Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Robert J. Latour
Eric J. Hilton
Jan R. McDowell
Wolfgang K. Vogelbein
Charles F. Cottonv
Batoid fishes are among the most threatened and least understood chondrichthyan species worldwide due to their large body size, conservative life-history characteristics, and predominantly coastal distributions where fishing and habitat degradation threaten the stability of populations. A lack of empirical life history data is widespread across batoid taxa — nearly half of all species are considered data deficient, thus hindering species assessments and the development of effective management strategies. Furthermore, many batoid taxa are in need of taxonomic re-examination. Increasing our understanding of life history traits that determine population productivity, such as age and size at maturity, growth rate, and fecundity is prerequisite to examining the potential for populations to increase or stabilize in response to fishing mortality.
The Butterfly Rays (Myliobatiformes: Gymnuridae) are comprised of 10 globally distributed species that inhabit shallow coastal regions and are commonly caught in benthic fishing gears targeting commercially valuable species. Two species are recognized in the western Atlantic: the Spiny Butterfly ray, Gymnura altavela (Linnaeus 1758), and the Smooth Butterfly Ray, G. micrura (Bloch & Schneider 1801). Previous life history studies on U.S. Butterfly Rays were often spatially and temporally limited, which may bias conclusions due to underrepresentation of some life stages, and lead to inaccurate biological characterizations. Furthermore, sexual dimorphism and ontogenetic variability in body shape, and inter- and intraspecific inconsistencies in taxonomic characters (e.g., disk coloration, tail banding patterns) have contributed to substantial taxonomic confusion in the Gymnuridae.
To address knowledge gaps in the life history and taxonomy of western Atlantic Butterfly Rays, this dissertation describes the age and growth of G. altavela, the reproductive biology of G. altavela and G. micrura, and the taxonomic status of G. micrura. The largest male and female G. altavela were estimated to be 11 and 18 yrs old, respectively. Disk width at maturity was 1278 mm and 946 mm for male and female G. altavela, respectively, and was significantly greater in Atlantic G. micrura (male: 390 mm; female: 551 mm) than Gulf of Mexico G. micrura (male: 298 mm; female: 448 mm). Maximum fecundity was seven in G. altavela, and ranged from six to 12 in G. micrura from the Atlantic and Gulf of Mexico, respectively. Based on geographical variation in life history parameters, morphology, and genetics, a re-description and proposed neotype for G. micrura is presented, and two new species and holotypes are described from the Atlantic (Gymnura n. sp. A) and Gulf of Mexico (Gymnura n. sp. B). In U.S. waters, Gymnura n. sp. A may be more vulnerable than Gymnura n. sp. B to indirect fishing mortality due to its larger size, potential later age at sexual maturity, and lower fecundity, since the probability of an individual encountering fishing gear before successfully reproducing is likely greater. This disseratation provides empirical support for the conservation and sustainable management of Atlantic Butterfly Rays. Careful consideration of species-specific taxonomy and biology is required to accurately assess the vulnerability of contemporary populations to extinction risk, and to document and maintain the true biodiversity of this taxon.
© The Author
Parsons, Kristene Teal, "Age, Growth and Reproduction of Western North Atlantic Butterfly Rays (Myliobatiformes: Gymnuridae), with the Description of Two New Species" (2017). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1516639565.