The Spatiotemporal Ecology of the Bonnethead, Sphyrna tiburo: Migration, Parturition and Magnetic-Based Navigation
Keller, Bryan Andrew (author)
Grubbs, R. Dean (professor directing dissertation)
Elsner, James B. (university representative)
Cotton, Charles F. (committee member)
Fuentes, Mariana (committee member)
Chanton, Jeffrey P. (committee member)
Florida State University (degree granting institution)
College of Arts and Sciences (degree granting college)
Department of Earth, Ocean, and Atmospheric Science (degree granting department)
Elasmobranch fishes are a highly vagile group that often cross state, regional or international boundaries. Delineating migratory patterns that cross jurisdictional boundaries is essential for effective management, especially when confronted with fishing pressures and regulations that vary over space and time. In addition to migrating individuals, studying critical life stages, such as neonates, is important for fisheries management. The broad goal of this dissertation was to develop a robust understanding of the spatiotemporal ecology of the bonnethead, Sphyrna tiburo. Our objectives were to quantify migration metrics (duration, distance, etc.) and assess the effects of ocean temperature on these metrics, to delineate the species’ pupping grounds and to determine if the bonnethead uses magnetic-based navigation. We used passive acoustic telemetry to track the migrations of 44 bonnetheads for up to 1506 days across ~ 1070 km of Southeastern United States Atlantic coastline. Bonnetheads are a widely distributed small coastal shark in the western hemisphere. The majority of these sharks exhibited strong site fidelity to the North Edisto River in South Carolina and returned annually, residing in the estuary from April to November. Climatic migrations then occurred and overwintering habitats were located throughout Georgia and mid-Florida waters. We present the interannual effects of varying ocean temperature on the seasonal migrations of individual sharks. We found that interannual variations in sea surface temperature had significant effects on the migration distance, duration, and date of arrival to overwintering habitats of individual sharks. An electivity analysis suggested that overwintering locations are selected based upon ocean temperature and not physical location. Given the significant effects of sea surface temperature on migration, future migrations may be altered due to climate change and warming seas. Given the degree of site fidelity observed, our work also has robust implications for the management of this species, as animals with strong ties to specific locations are put at high risks of localized extirpation. The data generated from this study can be used to generate predictive models that can estimate overwintering areas based upon sea surface temperature. These areas can then be protected during migration should the stock become imperiled. While monitoring the residency of S. tiburo in the North Edisto River (NER) we noticed a repeated absence of acoustic detections that aligned with the temporal span when parturition occurs. This absence from the NER suggested they may be giving birth outside of the estuarine waters. We used acoustic telemetry to track pregnant females and found they left the NER for ~2 weeks every year. During this time, they underwent a migration down the coast of the United States into the coastal waters of southern South Carolina and Georgia. We also analyzed fishery-independent surveys and found neonates began to appear in nearshore trawls during this same time period. The mature females caught in these surveys also lost ~1 kg of total body weight from summer to early fall, which we suspect indicates a birthing event. Additionally, an independent study from the same region demonstrated that bonnetheads were post-partum in September. Collectively these data illustrate that bonnetheads emigrate from the NER while pregnant, migrate down the coast, give birth and return to the NER. These data are important for the management of the bonnethead because their primary source of mortality is as bycatch in the shrimp fishery, which has spatiotemporal overlap in these nearshore waters. In our first two research chapters, we highlighted the migratory behavior of the bonnethead and demonstrated that they are capable of migrating over 1,000 km and successfully returning to a target location. In order for these movements to be possible, having a map sense may be important, and the earth’s magnetic field appears to be an important component of this behavior for other taxa. Whilst sharks have been at the forefront of sensory physiology research on sensitivity to electromagnetic fields, whether they use Earth's magnetic field for navigation is unresolved. Here we use a “magnetic displacement” experiment to show that sharks can use the map-like information from earth’s magnetic field to orient. Bonnethead sharks captured along the panhandle of Florida and exposed to a magnetic field mimicking a southern latitude within their range in the Gulf of Mexico oriented homeward, whereas orientation was random when exposed to a field that mimicked a northern latitude outside of their range (in the continental U.S.A.). Likewise, orientation was random when sharks were exposed to a field that controlled for nonspecific effects of the testing procedure. Given that sharks have prior experience of more southern magnetic fields than the control site, but never of more northern fields, our findings are consistent with the hypothesis that sharks learn magnetic features for navigational tasks. The apparent ability of sharks to discriminate position using magnetic cues may also explain the sensory basis by which genetic structure of shark populations are maintained and how cryptic speciation can occur - despite no geographic barriers among reproductive locations.
climate, migration, navigation, parturition, shark, site fidelity
April 2, 2020.
A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
R. Dean Grubbs, Professor Directing Dissertation; James Elsner, University Representative; Charles Cotton, Committee Member; Mariana Fuentes, Committee Member; Jeff Chanton, Committee Member.
Florida State University