Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
John E. Graves
John A. Musick
The sandbar shark, Carcharhinus plumbeus, has a discontinuous cosmopolitan distribution and is exploited throughout much of its range. In the western North Atlantic, it constitutes the majority of the directed commercial fishery. The stock has declined greatly since the fisheries' inception and has not shown signs of recovery despite the implementation of management practices. Like many highly vagile marine species, it is difficult to obtain information about the sandbar shark through direct observation. Therefore, the goal of this dissertation is to use a molecular approach to examine aspects of behavior and reproduction, providing information useful in conservation and management. to this end, I examine the prevalence of genetic polyandry in the western North Atlantic and estimate effective population size and effective number of breeders for the Delaware Bay and Eastern Shore of Virginia nursery grounds. In addition, I look at patterns of philopatry and reproductive periodicity, while on a worldwide scale, assessing both historical and contemporary gene flow. Paternity analysis using microsatellite markers reveals that most females are mate with multiple males during one reproductive period. Despite the high prevalence of genetic polyandry, no direct benefits are detected. The data, however, suggest that males benefit by excluding other males from mating, intimating strong intrasexual competition. The effective number of breeders per nursery ground, estimated using the linkage disequilibrium method, is fairly consistent across years. Comparisons with census size estimates made for Delaware Bay reveal that the two measurements are tightly coupled. The ratio of effective size to census size is 0.45 or higher. This suggests that monitoring of effective population size may be a useful methodology for tracking abundance, and that exploitation may have a direct negative impact on the level of genetic variance. The results suggest that females may stray between nursery grounds found in Delaware Bay, the Eastern Shore lagoons and Chesapeake Bay, as phi st values are nonsignificant and kin groups are detected between as well as within samples. However, true kin groups can not be distinguished from erroneous kin groups because sample size is too small and the loci employed do not have enough power. Even so, the results suggest that female reproductive periodicity in this species needs to be reevaluated. Different patterns of historical dispersal and contemporary gene flow are observed when markers with different modes of inheritance are used to evaluate historical phylogeography. The results suggest that, although females show regional phylopatry, pulses of female dispersal during the Pleistocene may have created the species' current distribution. This dynamic may have been mediated by the changing distribution of nursery habitat caused by the rise and fall of sea level associated with climate change rather than by fluctuating temperature. This idea is supported by the results, which suggest that male mediated gene flow persists long after female gene flow has stopped.
© The Author
Portnoy, David S., "Understanding the reproductive behavior and population condition of the sandbar shark (Carcharhinus plumbeus) in the western North Atlantic: A molecular approach to conservation and management" (2008). Dissertations, Theses, and Masters Projects. Paper 1539616815.