 12H). Of these 23 were adult females with yolk sac young that had grown to about 125 mm DW (see reproduction section). 32
Figure 12. Disc width frequencies of Rhinoptera bonasus by sex collected in the lower and upper York River by pound nets, gill nets and seines. A. May 1977. B. June 14-29, 1976 and 1977. C. July 1-14, 1976. D. July 15-26, 1976. E. July 28, 1976 and July 21, 1978. F. August 3-6, 1976 and August 4, 1977. G. August 16-29, 1976 and August 25-31, 1977. H. September 8 , 1976, September 21, 1977 and September 7, 1978. FREQUENCY (%) FREQUENCY (%) FREQUENCY (%) FREQUENCY (%) 60 0 4 40 40- - 0 6 20 0 2 50- 0 2 30- 0 2 70 J 70 20 30 20 50- 20 30 0 1 10 0 1 10 10 10 10 10 - - - - - ' 0 50 0 90 1100 900 700 500 300 -_t —I—i—I—i 0 50 0 90 1100 900 700 500 300 0 50 0 90 1100 900 700 500 300 — 0 50 0 90 1100 900 700 500 300 y 1 | ( | ■ f f f I f f I f f f ■ | ( | If "I—f j I f] F ] - ' I, . r . R1-R Lt | I I I I I ' I ' I ' I 1 I | | ■ r — I - | r - j - i L...i . rT~, ■r T 0 60 0 10 1200 1000 800 600 400 0 80 00 1200 1000 800 600 0 0 4 0 0 80 00 1200 1000 800 600 400 0 60 0 10 1200 1000 800 600 400 ■fR-
IC IT (mm) WIDTH DISC IC IT (mm) WIDTH DISC 1 I. | —I—i—j— |— | j I i-l. I . 1 I ‘ N N = 5 N = 26 N =29 =4 U N=34 N=l N = 3 1 d 9 d d
9 9 1 1 f f f f
p 1 i I ! I j t R i I I i_J I i — —I— R 1 I t Z 1 1 1 r | r , I' ' I
1 — l- 1 1
20 ^ J 30- CJ Co 30- o' o >- o >- O 10 § LU LU a LU LU a ZD IU ~z. Ll_ cn LU ZD LU i- L
40 40- - 0 4 20 50 40 0 2 60 0 2 30- 20 0 2 0 2 60- 0 2 50- 10 10 10 10 10 10 - ' ' H -
0 50 0 90 1100 900 700 500 300 0 50 0 90 1100 900 700 500 300 , f? r - . f-? ■ ,■ 0 50 0 90 1100 900 700 500 300 0 50 0 90 1100 900 700 500 300 iL J ' T !T ' ' T' I T~' _r~l P ! - '' !'T I ' I 'T R I' I . ■ I . I B D H F - 0 60 0 10 1200 1000 800 600 400 0 60 0 10 1200 1000 800 600 400 0 60 0 10 1200 1000 800 600 00 4 0 60 0 10 1200 1000 800 600 400 F-- I -T- -F H 3 R 1 I IC IT (mm) WIDTH DISC R IC IT (mm) WIDTH DISC [_j n- J I . l_ .1 I. I -J 1 Li 1 _ 1 1 L_t 1 i- L L I LJ I I J I-.-J . J i I . I . L_J i i '-rn-l 1 I ’ I ' I N =20 N =28 N = I6 N=34 N=63 N = 5 N=3 N=28 d 9 9 d 9 d HI ■ I — T ■ , 90C t 1 ‘bt- J 1 1 I n T 1 f — 1 I - r ~\ r - 1 1 ■ I
1
33
Pound netters in the lower York cited no catches of R. bonasus after the third week of September in 1976 and 1977. Feeding rays were last seen on the flats at Claybank on September 23 and September 11 in
1977 and 1978, respectively.
Potomac River Collections
Potomac River collections were primarily limited to the latter half of the summer of 1976. Young-of-the-year cownose rays dominated four pound net collections from the Smith Point area between July 30 and August 26, 1976 (Fig. 13A). These samples also included 9 adult females and 2 large males. The collection of September 13, 1976 was composed of almost entirely young-of-the-year rays (Fig. 13B). A haul seine set in the upper Potomac River (Kingcopsico Point) caught 9 mature males on July 28, 1978. The latest fall collection of
_R. bonasus from Chesapeake Bay included 4 rays (410-495 mm DW) from the Smith Point area during the week of October 25-29, 1976.
Lynnhaven Tnlet Collections
A total of 11 collections (10 pound net, 1 otter trawl) were made near Lynnhaven Inlet. Pound net collections (3) in July were dominated by young-of-the-year and adult females. Adult females
(n = 8 ) were caught on July 7, 1976 (Fig. 14A) and young-of-the-year rays (n = 21) were taken in the catch of July 15, 1977. September and
October collections (5) consisted almost exclusively of young-of- the-year rays (Fig. 14B) Figure 13 Disc width frequencies of Rhinoptera bonasus by sex collected in the Potomac River by pound net. A. July 1976 and August 13-26, 1976. B. September 13, 1976. FREQUENCY (%) 0 2 - 0 3 -i - 0 3 0 2 0 2 i - 0 3 0 2 0 1 0 1 - Q I IQ- 0 0 ------
0 0 0 4 300 0 4 0 0 70 0 90 00 1100 1000 900 800 700 600 500 0 0 4 300 J b ■J— 0 60 0 0 0 1000 900 800 700 600 500 [
1 I = 4 7 =N = 9 6 = N 38 3 =N f O 9 * O r | r | I M.O Q i inn J , W I D T H ( m m ) 1 WIDTH (mm) DISC DISC DISC 35
Figure 14. Disc width frequencies of Rhinoptera bonasus by sex collected near Lynnhaven Inlet by pound net and otter trawl. A. July 7 and 26, 1976 and July 15, 1977. B. September 10 and 23, 1976, October 7, 1976, September 23, 1977 and October 12, 1977. FREQUENCY (%) 20 i - 0 3 -1 0 3 20 - 0 3 20 20 1 - 0 3 10 10 10 0 ------0 500 400 0 0 3 0 70 0 90 10 1100 1000 900 800 700 600 = 4 3 = N = 12 = N = 7 2 = N = 8 2 = N 0 800 900500 0 0 8 700 600300 400 9 -* -- 1 —i —H I— i— i— — 00 "00 .IT
36
A sample of 18 R. bonasus (< 615 mm DW) was examined from a pound net catch of approximately 150-160 small cownose rays on September 10,
1976. Another pound net catch on September 26, 1978 contained approximately 100 small R. bonasus and no adults (no weights or disc widths recorded).
The latest catch of adult females (n = 2) in the Bay was at
Lynnhaven on October 12, 1977.
Eastern Shore Collections
Samples from eastern shore were very limited and five of the six collections were made in July and August 1976 (Fig. 15). No single size class dominated these collections, although the collection of
July 21, 1976 consisted of all males (7 adults, 6 juveniles). Eastern shore collections showed a slight increase in the number of juveniles present (600-800 mm DW), as opposed to the other collecting sites in the Bay. A gravid female was captured on a longline on October 5,
1977 along the oceanside of Wreck Island.
Fall - Virginia-North Carolina Coast
Th e fall haul seine fishery between Sandbridge, Virginia and Cape
Hatteras, North Carolina traditionally begins about September 1 as sciaenids, bluefish and flatfish begin their migration south to wintering grounds off the Carolinas. Haul seiners reported that
R. bonasus are rarely caught during the fall, rather they are most abundant along the beach in spring (late April through May). Negative results were obtained in three attempts (5-X-76, 27-IX-77, 10-X-78) to 37
Figure 15. Disc width frequencies of Rhinoptera bonasus by sex collected near Kiptopeke by pound net July 20-29, 1976 and August 17, 1976. FREQUENCY (%) 2 0 -i 0 2 20 10 10 10 0 0 — - i — 0 30 0 40 0 50 0 60 0 70 0 80 0 90 0000 i100 10001050 950 900 850 800 750 700 650 600 550 500 450 400 350 300 I I I I J --- 1 ---- 1 ------______---- I DISC WIDTH (mm) ---- _____ I ---- ______N = 29 N = I 0 Cf 9 I --- _____ ---- ______---- I ------_____ | _____ | ---- _____ I I I | 1 --- 1 ---- 1 38 sample cownose rays from this stretch of beach. One young-of-the-year ray (DW = 505 mm) acquired from a pound net in Pamlico Sound (near
Hatteras Village, North Carolina) on November 3, 1976 represented the latest catch of cownose rays in the fall over the three years of study.
Wintertime records of R.. bonasus (Table 3, Fig. 16) in the South
Atlantic Bight were supplied by the National Marine Fisheries Service
(Pascagoula, Miss., unpublished data) and the North Carolina
Department of Natural Resources and Community Development, Division of
Marine Fisheries (R/V Dan Moore Cruise Reports, Number 11, December
1975 and Number 13, February 1976).
Food Habits
In 1976, 337 stomachs from a wide size range of R. bonasus collected during June and July 1976 from pound nets were examined for food items. A total of 199 (59.1%) were empty. Of the 138 (40.9%) with food items 110 (79.7%) contained either the bay anchovy, Anchoa mitchi Hi (35), unidentified teleost remains (17), coelenterates (16) or teleost scales (73). I found no molluscs in the stomachs of these initial pound net samples, therefore, I assumed the rays purged their digestive tracts while entrapped and also fed in the net on anchovies and teleost remains. These stomachs were discarded as non-representative. Efforts during the remaining portion of this study were directed toward samples of actively feeding rays. LATE FALL AND WINTERTIME RECORDS OF RHINOPTERA BONASUS 3 J < ° x CO X) Q X CO 2 n X 4-) o id o o o o o m 0 0 vO ON i£>r"-ooor'--oor'»ocNr^ooonvo vO O C O C O C »— - r O C O C n i " d - m —H r r-~ 2 2 • • 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 2 x 2 2 2 CN N C CN r ■ r r-H OC OC OC OC OC OCO CO CO CO CO CO CO fefLhfLfefLfLfLlLlLfLFLUO CO CO CO CO CO 2 > o • • —1 1— i o ON 1——I OO vO i—H m m I"- N C O C N C m NCN CN —i i— *— 2 O > 1 o oo O co vo o in vO 1 r-~- r~~ - r t—t O C CO t N O C m O 25 — > o n • f o oo OvD vO in in r^- r—H O C O C o o o vO m m O C vD n -H r r"» o o n o vO N C 1— X CN 2 C p p O) i -i r • rH • • rH • rH * • P • • • X ,D d jc — i X CO CO CO P ( 4-J I 2 2 - cu 4-) CO 4-) 4-4 p CU o ) a cd CO id n t cu id CU 5-i M CU 5 o cu CO CD CU i—H CO cd o cu P CO CU p w cd o id cd cd u fd cu o u o cd p p j r\ t • rH • • rH • X X d n 3 T !— P 2 4-J C cd L C CO cu CO cd rH * CO p cd co CJ cd 60 o p P h 1— 1 • ♦ r% 1 • P • 3 T • rH • o p i r-H d P X 2 id 2 CP o 4-J 4-J 4-J 4-J 4-J UO o CU cd a CO CO CO O 5-i OJ cd B O 5-i cd P p cd * rH * cd 5-t cu td cd L C u o id cd O 5-4 * rH * • p • • p * • rH • X p i X o d J i— 2 p i 2 Q 2 Q 4-4 4-J cd cu cu 4P 54 OJ 5-4 CU o cu 54 p CO cu > o o id 2 > CO cu d cd cu CL B ** r\ i ~~— f-H CO X • rH • X *—4 —i i— o 2 > Cd Pd CP PC 4-4 id CU p cd CO B o u p Figure 16 Late Fall and Winter records of Rhinoptera bonasus from the South Atlantic Bight. 82 ° 81° 80° 79° 78° 77° 76° 75° 74° ^sJCapo H atteros NORTH CAROLINA 2 FEB'76 = ^ £ ^ •8 DEC*75 •2 3 NOV '59 Wilmington • 8 DEC '59 ^ 2 , _JY Capo SOUTH CAROLINA , Fear 7 NO V'6 2 NOV 6 2 Charlostoj 26 JAN 63 Savannah 3 2 ° GEORGIA I NOV 62 \« I8 J A N 66 %f t 27 N O V'66 FLORIDA ' • 23 FEB 63 ,«• 19 JAN'65 i I DEC 65 2 8 ° 8 2 ° 8 0 79 7 8 77 75 74 41 In 1976, 28 stomachs with representative food items were collected by otter trawl (9), gill net (8 ), pound net (7), commercial haul seine (3) and rod and reel (1). A total of 111 stomachs were examined during 1977; 29 (26.1%) of these contained representative food items. Methods of collection included gill net (21), rod and reel (4), pound net (3) and haul seine (1). Nine species of bivalve molluscs and one species of echinoderm were identified as representative food items of R. bonasus from a total of 57 stomach samples (Table 4). Trace amounts of unidentified shellfish and a partially digested teleost were also present. Seven species of shellfish were identified as food items of R. bonasus from shell fragments in the spiral valve (Table 5). An index of relative importance (IRI) was computed for 40 stomach samples collected in the York, Poquoson and Ware Rivers (Table 6 ). Food items included six species of bivalves. Mya arenaria contributed the greatest frequency of occurrence (45.0%), numerical dominance (57.8%) and volume (6 6 .6 %) of all food items and had an IRI of 5598. The remaining five food items were: Macoma balthica (IRI = 1476), Tagelus plebeius (IRI = 398), Mercenaria mercenaria (IRI = 64), Geukensia demissa (IRI = 14) and Crassostrea virginica (IRI = 2). Small numbers of samples precluded computation of IRI1s for other areas of lower Chesapeake Bay. Food items discovered in rays collected from those areas are listed in Table 7. TABLE 4 LIST OF REPRESENTATIVE FOOD ITEMS FROM RHINOPTERA BONASUS STOMACHS Phylum Echinodermata Class Echinoidea Me 11ita quinquiesparforata Phylum Mollusca Class Pelecypoda Geukensia demissa Mytilus e du1i s Crassostrea virginica Mercenaria mercenaria Mulinia lateral is Macoma balthica Tagelus plebeius Ensis directus Mya arenaria TABLE 5 LIST OF SHELL FRAGMENTS FROM RHINOPTERA BONASUS SPIRAL VALVES Phylum Mollusca Class Pelecypoda Noetia ponderosa Mytilus edulis Mercenaria mercenaria Mulinia lateralis Macoma balthica Tagelus plebeius Ensis directus A3 I—I OO vO 00 CN »—H rH cd o\ r~- c^ NO rH w LO Mt CO m rH Q !Z < 3 CO E=> CO Pd 00 o CO in 0 0 CO co CO S cd !Z • • • • • ••• cd Pd < 3 i"-. CN in CN i—i o o o Pd H & m CO cq i—I S Q do n !Z JZ o < 3 o H Pm Pd JZ !Z cn ►J w Pm O m i—i PQ cj O co o co < 3 Cd O d H Pd Pd *4 TO P m )—l O' CU • H i—4 00 »—i O r-» <— 1 i-H r —t cd O rJd > CO CN CN i— i Cm 6 • iH CN »—1 d TO Pm !Z d Pd Pd w o cd w 3z O o < 3 PH !Z Eh w Cd Pd cd o Pd o n in in O in in in cd CM EH Pm • •• • •••• do s m CN CN r- in CN CN CN o X CO CN c j O o Pd cd > Pm pH HH o Eh CO SZ <3 Pd Pd ►d O do m O' a o cn Pd o X cd Eh d u Pm o CO cu co 00 co 0"\ CO CN r—l »—H r-H 40 e '- < i—i H X 6 o Pd d J-J Pd Q 5Z CO o sz cd t—i w PM jd cn cn CO cu • H CO CO «u • ?H MH • rH e •H CO d MM r-H CO CU d • rH 4-J •rH rH E 4-1 in cO w cn 4-J M-l g M-l Jd X -5 3 CP 53 3 co !3 CU CU O u co cu COO o B l-lP a p o P o P e E * rH • H r-H r-H X jd CJ CO P 1—1 CJ 4-1 4 4-J 4-1 4-J p 0) cO B CO o o cu co cu o CU U p o -J m /-■s. v_^ • P • P — * p • i—l • rH i—i i—I 1—1 X X P P on S CO 4J 4-J 4-J 4 4—1 O P CO (X P CU Oco P CO P uCU cu o P P CO' CO.- o cO CJ 6 B P J m m • • \ • P * rH • rH ♦ rH * P —1 r— 1 —1 * H ‘ H * 1—1 i—i i—i -P un P P on s S _ 4-J 4J 4J cO o cn CJ CO P QJ S P CO P CO P p P p r-H • P • p • rH P • • p i i— Csl O X X MM i-H »— X d j CJ 'O W PP S3 - 4-1 4-J 4J p o co a> P H P CU ■P x P P E Q 5*4 PQ Q O PQ a p - ca (X CUP 00 CU u d j p cu p W ^ X X on H 4-J _ 4 p Q- p o cu p o CO * rH CJ O ’r-H CO’ CU cu p p r c P p CU P-. -J ' • * rH ”1 1” Jd MM P f-M , S 53 CJ CJ 4-1 4-J P cO CO 0) rH O cO CO cO tU CO r c CU CU P P CO p o P o p CM p p • • 44 45 Rhinopt era bonasus (3) collected in a pound net in the lower York River had consumed a nereid worm (1), Neomysis americana (1), Crangon septemspinosa (7), and Oxyurostylis smithi (1). These may have been either representative food items or products of net-feeding; these are listed in Table 8 . The digestive tract of most young-of-the-year and juvenile rays were found to be empty, however, 10 small rays (< 560 mm DW) consumed My tilis while four small specimens (< 660 mm DW) fed on Mulinia. Feeding Behavior Most schools of R. bonasus observed from the air were cruising shoal waters (1-3 m deep) in compact formations. On four flights I observed large sediment disturbances in shallow water adjacent to the shoreline. The most notable was the flight of June 2, 1977 along the southern shore of the Potomac River. Approximately 50-60 schools of R. bonasus (ca. 10-50 individuals per school) were observed in shoal water (ca. 100-^00 m from the shoreline) from the Yeocomico River to Smith Point (Fig. 1). The bottom was dotted with hundreds of circular depressions approximately 1 m across and several meters apart. A total of 8 - 1 0 separate sediment disturbances were observed along this route. These were characterized by well-defined sediment clouds at the head of the disturbance and a plume of dispersed sediments downriver. Individual cownose rays were observed at the heads of two such disturbances. Orth (1976) has recorded sediment plumes about 2 km long created by feeding cownose rays. 46 TABLE 8 A LIST, FREQUENCY OF OCCURRENCE, AND NUMBER OF ACCIDENTAL AND QUESTIONABLE INGESTIONS FROM RHINOPTERA BONASUS STOMACHS Frequency of Number of Item Occurrence I terns Remarks Bottom detritus- sand & wood chips 21 Zostera blades and rhizomes 4 Class Bryozoa 6 Phylum Annelida Class Polychaeta Spiochaetopterus costarum oculatus tubes Nereis sp. Phylum Arthropoda Stomachs from Class Crustacea rays collected Order Mysidacea in an area of Neomysis americana 19 patchy Zostera Order Isopoda distribution, Idotea balthica 3 south side of Edotea triloba 1 Goodwin I . Order Amphipoda .Ampithoe longimana 1 Caprel1a penantis 30 Order Decapoda Probably an P innotheres sp. inquilin; ray had injested Phylum Annelida My t i1u s. Class Polychaeta Nereis sp. Phylum Arthropoda Class Crustacea Order Mysidacea Neomys is americana Questionable Order Cumacea inj es t ions, Oxyurostylis smithi pound net- Order Decapoda caught rays. Crangon septernspinosa 47 Shore-based observations of cownose ray feeding activity occurred exclusively in nearshore, shoal water (1 - 2 m deep) during high tide. The rays invaded the flats during the first half of the flood tide and departed before the second half of the ebb. Exposed pectoral fin tips and large boils (1 - 2 m in diameter) on a calm surface typified cownose ray shallow water feeding activity. Perhaps this behavior was part of the "specific glide pattern" Schwartz (1967) referred to as an indication of feeding. Ten sets of a 91 m gill net around nearshore ray activity accounted for 62 R. bonasus. Of 32 stomachs examined, 29 (90.6%) contained food items. All but one set were made during the second half of the flood or the first half of the ebb tide within 300 m of the shoreline and in less than 2 m of water. A "blind" set of the gill net at Pages Rock (2 m deep) during slack low water accounted for one R. bonasus which had ingested 8 M. mercenaria. In areas actively feeding rays the bottom was pock-marked with numerous feeding excavations. Dimensions of these depressions approximated those reported by Orth (1975), that is, circular, ca. 1 m across and 20-40 cm deep. The depressions were generally bowl-shaped with a central steep-sided cavity approximately 15-20 cm in diameter which extended 20-40 cm below the horizon of the natural bottom. Howard et al. (1977) reported this central cavity from ray feeding depressions (probably Dasyatis spp.) in a Georgia estuary. I concur that wave and tidal action quickly fill in the central cavity with sediments. Shallow bowl-shaped depressions remained at low tide in 48 areas where JR. bonasus had been actively feeding during the preceeding high tide. The sediment disturbances created by actively feeding rays precluded in situ observations of the actual excavation process. However, a small cownose ray (510 mm DW) kept in an aquarium during August 1977, exhibited what I assume was a feeding response. Although no prey items were present, the ray glided towards and rested upon the gravel bottom. The subrostral fins probed the bottom for 5-10 seconds. The pectoral fins performed stirring motions for about 5 seconds. Both jaws were then protruded into the bottom and some gravel (ca. 2—3 mm diameter) was inhaled into the mouth. The ray then left the bottom, venting approximately 20-30 grains of gravel from the gill slits as it swam across the tank. The entire sequence lasted about 2 0 seconds. Reproduction Size at Maturity Males - Data included 188 male R. bonasus from Chesapeake Bay and vicinity. The ratio of clasper length to disc width increases noticably at a disc width of 750-850 mm indicating the onset of sexual maturity (Fig. 17). All male specimens < 755 mm DW were staged as immature. Nine males staged as maturing for the first time had disc widths ranging from 755 to 835 mm with a mean (DW) of 802 mm and 95% confidence limits (0 1 ^9 5 ) of 777 and 827 mm. A total of 115 mature males ranged 49 Figure 17. Clasper length (mm)-disc width (mm) relationship for 188 male Rhinoptera bonasus. M — CM O li n Va I ues • ♦ xxx*x xx. CD CD ro CD ro CM ) J U W ( CM oo 1N1 d3dSV10 H19N31 CM CM o CD CM *: 00 • X • V • •X X jc X <& X X X X • X I •XX X • f X • X f xx x • o — — O o ro O O O o O O O o o o CD O CD 10 O O o o CT> DISC WIDTH (m m ) 50 from 800 to 981 mm with DW = 898 mm and 0 1 ^ 9 5 limits of 8 6 8 and 928 mm. Both testes of mature males were greatly swollen and a milky, pinkish white color. The left testis was generally larger than the right. Sperm exuded freely from the testis upon incision. The seminal vesicles of freshly-caught males collected in lower Chespaeake Bay from May 12 to August 31 were distended with seminal fluid. The vesicles of two mature males collected at Corolla, North Carolina on May 5 showed no distention. No fresh, mature males were examined during September or October. Females - The smallest female R. bonasus specimen classified as mature measured 845 mm DW; the largest was 1050 mm DW. The mean disc width of 117 mature specimens was 960 mm with 0 1 ^ 9 5 limits of 920 and 1000 mm. The smallest gravid specimen measured 870 mm DW (Table 9). All specimens < 840 mm DW were judged immature. Eight females with DW = 8 6 8 mm and 0 1 ^ 9 5 limits of 837 and 899 mm were considered maturing for the first time. Only the left ovary and uterus (from a dorsal perspective) were functional in all mature specimens. There was no evidence of follicular development in the right ovary. The right uterus in mature specimens was slightly dilated (ca. 3 cm wide) but the trophonemata generally never exceeded about 0.5 cm in length and uterine eggs and/or embryos were not present. There were two groups of eggs present in the left ovary of R. bonasus: (1) maturing eggs, those > 1 cm diameter, which were due DATA FOR RHINOPTERA BONASUS YOUNG, AS COLLECTED CHRONOLOGICALLY \ o ' O 00 r"- r-. cd g 1 — I O S O 1— I • G • p ! i i— >■< m E m ^ CP --- 4-J o O > d c G o o U ) a 1 —i e •H • rH G • x: • G • -G S IS P Q P 4-1 CO 4-1 o ) 0 • G r-tcd c cd o D ' CD a) 0 0 G G DCM • ! •“ M C CD S o a> G - o - • i— P S I P O C C 2 i P ►J 4-> 4-1 d c < a CO & o o cd g o a 00 cd a> X M C G G e 1 > < x * - - x - x - x - x - -x -x -x -x -x -x -x -x -x -x * -x P CTN ' O -X i —I CM O I C •— O t < O'' M C CM " j < ‘ i < ‘ J < ' l < O C M C M c M C M C N C M c M C C'J'Dinr'M'OvDCOOOMONM r-H i—1 - r i"- - n - n p r*H p CM > CP —lOco^DLnuomoa-'OOLncnr^ d e PCP CP cd O G o 1 I 1 I o o o -X -x -X -X -X -X -x -X *1I 1 I- r*—1 r—H r"~ CO »—H - ~ r CP CM > P d c o g o 1 1 1 i 1 0 * CT o P - n O' r—I - n r"- •—H m m i— > CP P Pu CP cd o g Td o 1 1 i 1 n M C O L • rH p to P p n- p p MCM C CM “4 »“ > CO P p oo CD g cd C I > 1 1 i * c oo ' o co co co cr> M C r~- - r O O - n t - r CM - ~ r dd >-! 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OO NO no r ~ . x I I I QJ X X X 4-1 i—i w cd I I Q OO GG o QJ •—l CM CO CO 5-1 a> X tS O O O CO n> Q oo O c n r~- O 0> O 0> O S G X o * -}« * -a G 03 • H eq Pn o G qj CO 6 ^-s 3 .H x •-) e r-H O ^ o > 4 -1 to X! QJ 60 ✓'o co o on oo X) • H b o O < f H oo G X G X • G cj o G M QJ 4J • H cd > G 4-) x x cd O cd to o X, a G QJ C G o rH »H C * rH XI O ^ to X QJ cd f"- r-~- B r^> r>* r-. QJ 1 i 1 MH X X X M 1 1 cd 1 m CN Q O CS G X g g to Xi 4 -1 X) QJ 4 -1 G QJ X g G G QJ 4 -J CJ o QJ »—H t- H *-H CN QJ CO 5-4 G • rH S Sh i 1 1 1 CO QJ O 4 -J 4-1 • H QJ o CN CO X 5h X QJ CJ CJ CN CN CN G CL G X ■ I X) QJ QJ e 1 l 1 i—H CL a ro ro. r~- It II II II r-H CO 3: r-- r ~ - - r-'- o OeJC *3 0 3 ■K * G X CJ ^3 O CJ CJ •S 55 for release during the next ovulation period, and (2 ) reservoir eggs (sensu Bearden 1959 and Hess 1959) which were < 1 cm diameter and probably destined for release during subsequent breeding seasons. Ovulation occurred in mid-July. Mature females contained shell encapulated ova in the left uterus on July 21, 1978. Embryonic Development A total of 67 cownose ray young ranging in disc width from 18-440 mm were collected from gravid R. bonasus between early May and mid-October. Collection data for the young and their mothers are listed in Table 9. Additional parent-young data (Table 10) collected near Beaufort, North Carolina on April 24, 1978 were supplied by Dr. W. Stephen Otwell. Only one young per gravid female was observed. Gravid females often aborted the young upon capture, but in all such cases the number of aborted young equalled the number of females with post-partum uteri. The sex ratio of R. bonasus young did not differ significantly from 1:1. Of 83 young (Otwell's data included) for which sex was determined, 48.2% (n = 40) were male. Collections in Chesapeake Bay and vicinity revealed two broods of cownose ray young (Fig. 18). The first brood, three-quarter to full term young, are referred to as fetuses, whereas the second brood, yolk sac young, are referred to as embryos. Gravid females collected at Corolla, North Carolina, Sandbridge, Virginia, and in the York River during early May contained 56 TABLE 10 DATA FOR RHINOPTERA BONASUS YOUNG SUPPLIED BY W. S. OTWELL FROM A HAUL SEINE SET IN CORE SOUND, N.C., APRIL 24, 1978 Weight DW (mm) (g) Sex Mother's DW 281 340 % 970 266 283 %■ 938 2 2 2 o * 946 272 326 0* 940 241 240 £ 940 265 318 950 275 354 & 960 275 357 $ 960 268 305 cf 950 271 283 & 965 271 311 cT 940 No data given for 8 other embryos (4c? 4£) aborted in this subsample. 57 Figure 18 Rhinoptera bonasus embryo disc width (mm) versus date of capture, 1976, 1977 and 1978. m CD m o | n ii 0 0 - CD o o j o*+-* r * CD I-CD DISC DISC WIDTH OF EMBRYOS (mm) y* — ro ro oj oj cjn > > cjn • ro i ro I cn cn Q ui O cn O cn O cn ------I— • o o o o o o o o o o JAN 1 FEB 1 MAR 1 APR 1 MAY ' JUNE 1 JULY ' AUG ' SEPT 1 OCT ' NOV 1 DEC 58 three-quarter term fetuses (DW = 259mm, Cl 9 5 limits of 240 and 278 mm, n = 7, Figs. 18 and 19). Data supplied by Otwell (Table 10) for 11 young (DW = 264 mm, C I 9 5 limits of 247 and 281 mm) collected on April 24, 1978 are consistent with my findings. The fetuses were full term (DW = 413 mm, n = 4, C I 9 5 limits of 395 and 431 mm, Figs. 18 and 19) by late June and early July and parturition occurred in Chesapeake Bay as evidenced by the first pound net collections of small, free-swimming R. bonasus (DW = 437 mm, n = 8 , 0 1 ^ 9 5 limits of 396 and 478 mm, Figs 12B-D; see distribution section). Following parturition, ovulation occurred and the gestation of a second brood of embryos began (Fig. 18). Encapsulated uterine eggs were aborted by females captured by haul seine on July 21, 1978. By early August, embryos were about 20-30 mm wide (Fig. 20) and devoid of the shell capsule. Development was rapid thereafter; specimens collected in late August averaged 125 mm DW (n = 12, C I 9 5 limits of 106 and 144 mm). By late September and early October when R. bonasus left Chesapeake Bay the second brood embryos attained a relatively large size (Fig. 18). The largest of these were collected on October 12 in the Bay at Lynnhaven Inlet and measured 210 and 220 mm DW, respectively. No collections of adult R. bonasus were made in lower Chesapeake Bay between mid—October to the following May. Embryonic disc width-weight relationships, where W is weight in grams, were: W = 1.2 x 10"^ DW^*06 (r^ = 0.98; n = 19) for three-quarter to full term fetuses and 59 Figure 19 Three-quarter term Rhinoptera bonasus young collected May 19, 1977 (DW = 275 mm). Nearly full term Rhinoptera bonasus young collected June 14, 1977 (DW = 380 mm). 60 Figure 20. Series of second brood Rhinoptera bonasus embryos ranging in size from 18-140 mm DW. 61 W = 2.00 x 10- 3 DW2 * 1 0 (r2 = 0.98; n = 50) for second brood embryos. Three-quarter term fetuses collected in May were situated in the left uterus in an upright position (ventral surface of the embryo on the ventral floor of the uterus) with the rostrum pointed anteriad. The pectoral fins were folded dorsally upon themselves. The tail was conspicuously long and was bent forward along the dorsal surface of the disc. The dorsal surface of the disc was gray and the ventral surface white. The yolk sac and stalk were almost completely absorbed; only about 3 mm of the umbilicus protruded from the abdomen. The position of the full term fetus was unchanged from the three-quarter term position. The dorsal surface of the disc was grayish-brown. The umbilicus was totally absorbed leaving only a small scar, which was quite evident on many free-swimming young-of-the-year rays. Several tooth plates were discovered in the left uterus from which a full term fetus was removed, confirming Bigelow and Schroeder's (1953) report that tooth replacement in R_. bonasus begins in utero. On July 21, 1978, 19 adult female R. bonasus were subsampled from a haul seine catch. Shortly after being brailed into the catch boat, the rays aborted shell capsules from the left uterus which contained no greater than 3-4 large eggs. Most of the capsules burst upon ejection and their contents quickly deteriorated. Two capsules, each from a separate female, were delivered intact. The greenish, amber 62 capsules measured about 1 0 cm long, and were composed of a thin, diaphanous material. One capsule held a single yellowish egg; the other contained 3 eggs. The eggs were extremely flaccid and about 3-4 cm in diameter. Small embryos were not evident upon macroscopic inspection. A shell capsule devoid of ova was found in the right, non-functional uterus of several females from the above collection. Sixteen females with more advanced later stage second brood embryos also had an empty shell capsule in the right uterus. These capsules, although deflated, appeared identical to those in the left uterus which contained ova. All second brood embryos possessed yolk stalks and yolk sacs, although these structures often became detached during collection. The smallest undamaged specimen of second brood embryos measured 20 mm wide. At this point in gestation the shell capsule was absent and the embryo was unmiscakenly batoid (Fig. 20). Most conspicuous were the numerous external branchial filaments (ca. 15-30 mm long) which emanated from the gill slits on the ventral surface. Except for the lack of pigmentation, embryos greater than about 120 mm DW resembled the adults (Fig. 20). Embryonic Nutrition The gradual depletion of yolk material is shown for 33 second brood embryos collected between August 4 and October 12 (Fig. 21). Additionally, the embryos of the myliobatoid rays are nourished by a 63 Figure 21. Yolk sac volume (ml) of second brood Rhinoptera bonasus embryos-disc width relationship. YOLK SAC VOLUME (ml) 0 1 4 - 14 I i 6 — 2 1 4 — 4 8 6 - - — 0 0 0 0 0 0 10 4 10 8 20 2 240 2 220 200 180 160 140 120 100 80 60 40 20 MRO IC IT (mm) WIDTH DISC EMBRYO 12.59 - 0 . 0 6 X 6 0 . 0 - 12.59 3 3 73 .7 0 - 64 secretion from finger-like villi (trophonemata) which line the uterine wall (Bigelow and Schroeder 1953). Both uteri of immature cownose rays were narrow (ca. 0.5 cm wide), appeared undifferentiated from the oviduct, and had a rugous lining. Only the left uterus of R. bonasus was functional. In two specimens classified as maturing (826 and 900 mm DW), the left uterus measured about 2 cm wide and had trophonemata between 3-5 mm long. The trophonemata of mature females were elongate, deep red in color, flattened in cross-section and had a spatulate distal end. The villi were largest (ca. 2-3 cm long) in females with three-quarter and full term young. The uterus of R. bonasus was a tough, thick-walled organ during the early stages of gestation which gradually expanded to accommodate the developing embryo. Prior to parturition, the uterus was extremely distended (ca. 15 cm at its greatest breadth), thin-walled and flaccid. During all stages of pregnancy, the R. bonasus young appeared to lie in close contact with the trophonemata. Invasion of the gill slits by the trophonemata was observed in several three-quarter and full term fetuses. The trophonematic secretion (histotrophe) is a viscid, yellowish fluid, as cited by Schwartz (1967). The amount of histotrophe in the left uterus increases considerably as gestation progresses but I did not take quantitative measurements. External branchial filaments were present in all 14 embryos between 18-75 mm in disc width. These structures were absent in all embryos > 89 mm DW (Table 9). 65 Age and Growth Back-calculation of fish body lengths from hard parts assumes it is possible to correctly interpret the pattern of growth zones deposited on the hard parts (Rounsefell and Everhart 1953). To be a reliable tool for age determination, the hard part must remain present for the life of the fish, grow proportionally to the growth of the fish, and have annual checks formed at approximately the same time each year (Everhart et al. 1975). Vertebrae were found in all sizes of free-swimming R. bonasus examined. They are obviously not lost or regenerated during the life of the fish. Regression analyses indicated a linear relationship between DW and vetebral radius (VR, in ocular micrometer units) for each sex. The relationships are described by the equations: DW = 57.59 + 19.72 VR for males, and ' = 0.96; n = 69; Fig. 22) DW = 78.12 + 19.02 VR for females (r^ = 0.97; n = 60; Fig. 22) Preliminary sections indicated that the number of opaque and hyaline zones on each half of the hourglass-shaped vertebrae were consistent. Similarly, in individual rays the number of zones on vertebrae from various positions on the vertebral column remained cons tant. Specimens were only available May through October. The outer margins of vertebrae from young-of-the-year rays collected July 66 Figure 22. Disc width (mm)-vertebral radius (ocular micrometer units) relationships for 69 male and 60 female Rhinoptera bonasus (1 o.m.u. = 0.131 mm). ( micrometer ) s t i n u r e t e m o r c i m r a l u c (o RADIUS VERTEBRAL LO m m in ,3' in in m ro c\j o CM “ ro “ _ “CM in _ _o _ o _ -ro - m _ ro ~ - in _ -O in - r in r o J o o l ZD CO L O co CM o o cr>- m tr LU >- cn UJ oc CD oo CM CM Q ^ cr> o c h-' in o h-' in CD r o o o - m 8 m o o H1QIM OSIQ o CO h - o o o (uiuj ) ) (uiuj 0 0 o CT) o o o UJ UJ Li_ 0011 67 through October had only light opaque zones. The smallest rays (460-560 mm DW) collected along the Virginia-North Carolina coastline the following spring were presumably returning yearlings. A dark, hyaline zone was evident on or close to the outer vertebral margin of twelve of the yearlings examined. Vertebrae from thirteen larger rays (> 560 mm DW) collected in the Bay late May through October had narrow opaque outer margins, while 8 6 % (n = 19) of returning juveniles and adults taken in early May.along the coast had hyaline zones on or near the outer margin of the vertebrae. The evidence above suggests that: (1) two zones, one hyaline and one opaque, are deposited annually on the vertebrae of R. bonasus, (2) annulus (hyaline zone) formation occurs sometime between November and April, and (3) the wide opaque zones reflect greater growth during the summer months. Conditions upon which the hard part method of back-calculating fish size is based appear to be satisfied. Mean back-'alculated disc width at age for male and female JR. bonasus are given in Tables 11 and 12 respectively. Intercept values for the regressions of DW on VR represent correction factors and were incorporated into computations of the above. A von Bertalanffy growth equation for each sex was computed from weighted mean back-calculated disc widths. The equation has the form: DWt = D W ^ [l-e_K^t_to)] where MEAN BACK-CALCULATED DISC WIDTHS (mm) OF MALE RHINOPTERA BONASUS • • r-4 r-l • r4 * • • r r—i f—I d - TO 'O T3 X> M—( d r 1 < S u S Q ts CO & Q 4-1 J 4 4-J 4-1 CD CO d CO d O) CO d 4-J CJ d 3 rA O CO ) 0 o o o co oj CO H B CU O CO u 3 —1 3 CO d 3 n • r4 < l—l > t > (— o CO 1 1—1 1—1 M (—1 > > M > 1—1 1—1 M 1—1 4-> —t —1 CO CD o d CO 0 0 c o o c o c N L / o r - ^ r - r u o OcDO>uoC^ LO C3>in N M >— 1CO .— ICO CN \0 O O ^ O O h- O O ^ O O O T C O CO CT\ . ^ r CO o\ n --Cl- o< vOvO O' fc t CO vO O- O O o O v u O V - '- O c M C O O U U M C O C O C O v O O C O D v m t s O M H I— O U O O U O U O O u f < O U t O U < O O u D O O u O O U u O O O H O u M 'O O tO s M C O M C M C O v D v D v O C f O v < O D O v O U > a D v O v o o O D U v D v — C O iI D J V CM V0 OO co oo CO CO H 4-1 o CO • MO O O CT\ O C O C 00 O O O U CO 00 CTv o O'! vD O O uO vO CT\ o o u CM o - o - o - o CM Xi TO t a 4-1 60 co QJ c QJ QJ 2 O C cO vO D v 00 O O CO M C M C ov CM o u av o - o 1—t CM O O 4-i & 69 70 DWt = disc width at age t DW^ = theoretical mean maximum disc width K = rate at which DW is approached tQ = theoretical age when DW equals zero. Walford plots (Walford 1946) of DWt + ]_ on DWt yielded lines with the following equations: D W t + i = 190.7 + 0.8 DWj- for males (Fig. 23), and DWt + f = 157.2 + 0.9 DWt ^or females (Fig. 23). The slope of the Walford line is equal to k (= e “ ^ ) . The asymptotic disc width (DW^) is derived from the solution of y-intercept/(1 -k). Initial values of K = 0.212 and 0.148 and DWoc>= 997 and 1141 were obtained for male and female R. bonasus respectively. The plot of loge (DWoc- DWt) on t yields a line whose straightness is sensitive to changes in DW^,. Trial values close to the initial estimates of DW^were used to obtain the straightest line. The slope of this line is equal to -K. The formula (y-intercept - loge DW^/K yields tQ . Final von Bertalanffy growth equations obtained were: DW|- = 996 [1 — e~0 *215 (t + 2.55)] for male JR. bonasus and DWt = 1141 [1 — e~0• (t + 3.28)] for female JR. bonasus. The respective growth curves are shown in Figure 24. It should be noted that the hyaline zone considered the first annulus was actually the second hyaline zone encountered along the 71 Figure 23. Walford lines for male and female Rhinoptera bonasus with estimates of asymptotic disc widths (DW^) . O CM CM CM CD CO _ 8 O CM — O in 00 o LU < LU Ll . O O CM _ O WIDTH DISC O 00 CO — o 00 o o LU CM 00 in |+|96d id (LULU) H1QIM 0SIQ 72 Figure 24 Empirical lengths and Von Bertalanffy calculated lengths-at-age for male and female Rhinoptera bonasus. 1200 U L co l_ L LU LU CO O O o O O O O O o o o o o o o o n o (LULU) n o cd co cn H 1 Q I M O S 1 0 m LU LU X L X l J L u L U L ? o l . o o ro J C — CO I— - r \ < - ro - - m CD _ CJ — ro - LO - - CT) - CO o j AGE vertebral radius. A faint, initial hyaline zone was detected close to the center of the centrum in 87% (n = 79) of the vertebrae examined. This zone was not as pronounced on some of the larger vertebrae, probably due to vertebral thickness at this point. A hyaline zone was present on vertebrae excised from two three-quarter term embryos collected in the spring. On vertebrae from free-swimming young-of-the-year a wide opaque zone separated this initial hyaline zone from the vertebral margin. Apparently, this initial hyaline zone is formed in utero. Mean back-calculated DW's at time of formation of the embryonic hyaline zone (includes 24 vertebrae from young-of- the-year rays not used in the age analysis) were 289 mm for males (n = 55) and 299 mm for females (n = 48). Analysis of covariance indicated a significant difference between the disc width (DW, mm) - weight (W, g) relationships for male and female R. bonasus (Table 13). The relationship for males (Fig. 25) was described by the equation: W = 6.5 x 10- 6 DW 3 * 1 2 = 0.99; n = 391). The relationship for females (Fig. 26) was described by the equation: W = 4.5 x 1 0 - 6 DW (r^ = 0.99; n = 374). The exponents of both equations were close to 3 indicating growth in 74 CU Pi cd pj nD CM CM CM CO CT1 »-H CM CM CM <± CM CO o o O 1 O r-^ o o o O O O o o id ••••••• cd o o o o O o O QJ 323 LW ON CM »—1 i-M CM >-1 co Td 00 NO NO NO CO CO r-- 1- r-~ CO 33 O W i—I i-P CO LO NO »—i CM CO CO CO CM 00 CO CM ■CT NO Ml- W nd o~ oo NO i—1 I-'- 1— pj OeS nO On NO O NO o co pM O ••••••• o o i—H o i-H o f—i o S <1 H Pd w o CTN uo UO LO kp M JO CO CM r—1 O r^ CO • CM »—i 1 CO • • II pci Pd CO CO CO o H r— H Pm Q M M3 w w 3= NO UO i—M uo r-- h P cj CM CTN r-H CM CO !Z O ^n r—I uo I''- co <3 <3 CO O' CO o i—H H 1—I M •• • • Pd Q CO co r-~ oo uo CO *—H *—i o P3 pM c j CO <3 CM OO CM r- pM JZJ >-> o o ON NO -3< o O X oo CM o <1- CM CO o r-' o o OO CO •• oo • OO M o uo • NO CO £ CM CM UO >4 PP .-P H < ! CL. LM o CO CO Figure 25 Disc width (mm)-weight (g) relationships for male and female Rhinoptera bonasus. o o o o o CO o 5 £ o Q CD (O o X CD m o LU o _J < i i 5 $ LU U. o DISC WIDTH (mm) WIDTH DISC r~ 00° o X 1 0 \— CD O m- C\J o 00 00 CM CM CM (\J (0001 X 6) 1H9I3M DISCUSSION Distribution Bigelow and Schroeder (1953) listed the range of R. bonasus as the elbow of Cape Cod to the Gulf coasts of Florida and Louisiana. Reports of R. bonasus from Gulf coast states are primarily limited to summer and early fall (Springer and Woodburn 1960; Clark 1963; Wang and Raney 1971; Walls 1975; Hoese 1977). Bigelow and Schroeder (1953) noted reports of R. bonasus in Florida waters during the winter and spring. Dahlberg and Heard (1969) captured specimens from Georgia waters in late summer. The cownose ray is common along the South Carolina coast April through October (Bearden 1965). Smith (1907) reported the presence of _R. bonasus in North Carolina waters throughout the year, although they were most abundant in spring. Gudger (1910) collected specimens near Beaufort, North Carolina in May and July. Radcliffe (1914) considered the cownose ray as common in this area, however, Coles (1915) cited the ray as "not abundant" in North Carolina. Accounts of coastal incursions north of Cape Hatteras to Cape Cod have been limited to summer or early fall (Bigelow and Schroeder 1953; Bearden 1959; Hess and Miller 1963). Hildebrand and Schroeder (1928) considered R. bonasus a rare visitor to Chesapeake Bay. More than two decades elapsed before the 76 77 next published account of R. bonsasus from the Bay (Bayliff 1951). This specimen was taken near Solomons, Maryland and the author noted that residents of the area considered the occurrence of cownose rays as common. Joseph (1961) recorded several exceptionally large pound net catches of JR. bonasus from the lower Bay during the summer of 1960. In August and September of the following year, large schools were sighted near the Bay mouth (Hoese 1962). Musick (1972) listed Ft. bonasus as abundant to common in the upper and lower Chesapeake Bay in salinities as low as (13 °/oo) during the summer months. Schwartz (1965) tagged R. bonasus in the upper Chesapeake Bay (1958-1963) and his results constitute the most comprehensive account of R. bonasus movements to date. He indicated that large schools of R. bonasus occupy the upper portions of Chesapeake Bay from early June to early September and that they undergo steady southward migration in the fall with schools off Cape Hatteras in mid-October and off northern Florida in early December. Citing tag returns from Lake Maricaibo, Margarita Island, Trinidad and northern Brazil, he claimed that the schools of cownose rays arrive off northern South America in mid-January. They remain there until early March when the northward spring migration begins. Further, he cited another R. bonasus population in the Gulf of Mexico. This population migrates seasonally in a clockwise direction from the Yucatan Peninsula around the Gulf to Florida and then undergoes a fall migration from western Florida to Yucatan. This migration often involves schools consisting of up to 10,000 rays. Clark (1963) has also recorded an enormous school of 78 R. bonasus (ca. 4,000 to 6,000 rays) near Sarasota, Florida in mid-July. My data confirm that R. bonasus has a migratory pattern similar to those of many elasmobranch populations in the western North Atlantic, i.e., they migrate north with the vernal warming of coastal waters (Bigelow and Schroeder 1948, 1953; Springer 1960; Strushaker 1969). Schools of adult R. bonasus, some with several thousand members (Fig. 8), arrived in the Cape Lookout-Beaufort, North Carolina area in mid-April. The sex ratio of these schools appeared equal. Schools of adult cownose rays were the vanguard of the spring migration and they traversed the Virginia-North Carolina Outer Banks in late April. Juvenile cownose rays ascended the North Carolina coastline throughout May. Interviews with North Carolina commercial fishermen suggest that a majority of the adult spring migrants moved northward through the sounds rather than along the ocean beaches of the barrier islands. Food resources such a bay scallops, oysters and clams are abundant in these inside sheltered areas. Schools probably exited the sounds via Oregon Inlet, N.C. Rhinoptera bonasus entered Chesapeake Bay by the first week of May. Entrance and dispersal was probably via the high salinity waters of bayside eastern shore since pound netters at Lynnhaven reported insignificant catches of R. bonasus in May. Initial movement of cownose rays within the Bay appeared to be north- and westward with entry into western shore river systems by mid-May. The massive schools of rays, as cited off Beaufort, North Carolina in April, 79 became fragmented into schools of no more than several hundred members as they entered the rivers of Chesapeake Bay. Springer (1967) cited parallel trends for shark populations: stronger tendancy to form large groups during migration than at the terminal points of the migratory route. Juveniles and adults were present in collections from pound nets in the lower York River during May 1977. Adults gradually moved upriver during the latter half of May. Juvenile cownose rays were conspicuously absent from similar collections from June through September. By the first week of June aerial and shore-based siting confirmed the presence of schools of adult cownose rays up to Claybank in the York River, Jones Point in the Rappahannock River, and the Yeocomico River in the Potomac River (Fig. 1). These data are consistent with the report of cownose ray arrival at Solomons, Maryland on June 5 (Schwartz 1965). Rhiqoptera bonasus remained in Chesapeake Bay and its tributaries throughout the summer. Schools of feeding rays exhibited a shoalward movement with the rising tide and retreated during the ebb (see food habits section). West Coast bat rays, Myliobatis californica, underwent similar movements into forage areas (F. M. Douglas, pers. comm.). Some portion of the cownose ray population may inhabit deeper areas of the Bay which were not sampled during this study. The paucity of R. bonasus records in VIMS' trawl records argues against major deep water aggregations during the summer but gear avoidance bias may be great. The most upriver sightings of cownose rays in my 80 aerial and shore-based surveys was Claybank on the York River, Bowler's Wharf in the Rappahannock River and Kingcopsico Point in the Potomac River (Fig. 1). My collections indicated that parturition occurred during late June and early July and that young—of-the-year were abundant in the lower portions of the river systems throughout the summer. My data did not allow an estimate of the proportion of the young-of-the-year rays that moved upriver. Adult males appeared to be the first group to have left the river systems of western Chesapeake Bay. They were conspicuously absent from collections in the York River during August and Septmeber. Adult females, on the other hand, remained in the rivers until they departed in mid- to late September . The large aggregation of cownose ray schools along bayside eastern shore on September 29, 1976 suggests emigration of adult cownose rays from Chesapeake Bay via the high salinity waters of bayside eastern shore. Collections from Smith Point and Lynnhaven in October suggest that small JR. bonasus are last to leave the Bay. The fall migration of cownose rays southward is probably not as closely aligned to the shoreline as their spring movement to the north, since haul seiners at Sandbridge and Corolla claimed R. bonasus are rarely caught in their September-November sets. 81 Environmental Factors Temperatures recorded at sites where R. bonasus was collected were 15-29°C. Unpublished records from the South Atlantic Bight (Table 3) and the Gulf of Mexico (NMFS unpubl. data, Pascagoula, Miss.) show a similar range except 13°C was the coldest bottom temperature at which R. bonasus was taken. Schools of cownose rays arrived in the Cape Lookout-Beaufort, North Carolina area in mid-April when inshore water temperatures warmed to about 16°C. Their movement northward appeared to advance with the 15-16°C isotherms. Cownose rays entered Chesapeake Bay in early May when waters rose to about 18°C. Northward spring migration of cownose rays was delayed in 1978. Rhinoptera bonasus were initially reported near Beaufort, North Carolina on April 19 (water temperature = 16°C; W. S. Otwell, pers. comm.). The 15-16°C isotherms hovered near Cape Hatteras until about May 11 (Fig. 2b;. The spring of 1978 was very cold; Chesapeake Bay and nearshore Virginia-North Carolina waters remained quite low through April. I recorded a surf water temperature of 9°C at Corolla, North Carolina on May 2. The first haul seine catches of R. bonasus at Corolla were on May 11 (E. Lawler, pers. comm.). This was 15 days later than the initial haul seine catch of cownose rays in 1977. Further, no JR. bonasus were sighted in lower Chesapeake Bay or its tributaries on the flights of June 1 and 2. Schools of cownose ray were well-established in these areas on the corresponding date (June 2) in 1977. Thus, low, nearshore, vernal water temperature 82 Figure 26 Spring 1978 sea surface isotherms along the Virginia-North Carolina coast. 75' 75c 35 35 20 ' 20 16-20 APR 78 27 APR -I MAY 78 75° 75° 35 35 20 20 5-11 MAY '78 -17 MAY '78 75' 75° 35 20 35' 20 17-22 MAY'78 26 MAY-1 JUNE '78 75' 75c 20 20 25 35° 35 1-7 JUNE '78 8-13 JUNE '78 83 (< 15-16 °C) appears to limit the northward movement (dispersal) of R. bonasus. Strushaker (1969) found that bottom waters below 15°C were a barrier to the northward dispersal of the roughtail stingray, Dasyat i s centroura. Bearden (1959) suspected that the late arrival of the eagle ray, Myliobatis freminvi1lei, in Delaware Bay during the summer of 1958 was due to below average water temperatures. Adult cownose rays withdrew from Chesapeake Bay in late September when water temperatures cooled to about 20-22°C. Young-of-the-year and juvenile cownose rays appeared to tolerate lower fall temperatures since some remained in the Bay unt il late October. Rhinoptera bonasus is euryhal ine. I collected specimens in 8.1 °/oo to 30 °/oo. Young-of-the -year cownose rays may be even more tolerant of low salinities. One s pecimen (not seen) (DW = 348 mm) was taken on the intake screens at Sur ry Vepco generating station (James River) in 5.6 °/oo (J. Govoni, pers . comm.). Upriver penetration (distance and incidenc e) may be correlated with decreased summer, freshwater runoff. One ques t ionnaire respondant commented that cownose rays were pre sent in the upper Rappahannock River, near Warsaw, Virginia only during "a very dry summer when the water gets very salty. 11 Other fishermen from the upper Rappahannock and Potomac Rivers reported greatest catches of II. bonasus in August and September. 84 Schooling by Size and Sex The general tendency for elasmobranchs of similar size to associate together and for adults to segregate by sex has been reported previously (Springer 1967) and is well-documented for sharks (Ford 1921; Hickling 1930; Templeman 1944; Ripley 1946; Bigelow and Schroeder 1948; Olsen 1954; Backus et al. 1956; Springer 1960; Bullis 1967; McLaughlin and O'Gower 1971). Backus et al. (1956) recognized two types of sexual segregation in the selachians: (1 ) "behavioral,1' in which mature adults tend to school separately by sex, while smaller individuals of both sexes school together, and (2 ) "geographical," in which the sexes are distributed unequally "as a function of area or depth." Although swimming speed, habitat and food preferences may influence size segregation in a shark species, Springer (1967) pointed out that small sharks appear to actively avoid larger ones. He suggested that (1 ) segregation by size is a social phenomenon and (2 ) since most sharks are indiscriminant predators, the young of some species are prcJuced in specific nursery grounds which are relatively free of other sharks of the same or other species. Batoid populations are less well-studied than sharks, yet segregation by size and/or sex was noted in several species or raj ids (Steven 1933), Urolophus halleri (Babel 1967), D. centroura (Strushaker 1969), M. californica (Herald 1953; Odenweller 1975; F. M. Douglas, pers. comm.) and in M. freminvillei (Bearden 1959). Since the dentition of batoids does not lend itself to a cannibalistic fare, factors other than intraspecific predation must influence size and sexual segregation in these fishes. My findings indicated that the population of R. bonasus which inhabits Chesapeake Bay during the summer months, exhibited a high degree of social organization. Spring migratory schools and schools in Chesapeake Bay segregated by size. This segregation could be due to differences in swimming ability with size. Also, stomach contents suggested that differences in food preferences by size of the ray may be responsible for the segregation witnessed in the Bay (see food habits section). Small cownose rays did not excavate and consume deep-burrowing molluscs. Their diet consisted of shallow or non-burrowing molluscs. The cohort of juvenile R. bonasus which were absent in my collections from the western shore of Chesapeake Bay may have been in deeper areas of the Bay and bayside eastern shore where the blue mussel, Mytilus edulis, is abundant (Turgeon 1968; Wass 1972 pers. observations). Collections of cownose rays from the York River suggest spatial partitioning of resources between sexes in adult R. bonasus. Adult males dominated samples from the lower York River in June and July. Increased sampling further upriver in those months may have confirmed presence of the adult females. From late July through September samples in both the upper and lower York River and vicinity (Poquoson and Ware Rivers) were almost exclusively composed of gravid females with early yolk-sac embryos. 86 Adult R. bonasus may exhibit what Robins (1971) termed "ecological dimorphism." It is hypothesized that adult male R. bonasus leave the river systems of the western shore of Chesapeake Bay by late July thereby avoiding competition with gravid females for available food resources. Location of the adult male cownose rays during the latter half of the summer is uncertain but bayside eastern shore, Chesapeake Bay proper, or south along the Virginia-North Carolina coastline are most likely areas. Winter Habitat Winter habitat of the R. bonasus population which resides in Chesapeake Bay during the summer is debatable. Schwartz (1965) provided little concrete data but from tag returns in Lake Maricaibo, Margarita Island, Trinidad, and northern Brazil he maintained that they overwinter off northern South America. Bigelow and Schroeder (1953) commented on the paucity of records from northern South America and the Caribbean. The cownose ray is not cited in Cervigon’s "The Marine Fishes of Venezuela" (1966), although the author subsequently recorded a specimen from Guyama (Gines and Cervigon 1968). Dr. Cervigon reports R. bonasus as extremely rare in northeastern Venezuela but they are said to be common in the western part around Lake Maricaibo. He has seen them at the latter site "in perhaps brackish water, of small size" (pers. comm.). Recently, Mr. Donald Taphorn of the University of Zulia (Maricaibo, Venezuela) is of the belief that R. bonasus are present in Venezuelan waters year 87 round . He collected gravid females in Lake Maricaibo during July, 1978 (pers. comm.). I question the feasibility of an inter-American migration by R. bonasus. From Schwartz's data (1965), I calculated distance traveled per day through the South Atlantic Bight during the fall migration (7.9 nautical mi/da; Table 14). Assuming the U.S. Coast Guard ray sightings off northern Florida in mid-March (Fig. 9) were actually R. bonasus, a rate for the spring migration northward through the same area was obtained (12.3 mau mi/da; Table 15). A remarkably similar rate (12.5 mau mi/da; Table 15) was calculated for the spring migrants between Portsmouth I., N.C. and Corolla, N.C. The faster spring speed through the South Atlantic Bight could be attributed to the influence of the Florida Current. Even at the faster pace cownose rays could not traverse the distance between northern Florida and northern South America (shortest route ca. 1200 nautical miles) twice in 3 months as cited by Schwartz (1965). I have witnessed several, apparently undisturbed schools of R_. bonasus swimming near the surface at an estimated 4-5 knots. At this speed and assuming their orientation is not impaired at night the time required for a Caribbean crossing might be as short as 10-13 days. The apparent leisurely pace through the South Atlantic Bight may be due to regular pauses at feeding stations on the shelf. Once south of Cape Canaveral, the continental shelf narrows and the hypothetical migration to South America might be a direct route across deep ocean areas. It is of interest to note that schools of cownose rays have been photographed TABLE 14 ESTIMATED SPEED (NAUTICAL MILES/DAY) of RHINOPTERA BONASUS BETWEEN CAPE HATTERAS, N.C. AND NORTHERN FLORIDA DURING THE FALL MIGRATION SOUTH (DATA FROM SCHWARTZ 1965) Approx. Distance Days Required Between Points To Traverse Speed Point Date (nau mi) Distance (nau mi/da) Cape Hatteras, 15 Oct N.C. 440 56 7.9 Northern 10 Dec Florida TABLE 15 ESTIMATED SPEEDS (NAUTICAL MILES/DAY) OF RHINOPTERA BONASUS BETWEEN SEVERAL POINTS ALONG THE U.S. EAST COAST DURING THE SPRING MIGRATION NORTH Approx. Distance Days Required Between Points To Traverse Speed Point Date (nau mi) Distance (nau mi/da) Northern 15 March 77 Florida 420 34 12 .3 Portsmouth 18 April 77 I., N.C. 100 12.5 Corolla, 26 April 77 N.C. 70 11 6.3 Lower York 11 May 77 River, Va. 89 near the Bahamas during bluefin tuna remote sensing studies (A. J. Kemmerer, National Fisheries Engineering Laboratory, NSTL Station, Miss., pers. comm.). I described above my occasional, aerial oservations of V- or wedge-shaped schools of 11. bonasus. Clark (1963) and Otwell and Lanier (1978) (Fig. 8 ) observed massive schools of thousands of cownose rays. By swimming in formation or in large schools It. bonasus might improve hydrodynamic efficiency as has been suggested for other fishes (Shaw 1962; Breder 1965). An obvious question is, why migrate to South America when the shelf waters of the South Atlantic Bight offer suitable overwintering habitat? Winter bottom temperatures on the outer half of the Continental Shelf remain warm (Strushaker 1969; Mathews and Pashuk 1977) and food resources in the form of calico scallop beds, Argopecten gibbus, appear abundant in the shelf waters (13-94 m) off North Carolina (Drummond 1969; Schwartz and Porter 1977) and Cape Canaveral, Florida (Cummins 1971). This is not unreasonable since present data suggest that several of the dasyatids (Strushaker 1969; Barans and Burrell 1976) the gymnurids (Daiber and Booth 1960) and possibly M. freminvillei (C. A. Wenner, pers. comm.) overwinter in this area. A recent trawl survey along a thermal front between Oregon Inlet and Cape Lookout, North Carolina (winter cruises 1976-1977 along the 30 m contour; D. Stewart, University of Wisconsin, pers. comm.) and 90 MARMAP winter surveys (1974-1977) between Cape Fear, North Carolina and Cape Canaveral, Florida (C. A. Wenner, pers. comm.) failed to capture R. bonasus in the South Atlantic Bight in winter. However, Bureau of Commercial Fisheries surveys (1959-1966) and the North Carolina Division of Marine Fisheries trawling efforts list 14 late fall and winter records of R. bonasus in water depths of 7-33 m within the South Atlantic Bight (Table 3 and Fig. 16). Specimens for which weights were recorded (n = 9) were either young-of-the-year or juvenile R. bonasus. Springer (1967) noted that for sharks, "where migrations occur, the distance traveled by the immature individuals seem to be appreciably shorter than those traveled by the adults." On the basis of the information presented above, it seems reasonable to conclude that some young-of-the-year and juvenile _R. bonasus overwinter in the South Atlantic Bight. They appear to prefer the shoaler shelf depths (10-35 m) and indeed may concentrate in areas where calico scallops are abundant (off Cape Canaveral and between Cape Romain and Cape Lookout). The lack of winter records for adult R_. bonasus from the South Atlantic Bight is not conclusive evidence for their absence, since they are probably successful in avoiding most trawl nets. From the present data base I suggest that four populations of R. bonasus exist in the Western Atlantic with centers of distribution in middle to southern Brazil, northern South America, the Gulf of Mexico and the East Coast of the United States. If Schwartz's report is accurate, the latter two may be highly migratory components of the 91 Venezuelan stock. Obviously, additional tagging, taxonomic, and electrophoretic work would be useful for delineation of the stocks and their migratory routes. Food Habits Numerous citations in the literature indicate that the diet of R. bonasus consists primarily of bivalve molluscs which are crushed between the ray's flat, pavement-like tooth plates. Specimens from southern New England contained clams, large gastropods, lobsters, and crabs (Bigelow and Schroeder 1953); those from New York waters consumed Mya (Mitchill 1815). Smith (1907) found that the razor clam and oyster were food items of R. bonasus in North Carolina. Radcliffe (1914) also noticed molluscs in cownose stomachs from this area. A small school of cownose rays sighted at Gasparilla Pass, Florida fed on the sun ray venus clam, Macrocall ista nimbosa (Wang and Raney 1971). A single specimen from Lake Pontchartrain, Louisiana "was packed with the bodies of small clams" (Darnell 1958). Recently, Otwell and Crow (1977) implicated R. bonasus in the destruction of bay scallop (Aequipecten irradians) beds in the sounds of North Carolina. In Chesapeake Bay, Bayliff (1951) found that a specimen captured near Solomons, Maryland "contained what appeared to be molluscs." Wallace et al. (1965) indicated that the cownose is a serious predator of the Bay's Mya stocks during the summer months. Dr. Frank Schwartz (pers. comm.) stated that in the upper portion of the Bay, cownose fed on Mya and Crassostrea. Orth (1975) identified Mya, Zostera roots and small invertebrates from the stomachs of 8 rays collected in the York 92 River. Recently (1972-1975), Rappahannock River shellfish growers reported increased cownose ray predation on their oyster beds during the summer months. The diet of the Javanese cownose ray, R. javanica, is also reported to be primarily molluscan (Shipley and Hornell 1906; James 1962, 1970). Rhinoptera bonasus and R. steindachneri from the Gulf of Mexico and Gulf of California respectively, consumed mainly bivalve molluscs, but also large gastropods, small lobsters and fishes (Aquirre 1965). My food analysis results were in agreement with the citations above: the diet of R. bonasus consisted primarily of bivalve molluscs. Rhinoptera bonasus appeared fairly catholic in its choice of bivalves, and seemed to feed on the most abundant shellfish in its habitat. In the York River area (including the Poquoson and Ware Rivers) the soft-shelled clam, Mya arenaria, (IRI = 5598) was by far the most important food item. In Chesapeake Bay Mya occurs from Hampton Roads at the Bay mouth to Maryland's Chester River (Haven 1970; Lippson 1973). It is abundant in sand to silty-sand bottoms where salinities range from 5-10 °/oo to 25 °/oo (Haven 1970; Wass 1972; Lucy 1976). Adult Mya defined as > 55 mm in length by Lucy (1976) are primarily concentrated in shoal waters just below the mean low water mark out to depths of 3-4 m (Manning and Pfitzenmeyer 1958; Pfitzenmeyer and Drolbeck 1963; Haven 1970; Lucy 1976). Juvenile Mya may be 93 distributed in deeper areas of the Bay (Boesch 1971; Lucy 1976). The widespread distribution of Mya suggests that the soft-shelled clam is an important item in the diet of R. bonasus throughout the Bay. The little macoma, Macoma balthica, ranked second in importance (iRI = 1476) as a cownose ray food item in the York River. In Virginia, M. balthica is abundant intertidally and out to a depth of 10 m in salinities of 5-15 °/oo (Turgeon 1968; Wass 1972). M. balthica is probably an important food item for rays which penetrate to the upper mesohaline zone of the Bay. Tagelus plebeius (IRI = 398) was the third most important dietary item in rays collected in the York River. The stout razor clam is a euryhaline species occupying intertidal flats and bottoms to a depth of 1.5 m (Turgeon 1968; Wass 1972). It is abundant in the river systems of the western shore of Chesapeake Bay (Turgeon 1968) and, as in the York, probably constitutes an important food resource for R. bonasus in tuese areas. The hard clam, Mercenaria mercenaria ranked a distant fourth (IRI = 64) in stomachs of York River specimens. However, it is a polyhaline species and is abundant in various sediments intertidally to channel depths (Turgeon 1968; Wass 1972). It is suggested that Mercenaria represents an alternative food resource during low tide when the predominantly intertidal, soft-shelled bivalves are inaccessible. In August 1976 VIMS personnel (R. K. Dias and R. J. Orth) documented extensive cownose ray damage to 94 a commercially planted bed of Mercenaria in the Back River (see Merriner and Smith 1979a). The clam grower (R. Davis, Poquoson, Virginia) claimed approximately 1.8 million "little neck" clams were consumed by feeding schools of cownose rays in several days. Geukensia demissa, the ribbed mussel, inhabits the high intertidal zone and is generally associated with Spartina root systems which fringe marshy shorelines (Turgeon 1968; Wass 1972). Its presence in two R. bonasus stomachs (IRI = 4) reflects the ray's ability to invade intertidal flats during high water. The oyster, Crassostrea virginica, occurred in only one stomach (IRI = 2) from the York River. This low incidence is not surprising, since the York does not harbor extensive oyster beds, nor were actively feeding rays successfully collected over existing oyster rocks. It is suggested that recently observed (1972-1975) cownose ray predation on commercial oyster beds in the Rappahannock River may be due to: (1) th: almost total destruction of Mya stocks in the Rappahannock caused by a massive infusion of freshwater into the Bay by Tropical Storm Agnes in 1972 (Haven et al. 1976). The effects of the freshet are reflected in the post-storm commercial landings of Mya in Virginia and Maryland (Fig. 27). Presumably, other soft-shelled mollusc populations in the Rappahannock were similarly affected; (2) the drastic decline of the oyster industry in Virginia due to the deliterious effects of Minchinia nelsoni (MSX), Dermocystidium marinum ("dermo"), and the oyster drill, Urosalpinx cinerea (Haven et al. 1978). Presently, Rappahannock River oyster growers have adopted a 95 Figure 27. Maryland and Virginia Mya arenaria landings, 1953-1976. Mya arenaria LANDINGS (thousands of pounds) - 4 8 9 n - 7 - 5 6 2 - - -1 92 4 6 8 0 0 4 6 8 0 2 4 76 74 72 70 68 66 64 60 60 58 56 54 1952 AYAD LANDINGS MARYLAND VIRGINIA LANDINGS YEAR 96 policy of planting only in areas of 12-15 °/oo or less where the effects of these pathogens and predators are severely reduced (Haven et al. 1978). Thus, in the absence of preferred food items JR. bonasus predation probably increased on an already impacted oyster stock (see Merriner and Smith 1979a). The common razor clam, Ensis directus was found in 2 stomachs from bayside eastern shore. It is a polyhaline bivalve which is abundant on the sand flats of seaside eastern shore (Wass 1972). Populations are sporatic in the lower Bay (Turgeon 1968). A set of 30,000 juveniles/m^ has been reported from off the mouth of the Rappahannock River (Wass 1972). Ens is therefore would be an important R. bonasus food item along both sides of eastern shore and in the lower Bay proper. Mytilus edulis occurred in nine rays collected near Lynnhaven and one near York Spit Light. Its distribution and abundance in Virginia are similar to Ens is (Turgeon 1968; Wass 1972), although it is commonly attached to hard substrates. Like Ens is, it is probably an important food item of R. bonasus along eastern shore and the lower portion of the Bay. A total of 15 rays from the Potomac River had consumed the coot clam, Mulinia lateral is. This small mactrid clam is abundant in silt to clay bottoms, where the water is shallow, warm, and the salinity is greater than 8 °/oo (Turgeon 1968). Summer densities at subtidal sites in the York averaged 20/m^ and 37/m^ (Virnstein 1977). Up to 97 2 0 ,0 0 0 /m^ have b een reported from Tangier Sound (Wass 1972). Presumably, Mulinia is important in the diet of the cownose ray in other areas of the Bay in addition to the Potomac River. Rhinoptera bonasus may feed on teleost remains encountered along the bottom. One specimen (DW = 1000 mm) was taken on longline gear baited with cut spot (Leiostomus xanthurus) near Wreck Island, Virginia and chum of L_. xanthurus and Micropogonias undulatus was found in its stomach. Dahlberg and Heard (1969) also reported taking R. bonasus on cut bait (Lagodon rhomboides). Seventeen rays from pound net samples had ingested teleost remains (presumably fish that died in the net) and 35 R. bonasus had consumed Anchoa mitchilli. Capape (1977) recently indicated that small pelagic teleosts are important in the diet of the bull ray, Pteromyleaus bovinus. Food habits of R. bonasus seemed to change with growth. Small cownose rays (DW < 800 mm) collected with representative food items had consumed My'ilus (n = 10) and Mulinia (n = 5). Karl and Obrebski (1976) found that as M. californica increased in size, larger, deep-burrowing organisms became increasingly important in their diet. I suggest that young-of-the-year and juvenile R. bonasus are unable to extract deep-burrowing bivalves from the substrate, thus their diet is limited to shallow- or non-burrowing organisms. Juvenile II. bonasus which were scarce in the river systems of western shore may congregate along eastern shore or in deeper stretches of the Bay proper where the non-burrowing blue mussel (M. edulis) is more abundant. 98 Many small mysids, isopods and amphipods (Table 8 ) as well as Mya, were in the stomachs of actively feeding JR. bonasus captured in a patchy Zostera bed (Goodwin I., mouth of the York River). Orth (1975) cited "small invertebrates" and Mya in cownose ray stomachs from a similar habitat. These small invertebrates (in- and epifauna of Zos tera beds; Marsh 1973, Orth 1973) were probably non-selectively inhaled during feeding activity in the eelgrass beds. The extent of JR. bonasus feeding during their spring and fall migrations along the East Coast of the U.S. is uncertain. Otwell and Crow (1977) documented extensive R. bonasus damage to bay scallop (Aquipecten irradians) beds upon the ray's spring arrival in the Beaufort, North Carolina area. Only one of 32 stomachs collected at Sandbridge and Corolla in May contained food, the fragmented test of a sand dollar (Mel1 ita quinquiesperforata). Several JR. bonasus were observed in a feeding disturbance about 0.5 km offshore of False Cape State Park, Virginia on April 20, 1976. Captain Fred Feller (Rudee Inlet, Virginia; pers. comm.) cites schools of cownose rays feeding on clams over the "Lumps," southeast of Virginia Beach (Fall 1978). On this point it is interesting to speculate on the use of R. bonasus as a biological indicator for potential offshore shellfish beds in the South Atlantic Bight (W. S. Otwell, pers. comm.). Feeding Behavior The movement of rays onto shoal areas with the rising tide has been recorded on several occasions. Coles (1910) noted feeding incursions of the spotted eagle ray Aetobatus narinari, in shallow water at high tide along the North Carolina coast. Bigelow and 99 Schroeder (1953) reported that many of the dasyatids follow the flood tide onto shallow flats and retreat during the ebb tide. Ray feeding depressions (probably dasyatids) observed at low tides in a Georgia estuary were concentrated primarily on tidal flats and sand bars (Howard et al. 1977). Schools of M. californica along the California shoreline, invade intertidal oyster beds at high water and withdraw via adjacent channels as the tide falls (F. M. Douglas, pers. comm.). Karl and Obrebski (1976) discovered M. californica feeding excavations on intertidal sand flats. Aerial and shore-based observations along with gill net collections confirmed nearshore feeding activity of R. bonasus. In the lower Chesapeake Bay schools of R. bonasus invaded inter- and subtidal flats to feed during high water. Feeding however, is by no means restricted to shoal areas. Shell fragments of Noetia, a bivalve primarily limited to the deeper waters and channels of Chesapeake Bay (Turgeon 1968), occurred in the spiral valve of one specimen and My t i1u s, Ens is and Mercenaria were present in specimens collected from deep waters. The presence of these items suggests that JR. bonasus continues feeding even in the deeper stretches of the rivers and Bay. Elasmobranch fishes possess numerous sensory systems for the detection of prey (i.e., sight, olfaction, electroreception, acoustico-lateralis system). The exact sensory systems and stimuli involved in the detection of prey items by R. bonasus remain speculative. Possibly, injured or stressed shellfish and/or bivalve 100 excurrent produce a detectable odor field and aid in the general localization of prey items (Hobson 1963; Tester 1963; Kalmijn 1971). The subrostral fins appear to play an important role in the near location of prey. Cruising rays kept these structures pressed tight against the ventral surface of the rostrum, thus streamlining the body in flight. Schwartz (1967) noted that when feeding, the subrostrals "are used in a probing, creeping, and directing fashion." The juvenile cownose ray which I kept in an aquarium exhibited similar behavior during a feeding response. Macroscopic inspection reveals numerous ampullary pores on the subrostrals. Thus, by probing the bottom with the subrostral fins, the mechano-, electroreceptive ampullary system (Murray 1960, 1962; Kalmijn 1966, 1971) could detect: (1) the stream of excurrent from the siphons of burrowing bivalves [as Walford (1935) suggested for M. californica]; (2) the bioelectric fields produced by molluscs. The ampullae are responsive to electric fields of 1 V/cm in surrounding seawater (Murray 1962). Kalmijn (1972) demonstrated that several species of bivalves produce dc potentials of less than 10 V and that the values are appreciably higher for wounded specimens. (3) The burrows of infaunal bivalves and/or the valves of non-burrowing moluscs (Myti1us and Crassostrea) by tactile reception. Little information exists on the feeding dynamics of myliobatoid rays. Aetobatus narinari reportedly uses "its tough hog-like snout" (Coles 1910) or its lower jaw (Radcliffe 1914) to spade the bottom for shellfish. Walford (1935) reported that M. californica "flaps its 101 pectoral fins, creating a suction which digs out the clams"; feeding troughs up to 1 m wide, 50 cm deep and 4.5 m long have been reported for this ray (MacGinitie 1935). Van Blaricom (1976) indicated that the digging behavior of M. californica and U. halieri "involves rhythmic flapping of the rostrum and pectoral fins." The sediments then are dispersed to the front and sides of the ray. Bigelow and Schroeder (1953) considered the subrostral fins of R. bonasus too flaccid for plowing the substrate; an observation with which I concur since I found no excessive scratches or abrasions on these structures. They suggested rapid flapping motions of the pectoral fins as the mechanism used to uncover burrowing shellfish. Schwartz (1967) reported hydraulic mining of the bottom by the cownose ray, but did not elaborate on the mechanism. Analysis of stomach contents, field examination of feeding depressions, jaw morphology, and aquarium observations lead to a description of the feeding mechanics of the cownose ray by inference in the absence of in situ observations. The diet of the cownose ray reflects its ability to extract deep-burrowing bivalves (Mya and Tagelus) from the substrate. Adult Mya can burrow as deep as 30 cm depending on substrate composition (Lucy 1976 and pers. comm.). Tagelus (ca. 4 cm long) build permanent burrows about 30 cm deep (Webb 1942). I concur with Howard et al. (1977) that ray feeding depressions are circular and bowl-shaped with a central steep-sided cavity. However, their conclusion that rays obtain benthic food items by hydraulically eroding the substrate with 102 rapid flapping motions of their "wings," appears to be an imcomplete statement. Moss (1977) suggested most batoids, as well as heterodontid, orectolobid and pristiophoform sharks, are capable of suction feeding by virtue of their short ventrally positioned jaws and expansive orobranchial chamber. Indeed, Tanaka (1973) has experimentally shown that the nurse shark, Ginglymostoma cirratum can generate a relatively high oral suction pressure (maximum pressure = 1.03 kg/cm^). In batoids the loose connection of the palatoquadrate to the cranium by a long ligament (Bigelow and Schroeder 1953; Moss 1977) increases the protrusibility of the jaws. Suction appeared to be used in the feeding mechanics of the juvenile cownose ray in an aquarium. Theray protruded its jaws into the bottom and inhaled gravel into the orbranchial chamber. It then left the bottom and vented gravel out the gill slits as it swam across the tank. This corroborates an observation of cownose feeding behavior related to me by a commercial fishermen: having successfully excavated a shellfish, a cownose ray will swim away from the area of excavation and "begin breathing very hard to clean out its gills." Kalmijn (1971) observed similar behavior: Scyliorhinus canicula "removed the sand over his prey [plaice] by sucking it up and expelling it through the gill slits" and Raja c1avata dug out its prey "by blow and suction, the sand, however, coming out from below the ray [the gill slits]." I suggest that R_. bonasus (and possibly other batoids) employs the following feeding mechanism to obtain infaunal prey items. 103 1) Upon successful location of an infaunal food item, initial stirring motions of the pectoral fins serve to erode the upper sediment surface and create a shallow bowl-shaped depression. 2) Both jaws are protruded into the sediments, the orobranchial chamber is rapidly- expanded, and sediments and water are inhaled. 3) The mouth is then closed and the orobranchial chamber contracted. Sediments and water are prevented from passing out through the mouth by a broad transverse fold of the oral membrane on the roof of the mouth (Bigelow and Schroeder 1953) which functions as a one-way valve. Water and sediments are thus vented out the gill slits. 4) Repeated inhalation and venting of the sediments creates a central steep-sided cavity. 5) Continued movement of the pectoral fins aids in dispersing the ventrally evacuated sediments and increases the depth of the depression. 6) Deep burrowing prey items are eventually grasped and inhaled into the mouth. 7) The ray may then move away, clear its gills, and consume the prey. The proficiency with which myliobatids ingest molluscan vicera and reject shell fragements has been noted by several investigators (Coles 1910; Bearden 1959; Godfriaux 1970). I found that R_. bonasus stomachs contained only a few small fragments of Mercenaria shell, small to moderate amounts of Mya, Tagelus and Macoma shell, and large quantities of Ens is, Myt ilus and Mulinia shell. Bearden (1959) found shell fragments of Mytilus quite common in M. freminvillei stomachs. Shipley and Hornell (1906) reported finding "nothing but broken shell and viceral masses" of a small, mactrid clam 104 in a specimen of R. j avani ca. Likewise, in R. bonasus it appears that small, thin-shelled molluscs (Mulinia, Mytilus and Ens is) are simply crushed, the fragmented valves ingested, and passed through the digestive tract. Field examination of commercial Mercenaria and Crassostrea beds damaged by feeding schools of R. bonasus revealed that the valves of these molluscs are typically fragmented into several large pieces. Frequently, only the "bill" of the oyster is broken, leaving the dorsal half of both valves and the hinge intact. These observations suggest that once the valves of thick-shelled molluscs are crushed, the large, loose fragments may fall to the bottom after the molluscan vicera are inhaled into the mouth. The mechanism whereby the valve fragments of moderately thick-shelled molluscs are rejected is unclear. Perhaps as Bigelow and Schroeder (1953) suggested the buccal papillae are instrumental in sorting the shell fragments from the shellfish meats. Re p r o du c t i on Sexual Maturation Male - Male elasmobranch fishes possess highly developed structures on their pelvic fins called claspers, which function as intromittant organs during copulation (Hoar 1969; Gilbert and Heath 1972; Wourms 1977). At the onset of maturity, additional terminal cartilages develop on the distal end of the claspers (Bigelow and Schroeder 1953). Several investigators have used the allometric 105 growth of these appendages to determine the attainment of sexual maturity in sharks (Templeman 1944; Gilbert and Heath 1972) and rays (Bearden 1959; Hess 1959; Strushaker 1969; Mellinger 1971). Male elasmobranchs also possess secondary sexual structures called siphon sacs (Gilbert and Heath 1972). In batoids the sacs are modified into specialized clasper glands and are situated near the ventral surface of the pelvic fin close to the base of the claspers (La Marca 1964; Babel 1967; Wourms 1977). In concert with flexures of the claspers, contraction of the clasper glands expels a secretion which aids in forcing sperm through the clasper tube (La Marca 1964; Babel 1967; Wourms 1977). Bigelow and Schroeder (1953) noted considerable variation in the reported size at which sexual maturity is attained in male R. bonasus. I found the change in the size of the claspers relative to disc width and condition of the testes established that most male _R. bonasus began sexual maturation at about 800 mm and all were mature at disc widths greater than 840 mm. Judging from the amount of sperm present in the seminal vesicles, males were ripe in Chesapeake Bay from mid-May to at least late August. The clasper glands of ripe male R. bonasus which were freshly captured, contained a milky—white, viscous fluid which tended to form coagulated clumps. The clasper tubes contained the same material on several occasions. La Marca (1964) made similar observations in U . halleri and proposed that the coagulative properties of the clasper 106 groove facilitated the formation of a closed clasper tube. Wourms (1977) speculated on the fluid's role in the formation of sperm plugs. Female - Considerable uncertainty exists in the literature as to the size of female R. bonasus at sexual maturity. Gudger (1910) claimed that a female about 610 mm wide gave birth to a pair of young. Bearden (1965) reported four premature embryos from a gravid female measuring 712 ram (disc width?). Joseph (1961) and Orth (1975) collected gravid females of much larger size, about 890 and 900 mm, respectively. My data established that most female R. bonasus began sexual maturation at a disc width of 850-900 mm and all were mature between 900-950 mm DW. Thus, female R. bonasus became sexually mature at a larger size than males (ca. 100 mm greater DW). Only the left ovary and uterus (dorsal perspective) were functional in R. bonasus. A non-functional right ovary and uterus have also been observed in D. centroura (Strushaker 1969), D^. say (Gudger 1912; Hamilton and Smith 1941; Hess 1959; pers. observations) and in Myliobatis bovina ( = Pteromylaeus bovina) and M. nicuholfi (=Aetomylaeus nichofi) (Giacomini 1896, as cited by Babel 1967). Alternately, a non-functional right ovary and two functional uteri have been found in U. halleri (Babel 1967), Gymnura altavela and G_. micura (Daiber and Booth 1960; pers. observations), and M. freminvillei (Bearden 1959). The presence of an empty shell capsule in the right uterus of female R. bonasus with small yolk sac embryos 107 indicated the shell gland of the right oviduct was functional. However, the left uterus of R. bonasus containing a full term embryo was extremely distended and occupied the entire, left posterior region of the body cavity. The large spiral valve occupied the same area on the opposite side of the body cavity. It is suggested that the valvular intestine on the right side of the body may restrict distention of the right uterus; thus, the non-functional nature of the right uterus. Embryonic Development Most citations in the available literature concerning the development of R. bonasus embryos are limited to singular occurrences of gravid females along the East Coast of the United States. Smith (1907) claimed that the young, usually numbering two or three, are born in the spring and summer. Gudger (1910) found two embryos (ca. 220 and 340 mm DW) in a single female taken on May 31 near Beaufort, North Carolina. Study material utilized by Bigelow and Schroeder (1953) included specimens measuring 238 to 250 mm ("embryo") and 365 mm wide ("newborn") collected near Woods Hole (no dates given). Joseph (1961) discovered a normally pigmented embryo about 330 mm wide in an albino female cownose ray captured in lower Chesapeake Bay on May 23. Hoese (1962) reported that "a single fetal young near birth" was taken from a female (no DW given) on June 19 near Wachapreague, Virginia. "Four premature young" were recorded from a 712 mm (disc width?) female in South Carolina on August 16 (Bearden 1965). A gravid female (ca. 900 mm DW) collected by Orth 108 (1975) on August 6 in the lower York River carried three small yolk sac embryos (32, 53, and 54 mm DW, pers. observations). The most comprehensive description of cownose ray embryology however was reported by Schwartz (1967) in a brief abstract as follows: "Young are born tail first by being discharged as the female swims or jumps. Remating occurs within ten days by a ventral to ventral union.... Early development once fertilization has occurred, is rapid and by 15 August embryos are about 51.0 mm wide and resembling miniature adults, possess a yolk sac and elongated exterior branchial filaments. Embryos by 10 October average 133.0 mm in disc width and often are devoid of yolk sac and umbilicus but retain an umbilical scar. Term individuals are born with wings folded on themselves and average 305.0 mm disc width." The results of this study show that the breeding cycle of R. bonasus is well-defined. Gravid females collected along the Virginia-North Carolina coastline in early May carried three-quarter term fetuses. During mid-May and early June schools of R. bonasus entered Chesapeake Bay and its tributaries. By late June and early July the young reached full term; parturition occurred at this time. Fertilization took place in mid-July and the gestation of a second brood of embryos began. Growth of this group was rapid. Gravid females departed Chesapeake Bay in late September and early October with relatively large embryos (Fig. 18). The initial observations above for which embryo size and collection date are given (Gudger 1910; Joseph 1961; Hoese 1962; Bearden 1965; Orth 1975) compare closely with the results of my study. My findings closely parallel Schwartz's (1965) earlier observations, 109 however, we are in considerable disagreement concerning size at parturition. The latter author reports that term individuals average 305.0 mm DW. Full term embryos which I collected were considerably larger (DW = 413 mm) while the smallest free-swimming cownose ray taken during this study measured 323 mm DW. Perhaps, the embryos which Schwartz considered full term were collected in early June and were not yet ready for parturition. Alternately, as Ford (1921) noted for Squalus acanthias, young of larger than average size are born by females which give birth late in the season; thus my full term specimens may have been collected relatively late in the breeding season. Rhinoptera javanica also appear to have a well-defined breeding cycle. James (1962, 1970) witnessed large schools of mature Javanese cownose rays in the Gulf of Mannar, India from December to February. These schools consisted of mature specimens, a majority of which were gravid females. Five young for which data were recorded, ranged in disc width from 181-310 mm. Clearly, Chesapeake Bay is a principal nursery area for R. bonasus. Springer (1967) pointed out that the young of some shark species are produced in specific nursery areas which are relatively free of larger sharks of any species; thus absence of potential predators increases the chance of survival for the young. Large carcharhinids, of which batoids are proported to be a favorite food item (Gudger 1907; Darnell 1958; Budker 1971), are abundant seaward of the Virginia Capes during the summer months (Lawler 1976). Yet only 110 Carcharh inus milberti and C. leucas frequent the Bay proper (Musick 1972). The former utilizes bayside eastern shore as a nursery area (Lawler 1976); however, gravid females abstain from feeding while on nursery grounds and adult males fail to enter these areas (Springer 1960). Carcharhinus leucas, listed as occasional to common in the upper and lower Chesapeake Bay (Musick 1972) and large bluefish, Pomatomus saltatrix may represent the only major predators of young R. bonasus in the lower Bay. Although shell capsules containing as many as three and four ova were observed on one occasion, I found only one developing embryo per gravid female. Several reports of multiple young exist for R. bonasus (Smith 1907; Gudger 1910; Bearden 1965; personal observation of Orth's collection 1975). On the other hand, Setna and Sarangdhar (1949) found only one embryo per gravid female in two specimens of R. j avanica. James (1962, 1970) examined several commercial catches of the same species and reported that gravid females carried one embryo. The full term young of R. bonasus which I collected were relatively large when compared to the parent. As with other embryonic myliobatoids, spatial restrictions are surmounted by rolling the pectoral fins dorsally (or ventrally) along the anterioposterior axis (Gudger 1951). However, judging from the distension of the left uterus in the body cavity prior to parturition, there would appear to be little opportunity for two such young to occupy the same uterus simultaneously. Therefore, I suggest that multiple fertilizations may Ill occur but R. bonasus generally carry only one young to full term. Two full term fetuses in a single female may be possible but only with a reduction in the size of the end product. Possibly the smallest free-swimming young collected during this study (323-395 mm DW, n = 19) were the offspring of paired litters. Parturition Several authors have reported that parturition in myliobatoids occurs as the female jumps or leaps from the surface (Coles 1910; Gudger 1951; Schwartz 1967). I did not observe natural parturition during this investigation. Several local fishermen claim to have witnessed leaping rays, however, none could recall seeing embryos ejected from the ventral surface nor release upon impact. I induced the birth of two full term embryos (from separate females) in a manner similar to that which Gudger (1910) described as the "spawn taker" method. The pregnant female was placed dorsal surface down. Pressure was applied to the left side of the abdomen with posteriorly moving strokes. The young were born posterior end first with the tail bent anteriad. The pectoral fins were folded dorsally upon themselves. Gudger (1910) and Schwartz (1967) have also noted this position for R. bonasus. The ease with which the full term young pass through the cloacal oriface when the parent is manipulated in the above manner suggests that it is not necessary for R. bonasus to leap above the surface in order to liberate their young. Schwartz (1967) also reported that young—of-the-year rays exhibit "a dorsal back-riding unsuccessful copulatory behavior in late August or early September." I did not observe this behavior during my study. 112 Copulation Wourms (1977) stated that "although definitive evidence is lacking, rays seem to copulate as do the small skates," that is, by apposition of the ventral surfaces. Indeed, Schwartz (1967) cited copulation in R. bonasus via a ventral-to-ventral union. Conversely, Brockmann (1975) claims to have witenssed a mating pair of dasyatids (probably D. americana). The male was positioned on the back of the female and overlapped her for about a third of her length. As the pair rolled to the side, the pelvic region of the male appeared curved forward and covered that of his partner. Several commercial fishermen interviewed during this study claimed to have witnessed cownose rays "mating." Their descriptions, although anecdotal, invariably consisted of a pair or several rays creating a large disturbance near the surface. Occasionally, the behavior was described as "the male chasing the female." I witnessed what appeared to be this chase behavior on one occasion at Claybank in July, 1978. A large cownose ray broke the surface and with rapid flappings of its pectoral fins, swam rapidly in a straight line for about 10—15 m, then sank out of sight. I could not discern if a mate was present below the ray on the surface. This behavior is similar to that described by Hamilton and Smith (1941) along eastern shore Virginia. Although these authors could not make positive identification, they implied that D_. say were respons ible. Embryonic Nutrition Rhinoptera bonasus, as with all other myliobatoids (Bigelow and Schroeder 1953; Breder and Rosen 1966), exhibits aplacental 113 viviparity. Fertilization is internal. Embryos develop inside the uterus and are born as free-swimming individuals; there is no physical connection however, between the embryo and the mother. The uterine wall is lined with numerous villiform projections called trophonemata (Wood-Mason and Alcock 1891) which function as placental analogues (Wourms 1977). These structures secrete histotrophe, a viscid liquid, rich in organic content which serves as the principal nourishment for developing embryos (Bigelow and Schroeder 1953). In summarizing pervious chondrichthian, fetal-maternal relationships, Wourms (1977) noted that the efficiency of placental analogues far surpasses that of the yolk sac placenta exhibited by some carcharhinids. Wood-Mason and Alcock (1891) were first to suggest the nutritive value of histotrophe. Gudger (1912) expressed the belief that early stage myliobatoid embryos absorbed histotrophe through their external branchial filaments; he proposed that in the later stages of gestation when these filaments disappeared, the spiracles then conveyed histotrophe directly to the gut. Ranzi (1934a, as cited by Babel 1967) demonstrated that in early stage embryos of Trygon (=Dasyatis) violacea histotrophe was absorbed through the yolk sac and external branchial filaments. Using ink injections he also showed that the later stage embryos ingested histotrophe through the mouth and spiracles. Yolk material appears to provide the initial nutritional requirements for R. bonasus embryos. This is probably augmented somewhat by the absorption of histotrophe via the external branchial^ 114 filaments and yolk sac as was shown in D. violacea (Ranzi, cited above). Babel (1967) suggested that this was also likely for U. halleri. External branchial filaments were absent in R. bonasus embryos larger than about 90 mm DW. By late September and early October the yolk sac volume in R. bonasus was diminished to 1-2 ml (Fig. 21). Thus, histotrophe appears to supply nourishment for the remainder of gestation. Intrusion of the trophonemata into the gill slits of three-quarter and full term young was observed, however, infiltration of the trophonemata into the mouth and spiracles was not witnessed. Gestation Period Difficulty arises when attempting to define reproductive cycles of large elasmobranchs due to their schooling and highly migratory habits; hence certain stages of pregnancy are inaccessable (Holden 1974). Any definative conclusions concerning the length of gestation were precluded because I was unable to sample R. bonasus from mid-October till the following May and the dearth of available literature for the species during this period. Two hypotheses are described. 11-12 Month Gestation Period - Schwartz (1965) maintains that the population of R. bonasus which inhabits Chesapeake Bay during the summer months overwinters in northern South America. To be valid, the rapid embryonic growth observed between May and October in the northern latitudes must be retarded during the winter months. Retarded embryonic development would be due to the high 115 energy demand during such an extensive migration. Thus, the three-quarter and full term embryos collected from May to July may actually be the products of what I have termed the second brood embryos from the latter half of the previous summer - a gestation period of 11-12 months. A somewhat analogous situation may exist in temperate passerine birds. Klopfer et al. (1974) suggested that a relatively uniform pattern of abundance prevails in the tropics and "the resources available in any one day are insufficient to maintain the more rapid developmental rates of birds adapted to the seasonally richer temperate zones." Thus, these birds migrate out of the tropics annually in order to breed successfully. Clearly, a short residence time and the lack of a pronounced peak of resources in the tropics, weigh heavily against parturition by R. bonasus occurring in northern South America during the winter months. r-6 Month Gestation Period - Contrary to Holden's (1974) suggestion that gestation in most ovoviviparous (=aplacental viviparous) rays should last one year, several myliobatoids exhibit rapid embryonic development. Ranzi (1934b, as cited by Babel 1967) indicated a gestation period of 4 months for P. bovina and 2 months for D. violacea. Wilson and Beckett (1970) concurred with the latter finding. Several investigators have noted rapid growth of D. sabina embryos (Breder and Krumholz 1941; Murray and Christmas 1968). Dasyat is s ay requires about 5-6 months for development (Hess 1959 and pers. observations), while Daiber and Booth (I960) suggested a similar 116 gestation period for G. altavela and only "a few months" for G. micrura. Embryos of U. halleri attain full term in 3 months (Babel 1967 ). On the other hand, the large roughtail stingray, D_. centroura, has a gestation period of 9-11 months (Strushaker 1969). The presence of well-developed embryos in G. altavela collected in May from Delaware Bay and in February off the coast of North Carolina (27 fa.) led Daiber and Booth (1960) to propose two 5-6 month gestation periods per year for this species. Both Gudger (1912) and Hess (1959) observed rapid development of D. say embryos during the summer months and suggested that parturition may also occur on its wintering grounds. In light of the above observations and the fact that gravid R. bonasus vacate Chesapeake Bay in the fall with relatively large young, this species may have two 5-6 month gestation periods per year. Assuming gravid R. bonasus continue feeding during their autumnal migration through the South Atlantic Bight (see distribution discussion), the rapid embryonic growth observed in Chesapeake Bay during the summer may continue throughout the fall. Parturition might then occur on the wintering ground of R. bonasus, wherever this might be. The gestation of another brood of young may follow this hypothesized parturition. Copulation need not occur since sperm storage has been observed in other elasmobranchs (Richards et al. 1963; Babel 1967). 117 Until R. bonasus is collected in its wintering habitat and observations are made on the reproductive condition of mature females, the length of gestation in this ray remains speculative. Age and Growth Numerous authors have assumed annual ring formation on elasmobranch vertebrae (ishiyama 1951; Taylor and Holden 1964; Stevens 1975; Du Buit 1977). However, only Holden and Vince (1973), working with Raja clavata, properly validated the rings as annuli. They found that one hyaline and one opaque zone are deposited annually. Subsequently, Holden (1974) analyzed data of others (ishiyama 1951; Daiber 1960; Richards et al. 1963) and concluded that the rings on several other species of raj ids were annual. Causal factors such as changes in diet and/or temperature (Stevens 1975; Jones and Geen 1977) have been suggested to explain the annual nature of vertebral rings. Hyaline zones on the vertebrae of R_. bonasus were considered valid annuli because, 1) free-swimming young-of-the-year rays had a wide opaque zone on the outer margin of the vertebrae and returning yearlings, collected during the following spring, had a narrow hyaline band on or near the periphery of the vertebral centrum, and 2) weighted back-calculated mean disc width at ages were consistent with observed size at capture for corresponding age groups among the younger age classes. Patterns of modes in frequency distributions of disc widths for summer-caught cownose rays were difficult to detect. However, two modes were apparent on the disc width frequency distributions (sexes 118 combined) of small rays (450-650 mm DW) taken along the coast in early May (Fig. 28). The presence of a hyaline zone on the vertebrae of two three-quarter term embryos supports the hypothesis for an 11-12 month gestation period in _R. bonasus (see below). Thus, the first mode (508 mm DW) probably represented returning yearlings and the second mode (586 mm DW) age II rays. These modal values for one and two year old rays were consistent with back-calculated values for the same year classes (Tables 11 and 12). Weighted back-calculated means for ages I and II were slightly larger than the modal values of the DW frequency distribution and back-calculated disc widths within age classes tend to increase with older rays (reverse Lea's phenomena). This suggests a possible selection for the larger members of older age groups, although I cannot account for this bias in my age analysis. Nevertheless, weighted back-calculated means also showed a growth increment of 87 mm between ages I and II which is consistent with the growth increment for these ages derived from the springtime size frequency distribution. Comparison of weighted mean back-calculated disc widths between male and female R. bonasus showed little difference between ages I to V (Tables 11 and 12). Both sexes through age V show growth increments decreasing from 90 to 50 mm in width per year. Application of reproductive observations to growth curves (Fig. 24) indicated growth slowed in both sexes upon attainment of sexual maturity. Male and female II. bonasus matured at 800-840 mm and 900-950 mm DW, respectively. Corresponding ages at maturity were 5-6 years for males and 7-8 years for females. Thus, females matured later than males and 119 Figure 28. Disc width frequency of small Rhinoptera bonasus (< 630 mm DW, sexes combined) collected along the Virginia-North Carolina coast in early May 1976, 1977 and 1978. 2 0 -i N 469 489 509 529 549 569 589 609 629 DISC WIDTH (mm) 120 attained a relatively larger size. The oldest specimen aged was a female (not included in the age and growth analysis) with 13 or 14 vertebral hyaline rings and was also the largest specimen collected during the study. Reports in the literature of 2.1 m (7 ft) wide R. bonasus in Florida waters (Smith 1907) and 45 kg (100 lb) cownose rays from New York (Mitchill 1815) are highly suspect. Over 900 R. bonasus were examined during this study; maximum sizes recorded were 981 mm DW (16.16 kg) and 1070 mm DW (22.79 kg) for males and females, respectively. Walford curves predicted asymptotic disc widths of 996 mm for males and 1141 mm for females. These values were consistent with maximum observed sizes, but may be slight underestimates due to the small sample size of adult rays aged. Generally, elasmobranchs mature after reaching about 60% or more of their maximum size (Babel 1967; Holden 1974). Assuming mean disc widths at 50% maturity (DWm ) of 820 mm for males and 925 mm for females, the ratio D W ^ D W ^ for each sex was 0.82 and 0.81, respectively. In other words, both sexes of R. bonasus mature after attaining about 80% of their maximum size. Parturition in R. bonasus occurred when young attained a disc width of 400-440 mm. Von Bertalanffy predictions of size at birth « 7 = 420 mm DW; ^ = 440 mm DW, Fig. 24) agreed well with observed values; thus, the growth equations are considered accurate for the early years of growth. 121 The appearance of a hyaline zone on the vertebrae of two three-quarter term embryos indicated that an initial hyaline zone was deposited in utero. This zone was also detected close to the center of the centrum on a majority of vertebrae examined from free-swimming rays. Back-calculations suggested that size at deposition is 290-300 mm DW. Parturition occurred in June and July when embryos were 400-440 DW. Gravid females left the Bay in October with another brood of relatively large embryos. Pregnant females returned the following spring with three-quarter term young. Cownose rays were unavailable to me November through April, however Schwartz (1965) claims they undergo an extensive migration to northern South America during this period. Presumably, embryonic growth may be retarded during the winter months due to the energy expenditure of the parent during migration; thus, the embryonic hyaline zone may be deposited in response to a decrease in the supply of nutrients to the embryo. Rapid embryonic growth and a consequent deposition of opaque material on the vertebrae resumes in May when the parent re-enters the productive estuaries of the East Coast of the United States. Holden (1972b, 1974) pre sented available growth rate data for elasmobranchs and suggested that K values tend to be of similar order, that is, 0.1 to 0.2 for sharks and 0.2 to 0.3 for batoids. Growth coefficients for II. bonasus (K = 0.215 for males and 0.149 for females) derived in this study agreed favorably with Holden's predicted values. MANAGEMENT OF THE COWNOSE RAY IN CHESAPEAKE BAY In recent years (1972-1977) several Rappahannock River oyster growers reported substantial losses to seed and harvestable beds due to cownose ray predation. In spring 1975, eight major Virginia oyster growers solicited aid in the form of control measures to reduce ray predation. VIMS Advisory Service contacts indicated that the problem was a recurrent one in many areas and the ray population appeared to be increasing in the past decade. Concurrently, feeding cownose rays were observed to have a detrimental impact on eelgrass (Zostera maripa) beds (Orth 1975). The destruction of eelgrass habitat by rays was often considerable, resulting in reduced biological productivity of shoal areas, reduced sediment stability and localized erosion (Orth 1975, 1976). The elimination or reduction of certain predators from an area may be a desirable management practice when their numbers or predations have a negative impact on more desirable species (Rounsefell and Everhart 1953; Alverson and Stansby 1963). Reducing cownose ray predation on Chesapeake Bay stocks of commercially important shellfish may be accomplished by: (1) physical or mechanical barriers placed on or about shellfish beds to exclude rays, or (2) reduction of the cownose ray stock. 122 123 Generally, the shellfish industry's solution to ray predation has come in the form of mechanical barriers. Fences have been used in the Philippines (Villadolid and Villaluz 1938), California (Barrett 1963), and Eastern Shore, Virginia (Kraeuter and Castagna 1977, in prep.; D. Haven, pers. comm.). French shellfish growers implant arrays of pointed stakes on the oyster bottom (Korringa 1976). California oysters are planted in shallow, intertidal waters where defense against the bat ray, Myliobatis californica, is possible (Barrett 1963). Fences composed of large mesh netting material represent the best short-term method of protecting commercial oyster bottom or other planted bottom from cownose ray predation in Chesapeake Bay (Merriner and Smith 1979a). The use of fences at present is limited to intertidal or shallow subtidal beds. Many of the Bay's oyster beds cover several thousand acres and are located in up to 7.6 m (25 ft) of water (Haven et al. 1978). Obviously, the widespread application of any mechanical device on the Bay's oyster bottom would be impractical and expensive. Mechanical deterrents such as barbed wire and arrays of pointed stakes on the bottom would also hamper present harvesting practices and may serve to increase the siltation rate, thus smothering the oysters (D. Haven, pers. comm.). On several occasions, creation of a fishery for undesirable elasmobranchs has been suggested. During the early 1960's, extensive gear destruction and depredation on more commercially valuable species were attributed to increasing populations of spiny dogfish, Squalus 124 acanthias, along the Pacific Northwest and the northeast coasts of the U.S. (Alverson and Stansby 1963; Commonwealth of Massachusetts 1964). In each case, development of a commercial fishery for dogfish was recommended as the most practical solution. Likewise, shark attacks on commercial mackerel fishing operations in Florida stimulated interest in commercial shark fishing as a control and possible new fishery (Beaumariage 1968). Walford (1935) reported that the West Coast bat ray was the target of special exterminating parties of sportfishermen. Presumably, reduction of cownose ray numbers would decrease predation on commercially important shellfish. Therefore, development of a fishery for II. bonasus seems highly desirable. The pectoral fins or "wings" are the marketable portions of skates and rays. In the absence of a high domestic market demand for these items, there have been no directed fisheries for batoids in the U.S. Recently, Otwell and Lanier (1978) completed a study of the utilization of skates and rays in North Carolina. The clearnose skate (Raja eglanteria) and the cownose ray were the target species of the project. They reported that present "market trends in Europe are conducive for increased importation of skate and ray" and concluded that "foreign market trends, product characteristics of domestic skate, and fishermen/processors interests indicate potential for development of a skate and ray fishery in North Carolina." The report recommended that "a proper, cautious promotion directed toward researched markets should find market potential for the cownose ray." 125 The feasibility of harvesting schools of cownose rays with existing commercial gear (haul seines) has been demonstrated (Otwell and Lanier 1978; Merriner and Smith 1979b). Haul seine fishermen have also indicated a willingness to fish for rays if the price is competitive with that of other market fish in the area (croaker, spot, weakfish, and bluefish). Thus, development of a fishery for cownose rays appears to be the most practical and promising method for a long term reduction of cownose ray predation on commercially important shellfish beds. The recommendation to develop a fishery for R. bonasus in the Chesapeake Bay must be tempered with a word of caution. Elasmobranch populations are particularly susceptible to the effects of an intense fishery. Generally, elasmobranchs have a slow growth rate and low fecundity, hence recruitment cannot keep pace with a high rate of exploitation (Holden 1974). Historically, initial exploitation is followed by a rapid decline in catch rate or total collapse of the fishery (Holden 1974). Outstanding examples include fisheries for the soupfin shark, Galeorhinus zyopterus, along the West Coast of the U.S. (Ripley 1946), the Austr alian school shark, Galeorhinus australis (Olsen 1954), the Pacific Northewst stocks of spiny dogfish, Squalus acanthias (Alverson and Stansby 1963), the basking shark, Cetorhinus maximus (Parker and Stott 1965) and the Scottish-Norwegian stocks of S_. acanthias (Holden 1968). Rhinoptera bonasus is a classic example of an elasmobranch stock. Both sexes mature after attaining about 80% of their maximum size, 126 while adult females give birth to one, possibly two, relatively large young per year. It is a slow grower (K = 0.215 for males and 0.149 for females) and has a moderately long life span. Holden (1972b) discussed the possibilities of sustainable fisheries for sharks, skates and rays and concluded that the relationship between stock and recruitment in the unexploited phase is nearly linear for all elasmobranchs. Earlier, in assessing the effects of fishing on J3. acanthias, Holden (1968) showed that the female segment of the dogfish stock must be given considerable protection if recruitment is not to be affected. Rhinoptera bonasus possess little, if any, capacity to increase fecundity in the face of exploitation. Clearly, if a sustained fishery for the cownose ray is desired, quotas will have be be set to prevent overfishing. This will necessitate an accurate estimate of the size of the cownose ray population. Pending knowledge of mortality and population size, it is suggested that a directed fishery for cownose rays in Chesapeake Bay should begin only after July 15. This would allow for parturition in mid-June and early July, thus insuring at least partial recruitment. A ray fishery in the Rappahannock River should be unrestricted since this river harbors some of Virginia's most valuable oyster grounds. APPENDIX TABLE 1 COWNOSE RAY COLLECTION DATA - 1976, 1977 and 1978 Total CR76- 1 6 -V SB 1 2 6 2 1 3 2 2-V LI 1 1 3 26-V YK 1 1 4 26-V LI 4 1 2 1 5 17-VI YK 4 2 2 6 21-VI YK 2 2 7 22-VI YK 18 15 3 8 23-VI YK 7 4 3 9 24-VI YK 14 1 2 2 1 0 25-VI YK 13 2 2 8 1 1 1 29-VI YK 16 15 1 1 2 1-VII YK 11 1 8 2 13 6 -VII YK 1 1 14 7-VII LI 11 1 2 8 15 8 -VII YK 1 1 1 0 1 16 9-VII YK 7 2 5 17 14-VII YK 2 1 1 18 15-VII YK 1 1 19 16-VII YK 5 3 2 2 0 19-VII YK 2 0 7 9 2 2 2 1 20-VII ES 1 1 2 2 21-VII ES 13 6 7 23 22-VII ES 6 4 1 1 24a 23-VII YK 15 3 1 4 7 25 24-VII ES 1 1 26 26-VII LI 4 1 2 1 27 26-VII YK 1 2 6 6 28b 28-VII PQ 15 15 29 28-VII YK 3 3 30 29-VII ES 4 4 31 30-VII PO 3 1 2 32 3-VIII YK 5 1 4 33 4-VIII YK 1 2 1 2 34 6 -VIII YK 11 3 5 1 2 35a 13-VIII PO 16 6 5 1 4 36c 16-VIII PO 90 47 41 2 37 16-VIII WR 2 2 127 128 Appendix Table 1 (continued) Total 9 9- Collection Number of < 800 < 800 >_ 800 >_ 80( Number Date Locale Specimens mm DW mm DW mm DW mm DI CR76-38 17-VIII ES 15 8 3 3 1 39 25-VIII YK 8 8 40a 26-VIII PO 34 18 13 1 2 41 29-VIII YK 1 1 42 8 -IX YK 7 7 43 10-IX LI 18 1 0 8 44 a 13-IX PO 73 38 35 46 23-IX LI 11 7 4 47 1-IX ES 2 1 1 48 7-IX LI 9 4 5 49 3-IX CH 1 1 50d PO 4 2 2 CR77- 1 2-V CO 2 1 1 2 3-V CO 1 1 3 4-V CO 2 2 4 5-V CO 8 6 49 35 2 5 5-V CO 3 1 2 7 11-V YK 2 0 6 4 8 2 8 12-V YK 15 1 6 8 9 12-V LI 3 1 2 1 0 18-V YK 19 3 4 5 7 11 14-VI YK 1 1 1 2 27-VI PQ 4 3 1 13 15-VII LI 25 , 7 14 3 1 14 4-VIII YK ll' 11 16 11-VIII YK 1 1 17 25-VIII YK 1 1 18 29-VIII YK 1 1 19 31-VIII YK 15 1 14 2 0 21- IX YK 1 1 2 1 23-IX LI 2 0 5 15 2 2 5-X ES 1 1 23 12-X LI 3 1 2 CR78- 1 16-V CO 1 1 2 16-V CO 2 2 3 17-V CO 4 3 1 129 Appendix Table 1 (concluded) Total a C? $ Collection Number of < 800 < 800 >_ 800 >_ 800 Number Date Locale Spec imens mm DW mm DW mm DW mm DW CR78- 4 17-V CO 8 6 2 5 17-V CO 26 16 1 0 6 10-V YK 1 1 7 21-VII YK 2 2 1 2 19 8 28-VII PO 9 9 9 7-IX YK 19 1 3 15 SB = Sandbridge, Va. PO Potomac River LI = Lynnhaven Inlet WR Ware River YK = York River CH Cape Hatteras ES = Eastern Shore CO Corolla, N.C. PQ = Poquoson River a = one week’s catch b = subsample c = two day's catch d = caught last week of October, 1976 Note: 15 additional adult rays were collected during 1976 and 1977 and were used in a penning experiment. No DW's or weights were recorded. 130 Name and Address Area fished & depth of water______Number and kind(s) of nets fished______Date Cownose Rays first appear in your area______Dates when they are most plentiful______Latest date Cownose Rays are in your area______Estimated number and total weight of cownose rays which you catch in each of your nets during the peak period each day: ■ Number ______Total Weight______In your area, have they been increasing or decreasing in abundance over the past ten years? ______What is the type of bottom in the area you fish______Are the cownose rays feeding and digging up the bottom in your area?______Do you ever catch very small cownose r a y s ? ______ THANK YOU !’ Appendix Figure 1. Commercial fishrmen's questionnaire. LITERATURE CITED Aasen, 0. 1963. Length and growth of the porbeagle (Lamna nasus, Bonnaterre) in the North West Atlantic. Fisk. Dir. Skr . , Ser. Fiskeri 13(6):20-37. Alverson, D. L. and M. E. Stansby. 1963. The spiny dogfish (Squalus acanthias) in the northeastern Pacific. U.S. Fish and Wildl. Serv., Spec. Sci. Rep., Fish. No. 447, 25 p. Aquirre, J. L. C. 1965. Peces sierra, rayas, mantas y especies afines de Mexico. Anal, del Inst. Nacional de Invest. Biol.-Pesqueras. 1:246-249. Babel, J. S. 1967. Reproduction, life history, and ecology of the round stingray, Urolophus halleri Cooper. Calif. Fish and Game, Fish. Bull. 137:1-104. Backus, R. H., S. Springer and E. L. Arnold. 1956. A contribution to the natural history of the white-tip shark, Pterolamiops longimanus (Poey). Deep-Sea Res. 3:178-188. Bagenal, T. B. and F. W. Tesch. 1978. Age and growth. pp. 101-136. In: T. Bagenal (ed.), Methods for assessment of fish production in fresh waters. Blackwell Scientific Publ., Oxford. 131 132 Barans, C. A. and V. G. Burrell. 1976. Preliminary findings of trawling on the Continental Shelf off the sourtheastern United States during four seasons (1973-1975). S.C. Mar. Resour. Ctr . , Tech. Rep. No. 13, 16 p. Barrett, E. M. 1963. The California oyster industry. Calif. Dep. Fish and Game, Fish Bull. 123:18-19. Bayliff, W. H. 1951. A record of the cownose ray in Chesapeake Bay. Copeia 1951:100. Bearden, C. M. 1959. A life history study of the eagle ray, Myliobatis freminvillii Le Sueur 1824, in Delaware Bay. M.S. Thesis, University of Delaware, Newark, 37 p. _ . 1965. Elasmobranch fishes of South Carolina. Contrib. Bears Bluff Labs. No. 42:17. Beaumariage, D. S. 1968. Commercial shark fishing and processing in Florida. Fla. Bd. Conserv. Mar. Res. Lab., Educ. Ser. No. 16, 2 1 p . Bigelow, H. B. and W. C. Schroeder. 1948. Fishes of the Western North Atlantic. Part I, Lancets, cyclostomes and sharks. Mem. Sears. Fdn. Mar. Res. 1-576. ____ . 1953. Fishes of the Western North Atlantic. Part II, Sawfishes, guitarfishes, skates, rays and chimaeroids. Mem. Sears Fdn. Mar. Res. 1-588. Blacker, R. W. 1974. Recent advances in otolith studies. pp. 67-90 In: F. R. Harden Jones (ed.), Sea Fisheries Research. Wiley and Sons, N.Y. Boesch, D. F. 1971. Distribution and structure of benthic communities in the Hampton Roads area, Virginia. VIMS SRAMSOE 15, 112 p. Breder, C. M. 1965. Vortices and fish schools. Zoologica 50:97-114 . and L. A. Krumholtz. 1941. On the uterine young of Dasyatis sabinus (Le Sueur) and Dasyatis hastatus (De Kay). Zoologica 26:49-51. ____. and D. E. Rosen. 1966. Modes of reproduction in fishes, T.F.H. Publications, 914 p. Brockmann, F. W. 1975. An observation on mating behavior of the southern stingray, Dasyatis americana. Copeia 1975:784-785. Budker, P. 1971. The life of sharks. Columbia Univ. Press, New York, 222 p. Bullis, H. R. 1967. Depth segregation and distribution of sex-maturity groups in the marbled catshark, Galeus arae. pp. 141-148. In: P. W. Gilbert, R. F. Mathewson and D. P. Rail (eds.), Sharks, skates and rays. The Johns Hopkins Press, Baltimore, Md. 134 Capape, C. 1977. Etude du regime alimentaire de la Mourine vachette, Pteromylaeus bovinus (Geoffroy Saint-Hilaire, 1817) (Pisces, Myliobatidae) des cotes tunisiennes. J. Cons. Int. Explor. Mer. 37:214-220. Cervigon, F. 1966. Los peces marinos de Venezuela, Monografia No. 11, Tomo I. Estacion de Invest. Mar. de Margarita, Fund. La Salle de Cien. Nat., Caracas, 448 p. Clark, E. 1963. Massive aggregations of large rays and sharks in and near Sarasota, Florida. Zoologica 48:61-64. Coles, R. J. 1910.Observations on the habits and distribution of certain fishes taken on the coast of North Carolina. Bull. Am. Mus. Nat. Hist. 28:337-348. ______. 1915. Notes on the sharks and rays of Cape Lookout, N.C. Proc. Biol. Soc. Wash. 28:94. Commonwealth of Massachusetts. 1964. Special report of the Department of National Resources relative to the control and eradication of dogfish. House No. 3703:104-117. Cummins, R. 1971. Calico scallops of the southeastern United States 1959-1969. U.S. Dep. Commer., Natl. Mar. Fish. Serv., Spec. Sci. Rep. Fish. No. 627, 22 p. Dahlberg, M. D. and R. W. Heard. 1969. Observations on elasmobranchs from Georgia. Fla. Acad. Sci., Gainesville. Quart. J. 32:21-25. 135 Daiber, F. C. 1960. A technique for age determination in the skate, Raja eglanteria. Copeia 1960:258-260. ______and R. A. Booth. 1960. Notes on the biology of the butterfly rays, Gymnura altavela and Gymnura micrura. Copeia 1960:137-139. Darnell, R. M. 1958. Food habits of fishes and larger invertebrates of Lake Ponchartrain, Louisiana, an estuarine community. Publ. Inst. Mar. Sci. Univ. Tex. 5:353-416. Drummond, S. B. 1969. Explorations for calico scallop, Pecten gibbus, in the area off Cape Kennedy, Florida, 1960-1966. U.S. Fish Wildl. Serv., Fish. Ind. Res. 5:85-101. Du Buit, M. H. 1977. Age et croissance de Raja batis et de Raj a naevus en Mer Celtique. J. Cons. Int. Explor. M e r . 37:261-265. Everhart, W. H., A. W. Eipper and W. D. Youngs. 1975. Principles of fishery science. Cornell University Press, Ithaca, New York, 228 p . Ford, E. 1921. A contribution to our knowledge of the life-histories of the dogfishes landed at Plymouth. J. Mar. Biol. Assoc. U.K., N.S. 12(3) :468-505. Giacomini, E. 1896. Contributo all "istologia dell" ovario dei Selaci con speciale rigardo sopra ad alcune particolarita di struttura riscontrate nell 1 ovario di Myliobatis bovina. Geoffr. Ric. Labor. Anat. Roma. 5:221-274 (Cited by Babel 1967). 136 Gilbert, P. W. and G. W. Heath. 1972. The clasper-siphon sac mechanism in Squalus acanthias and Mustelus canis. Comp. Biochem. Physiol. 42A:97-119. Gines, H. and F. Cervigon. 1968. Exploracion pesquera en las costas de Guayana y Surinam. Estacion de Investigaciones Marinas de Margarita, Fundacion La Salle de Ciencias Naturales, Contrib. No. 29:5-96. Godfriaux, B. L. 1970. Food of predatory fish in Hauraki Gulf. 2:Five fish species associated with snapper. N.Z. J. Mar. Freshwat. Res. 4:248-266. Gosner, K. L. 1971. Guide to identification of marine and estuarine invertebrates, Cape Hatteras to the Bay of Fundy. Wiley-Interscience, N.Y., 693 p. Gregory, W. K. 1935. Winged Sharks. Bull. N.Y. Zool. Soc. 38: 129-133. Gudger, E. W. 1907. A note on the hammerhead shark (Sphyrna zygaena) and its food. Science 25:1005. ______. 1910. Notes on some Beaufort fishes - 1909. Araer. Naturalist 44:397-398. ______. 1912. Natural history notes on some Beaufort, N.C. fishes, 1910-1911. I. Elasmobranchii, with special reference to utero-gestat ion. Proc. Biol. Soc. Wash. 25:143-144. 137 1951. How difficult parturition in certain viviparous sharks and rays is overcome. J. Elisha Mitch. Sci. Soc. 67:56-86. Hamilton, W. J. and R. A. Smith. 1941. Notes on the sting-ray, Dasyatis say (Le Sueur). Copeia 1941:175. Haskell, W. L. 1949. An investigation of the possiblity of determining the age of sharks through annuli as shown in cross sections of vertebrae. Ann. Rep. Ma. Lab., Tex. Game Fish Oyster Comm., Fiscal Year 1948-1949:212-217. Haven, D. S. 1970. A study of the hard and soft clam resources of Virgina. VIMS Grant Rep., 69 p. ______W. J. Hargis, J. G. Loesch and J. P. Whitcomb. 1976. The effects of tropical storm Agnes on oysters, hard clams, soft clams and oyster drills in Virginia. pp. 488-508. In: (The Chesapeake Research Consortium), The effects of tropical storm Agnes on the Chesapeake Bay estuarine system. The Johns Hopkins University Press, Baltimore, M d . ______, W. J. Hargis and P. C. Kendall. 1978. The oyster industry of Virginia: Its status, problems and promise. VIMS Spec. Rep. N o . 168, 149 p. Herald, E. S. 1953. The 1952 shark derbies at Elkhorn Slough, Monterey Bay, and at Coyote Point, San Francisco Bay. Calif. Fish and Game 39:237-243. Hess, P. W. 1959. The biology of two sting rays, Dasyatis centroura Mitchill 1815 and Dasyatis sayi Le Sueur 1817, in Delaware Bay. M.S. Thesis, University of Delaware, Newark, 42 p. ______and L. W. Miller. 1963. South by south-west, a migration o cownose rays. Univ. Del. Estuarine Bull. 7(2&3):3-5. Hickling, C. F. 1930. A contribution towards the life history of th spurdog. J. Mar. Biol. Assoc. U.K. 16:529-576. Hildebrand, S. F. and W. C. Schroeder. 1928. Fishes of Chesapeake Bay. U.S. Bur. Fish. 43:70-71. Hoar, W. S. 1969. Reproduction. pp. 1-72. In: W. S. Hoar and D. J Randall (eds.), Fish Physiology, Vol. 3, Academic Press, New York. Hobson, E. S. 1963. Feeding behavior in three species of sharks. Pac. Sci. 17:171-194. Hoese, H. D. 1962. Sharks and rays of Virginia's seaside bays. Chesapeake Sci. 3:166-172. 1977. Fishes of the Gulf of Mexico, Texas, Louisiana and adjacent waters. Tex. A&M Press, College Station, Texas, 327 p. Holden, M. J. 1968. The rational exploitation of the Scottish-Norwegian stock of spurdogs (Squalus acanthias L.). Fishery Invest., Lond., Ser. 2, 25 (8):28 p. 139 1972a.The growth rate of Raja brachyura, R. clavata and R. montagui as determined from tagging data. J. Cons. Int. Explor. Mer. 34:161-168. ______. 1972b. Are long-term sustainable fisheries for elasmobranchs possible? pp. 360-367. In: B. B. Parrish (ed.), Fish stocks and recruitment, Rapp., Proc. Verb. Reun., Cons. Inter. Explor. Mer. 164. . 1974. Problems in the rational exploitation of elasmobranch populations and some suggested solutions. pp. 117-137. In F. R. Harden Jones (ed.), Sea Fisheries Research, Wiley and Sons, N.Y. _____ and P. S. Meadows. 1962. The structure of the spine of the spur dogfish (Squalus acanthas L.) and its use for age determination. J. Mar. Biol. Assoc. U.K. 42:179-197. ______and M. R. Vince. 1973. Age validation studies on the centra Raja clavata using tetracycline. J. Cons. Int. Explor. Mer. 35:13-17. Howard, J. D., T. V. Mayou and R. W. Heard. 1977 . Biogenic sedimentary structures formed by rays. J. Sed. Petrology 47:339-346. Ishiyama, R. 1951. Studies on the rays and skates belonging to the family Rajidae, found in Japan and adjacent regions. 2. On the age-determination of Japanese black-skate Raja fusca Garman (Prelimnary Report). Bull. Japan. Soc. Sci. Fish. 16:112-118. James, P. S. B. R. 1962. Observations on shoals of the Javanese cownose ray, Rhinoptera javanica Muller and Henle, from the Gulf of Mannar, with additional notes on the species. J. Mar. Biol. Assoc. India, 4:217-223. ______. 1970. Further observations on shoals of the javenese cownose ray, Rhinoptera javanica Muller and Henle, from the Gulf of Mannar with a note on the teeth structure in the species. J. Mar. Biol. Assoc. India 12(1&2):151-157. Joseph, E. B. 1961. An albino cownose ray, Rhinoptera bonasus (Mitchill) from Chesapeake Bay. Copiea 1961:482-483. Jones, B. C. and G. H. Geen. 1977. Age determination of an elasmobranch (Squalus acanthias) by X-ray spectrometry. J. Fish Res. Bd. Canada 34:44-48. Kalmijn, A. J. 1966. Electro-perception in sharks and rays. Nature Lond. 212:1232-1233. ___. 1971. The electric sense of sharks and rays. J. Exp. Biol. 55:371-383. ______. 1972. Bioelectric fields in sea water and the function of the ampullae of Lorenzini in elasmobranch fishes. Scripps Inst. Oceanog., Ref. Ser. 72-83, 21 p. Karl, S. and S. Obrebski. 1976. The feeding biology of the bat ray, Myliobatis californica in Tomales Bay, California. pp. 181-186. In: C. A. Simenstad and S. J. Lipovsky (eds.), Fish food habit studies. Proc. 1st Pac. Northw. Tech. Workshop, Astoria, Wash. 141 Ketchen, K. S. 1975. Age and growth of dogfish Squalus acanthias in British Columbia waters. J. Fish. Res. Bd. Canada 32:43-59. Klopfer, P. H.j D. I. Rubenstein, R. S. Ridgely and R. J. Barnett. 1974. Migration and species diversity in the tropics. Proc. Nat. Acad. Sci. 71:339-340. Korringa, P. 1976. Farming the cupped oysters of the genus Crassostrea. Elsevier Scientific Publ., N.Y., 224 p. Kraeuter, J. N. and M. Castagna. 1977. An analysis of gravel, pens, crab traps and current baffles as protection for juvenile hard clams (Mercenaria mercenaria). pp. 581-592. In: Proc. 8 th Ann. Meeting World Maricult. Soc. . In preparation. Effects of the ray Rhinoptera bonasus on the field culture of the hard clam Mercenaria mercenaria, 6 p. La Marca, M. J. 1964. The functional anatomy of the clasper and clasper gland of the yellow stingray, Urolophus jamaicensis (Cuvier). J. Morph. 114:303-324. ______. 1966. A simple technique for demonstrating calcified annuli in the vertebrae of large elasmobranchs. Copiea 1966:351-352. Lawler, E. F. 1976. The biology of the sandbar shark Carcharhinus plumbeus (Nardo, 1827) in the lower Chesapeake Bay and adjacent waters. M.A. Thesis, College of William and Mary, Williamsburg, V a ., 48 p . 142 Lippson, A. 1973. The Chesapeake Bay in Maryland, an atlas of natural resources. The Johns Hopking Univ. Press, Baltimore, 55 p. Lucy, J. A. 1976. Reproductive cycle of Mya arenaria L. and distribution of juvenile clams in the upper portion of the nearshore zone of the York River, Virginia. M.A. Thesis, College of William and Mary, Williamsburg, Va., 131 p. MacGinitie, G. B. 1935. Ecological aspects of a California marine estuary. Amer. Midi. Nat. 16:629-765. Manning, J. H. and H. T. Pfitzenmeyer. 1958. The Maryland soft shell clam industry: its potential and problems. Proc. Natl. Shellfish Assoc. 48:110-114. Marsh, G. A. 1973. The Zostera epifaunal community in the York River, Virginia. Chesapeake Sci. 14:87-97. Mathews, T. D. and 0. Pashuk. 1977. A description of oceanographic conditions off the Southeastern United States during 1973. South Carolina Mar. Resour. Ctr., Tech. Rep. No. 19, 6 p. McEachran, J. D. 1968. Age and growth of the striped searobin. M.A. Thesis, College of William and Mary, Williamsburg, Va., 47 p. ______. 1973. Biology of seven species of skates (Pisces: Rajidae). Ph.D. dissertation, College of William and Mary, Williamsburg, Va., 127 p. 143 McLaughlin, R. H. and A. K. O'Gower. 1971. Life history and underwater studies of a heterodont shark. Ecol. Monog. 41:271-289. Mellinger, J. 1971. Croissance et reproduction de la torpille (Torpedo marmorata). I. Introduction, Ecologie. Croissance generale et dimorphisme sexuel. Cycle. Fecondite. Lab. de Biol, animale, Fac. des Sci. F-51 Reims:165-218. Merriner, J. V. and J. W. Smith. 1979a. A report to the oyster industry of Virginia on the biology and management of the cownose ray (Rhinoptera bonasus, Mitchill) in lower Chesapeake Bay. VIMS SRAMSOE Rep. No. 216, 33 p. Merriner, J. V. and J. W. Smith. 1979b. Gear feasibility study for the cownose ray, Rhinoptera bonasus. VIMS SRANSOE Rep. No. 227, 27 p. Mitchill, S. L. 1815. The fishes of New York. Trans. Literary and Philosophical Soc. N.Y. (l):479-480. Moss, S. A. 1967. Tooth replacement in the lemon shark, Negaprion brevirostris. pp. 319-329. In: P. W. Gilbert, R. F. Mathewson and D. P. Rail (eds.), Sharks, skates and rays. The Johns Hopkins Press, Baltimore, M d . _ . 1972. Tooth replacement and body growth rates in the smooth dogfish, Mustelus canis (Mitchill). Copeia 1972:808-811. • 1977. Feeding mechanisms in sharks. Amer. Zool . 17:355-364. 144 Murray, J. A. and J. Y. Christmas. 1968. Growth of the uterine young of the Atlantic stingray Dasyatis sabina (Le Sueur) with notes on its ecology. J. Miss. Acad. Sci. 14:128. Murray, R. W. 1960. Electrical sensitivity of the ampullae of Lorenzini. Nature, Lond. 187:957. ______. 1962. The response of the ampullae of Lorenzini of elasmobranchs to electrical stimulation. J. Exp. Biol. 39:119-128. Musick, J. A.1972. Fishes of Chesapeake Bay and the adjacent coastal plain. pp. 175-212. In: M. L. Wass et al. (eds.), A checklist of the biota of lower Chesapeake Bay. VIMS Spec. Sci. Rep. No. 65. Odenweller, D. B. 1975. An unusual aggregation of bat rays, Myliobatis californica Gill. Calif. Fish and Game 61:159. Olsen, A. M. 1954. The biology, migration and growth rate of the school shark, Galeorhinus austral is (Mac Leay) (Carcharhanidae) in south-eastern Australian waters. Aust. J. Mar. Freshwat. Res. 5:353-410. Orth, R. J. 1973. Benthic infauna of eelgrass, Zostera marina, beds. Chesapeake Sci. 14:258-269. ______. 1975. Destruction of eelgrass Zostera marina, by the cownose ray, Rhinoptera bonasus, in the Chesapeake Bay. Chesapeake Sci. 16:205-208. 145 ______. 1976. The demise and recovery of eelgrass, Zostera marina, in the Chesapeake Bay, Virginia. Aquatic Bot. 2:141-159. Otwell, W. S. and G. L. Crow. 1977. Utilization of North Carolina skates and rays. UNC Sea Grant Mini-Project. Final Rep., 23 p. ______and T. Lanier. 1978. Utilization of North Carolina skates and rays. Final Rep. N.C. Dep. Nat. Res. Commun. Devel., Div. Mar. Fish, Morehead City, N.C. Spec. Sci. Rep. No. 31, 47 p. Parker, H. W. and F. C. Stott. 1965. Age, size and vetebral calcification in the basking shark, Cetorhinus maximus (Gunnerus). Zool. Meded. 40:305-319. Pfitzenmeyer, H. T. and K. G. Drolbeck. 1963. Benthic survey for populations of soft-shelled clams, Mya arenaria, in the Lower Potomac River, Maryland. Chesapeake Sci. 4:67-74. Pinkas, L., M. S. Oliphant and I. L. K. Iverson. 1971. Food habits of albacore, bluefin tuna, and bonito in California waters. Calif. Fish and Game, Fish Bull. 152:1-105. Radcliffe, L. 1914. The sharks and rays of Beaufort, North Carolina. Bull. U.S. Bur. Fish. 34:279. Ranzi, S. 1934a. Sulle funzione dei Leucocite nell 1 utero del Selaci. Arch. Zool. Italino 20:569-577 (Cited by Babel 1967). ___ . 1934b. Le basi fisio morphologiche dello sviluppo embrionale dei Selaci. Publ. Staz. Zool. Napoli., Parti II e III, 13:331-437 (Cited by Babel 1967). 146 Richards, S. W., D. Merriman and L. H. Calhoun. 1963. Studies on the marine resources of southern New England, IX. The biology of the little skate, Raja erinacea Mitchill. Bull. Bingham Oceanogr. Coll. 18:4-67. Ricker, W. E. 1975. Computation and interpretation of biological statistics of fish populations. Fish. Res. Bd. Canada, Bull. 191:382 p. Ripley, W. E. 1946. The soupfin shark and thefishery. Calif. Fish and Game, Fish Bull. 64:7-37. Robins, J. D. 1971. Differental niche utilization in a grassland sparrow. Ecol. 52:1065-1070. Rounsefell, G. A. and W. H. Everhart. 1953. Fishery science, its methods and applications. Wiley and Sons, Inc., N.Y., 444 p. R/V Dan Moore - Cruise Report No. 11, December 1975. N.C. Dep. Nat. Res. and Commun. Devel., Div. Mar. Fish., Morehead City, N.C., 32 p . R/V Dan Moore - Cruise Report No. 13, February 1976. N.C. Dep. Nat. Res. and Commun. Devel., Div. Mar. Fish., Morehead City, N.C., 23 p . Sage, M . , R. G. Jackson, W. L. Klesch and V. L. de Vlaming. 1972. Growth and seasonal distribution of the elasmobranch Dasyatis sabina. Contr . Mar. Sci. 16:71-74. Schwartz, F. J. 1959. White cownose ray, Rhinoptera bonasus, from Tangier Sound, Maryland. M d . Tidewater News 15:12. ______. 1965. Inter-american migrations and systematics of the western Atlantic cownose ray, Rhinoptera bonasus. Author's Abst 1 p., Assoc. Isl. Mar. Labs. Carib., 6 th Meet., Isla Margarita, Venez. Jan. 20-22, 1965. ______. 1967. Embryology and feeding behavior of the Atlantic cownose ray Rhinoptera bonasus. Author's Abst. 1 p., Assoc. Isl Mar. Labs. Carib., 7th Meet., Barbados, W.I., Aug. 24-26, 1966.9 ______and H. J. Porter. 1977. Fishes, macroinvertebrates, and their ecological interrelatioships with a calico scallop bed off North Carolina. Fish. Bull. 75:427-446. Setna, S. B. and P. N. Sarangdhar. 1949. Breeding habits of Bombay elasmobranchs. Rec. Indian Mus. 47:107—124. Shaw, E. 1962. The schooling of fishes. Sci. Am. 206:128-138. Shipley, A. E. and J. Hornell. 1906. Report on the cestode and nematode parasites from the marine fishes of Ceylon. Rep. Govt. Ceylon on Pearl Oyster Fish, Gulf of Mannar, Part V:60-61. Smith, H. M. 1907. The fishes of North Carolina. N.C. Geol. and Econ. Surv., Vol. 11:47. Smith, M. S. 1975. A^ P^ Knight Groundfish Cruise No. 74-2, Oct. 15-17, 1974 (Data Record). Fish. Res. Bd. Canada. MS Rep. Ser. No. 1336, Pac . Biol. Sta., Nanaimo, B.C., 13 p. 148 Springer, S. 1960. Natural history of the sandbar shark, Eulamia milberti. Fish. Bull. 61:1-38. ______. 1967. Social organization of shark populations. pp. 149-175. In: P. W. Gilbert, R. F. Mathewson and D. P. Rail (eds.), Sharks, skates and rays. The Johns Hopkins Press, Baltimore, M d . Springer, V. G. and K. D. Woodburn. 1960. An ecological study of the fishes of the Tampa Bay area. Fla. State Bd. Conserv., Profess. Pap. Ser. No. 1, 104 p. Steven, G. A. 1933. Rays and skates of Devon and Cornwall. III. The proportions of the sexes in nature and in commercial landings, and their significance to the fishery. J. Mar. Biol. Assoc. U.K., N.S. 18:611-625. Stevens, J. D. 1975. Vertebral rings as a means of age determination in the blue shark (Prionace glauca L.). J. Mar. Biol. Assoc. U.K. 55:657-665. Strushaker, P. 1969. Observations on the biology and distribution of the thorny stingray, Dasyatis centroura (Pisces: Dasyatidae). Bull. Mar. Sci. 19:456-481. Tanaka, S. K. 1973. Suction feeding by the nurse shark. Copeia 1973:606-608. 149 Taylor, A. J. and M. J. Holden. 1964. The preparation and use of vertebrae for age determination in rays. Internat. Coun. Explor. Seas, Near North Seas Committee, C.M. Pap. and Reps. No. 145, 4 p. Templeman, W. 1944. The life-history of the spiny dogfish Squalus acanthias) and the vitamin A values of dogfish liveroil. Nfld. Dep. Nat. Res. Res. Bull. 15:102 p. Tester, A. L. 1963. Olfaction, gestation and the common chemical sense in sharks. pp. 255-282. In: P. W. Gilbert (ed.), Sharks and Survival. D. C. Heath, Boston, Mass. Turgeon, D. A. 1968. Guide to estuarine and inshore bivalves of Virginia. M.A. Thesis, College of William and Mary, Willliamsburg, Va., 126 p. Van Blaricom, G. R. 1976. Preliminary interactions between two bottom fecJing rays and a community of potential prey in a sublittoral sand habitat in southern California. pp. 153-162. In: C. A. Simenstad and S. J. Lipovsky (eds.), Fish food habit studies. Proc. 1st Pac. Northw. Tech. Workshop, Astoria, Wash. Villadolid, D. V. and D. K. Villaluz. 1938. Animals destructive to oysters in Bacoor Bay Luzon. Philippine J. Sci. 67:394. Virnstein, R. W. 1977. The importance of predation by crabs and fishes on benthic infauna in Chesapeake Bay. Ecol. 58:1199-1217. Walford, L. 1935. The sharks and rays of California. Fish. Bull., Sacramento 45:61. 150 1946. A new graphic method of describing the growth of animals. Biol. Bull. 90:141—147. Wallace, D. E., R. W. Hanks, H. T. Pfitzenmeyer and W. R. Welch. 1965. The soft-shell clam... A resource with great potential. Atlan. States. Mar. Fish. Comm., Leaflet No. 3, 4 p. Walls, J. G. 1975. Fishes of the northern Gulf of Mexico, T.F. H. Publ., Hong Kong, 432 p. Wang, J. C. S. and E. C. Raney. 1971. Distribution and fluctuations in the fish fauna of the Charlotte Harbor Estuary, Florida. Charlotte Harbor Estuarine Study, Mote Mar. Lab. Contrib. No. 112:21. Wass, M. L. (ed.). 1972. A checklist of the biota of lower Chesapeake Bay. VIMS Spec. Sci. Rep. No. 65. Webb, W. F. 1942. Unites States mollusca. Ideal Print. Co., St. Petersburg, Fla., 224 p. Wilson, P. C. and J. S. Beckett. 1970. Atlantic ocean distribution of the pelagic stingray, Dasyatis violacea. Copeia 1970:696-707. Wood-Mason, J. and A. Alcock. 1891. Further observations on the gestation of Indian rays. Proc. Royal Soc. Lond. 50:202-209. Wourms, J. P. 1977. Reproduction and development in chondrichthyan fishes. Amer. Zool. 17:379-410. VITA Joseph William Smith The author, Joseph William Smith, was born in Philadelphia, Pennsylvania on December 16, 1953. He graduated from Father Judge High School in Philadelphia in 1971. He attended Saint Joseph's College in Philadelphia and graduated with a Bachelor of Science degree in Biology in 1975. The author was admitted to the Graduate Program in the Department of Marine Science of the College of William and Mary in September 1975 and held a student research assistantship in the Ichthyology Department of the Virginia Institute of Marine Science from January 1976 to March 1979. In April 1979 the author accepted a biologist position with the Recreational Fisheries Section of the South Carolina Wildlife and Marine Resources Department in Charleston where he is presently employed. The author is married to Rosemarie Fischer and they have two children, Joey and Katrina. 151