[Eoas-seminar] Oceanography Dissertation Defense - Bryan Keller - April 2, 1:00 TIME CORRECTION

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Fri Mar 27 11:05:21 EDT 2020

Join Zoom Meeting:  https://fsu.zoom.us/j/328373041

Oceanography Seminar

Bryan Keller

PhD Oceanography Candidate


Major Professor:  Dr. Dean Grubbs

Date: Thursday April 2, 2020                             Time: 1:00 PM (Note time correction)

Location: Zoom meeting ID:  https://fsu.zoom.us/j/328373041


 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 that vary over space and time. In addition to migrating individuals, studying critical life stages, such as neonates, is important for fisheries management. These neonates can be essential for recruitment to adult populations and proper management should be a priority.

We used passive acoustic telemetry to track the migrations of 44 bonnetheads, Sphyrna tiburo, a widely distributed small coastal shark in the western hemisphere. We monitored individuals for up to 1506 days across ~ 1070 km of Southeastern United States Atlantic coastline. 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, for the first time to our knowledge, the interannual effects of climatic variation 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 thermal range 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. The effect of shorter migrations or the overwintering presence of S. tiburo in more northerly habitats is unknown. 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 migration/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 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 gametic 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. A separate study on the same population of sharks identified that bonnetheads captured in mid-September were post-partum. Collectively these data illustrate that bonnetheads emigrate from the NER while pregnant, migrate down the coast, give birth and return to the NER where they stay until early November. These data are important for the management of the bonnethead because their primary source of mortality is the shrimp fishery, which has heavy spatial overlap in these nearshore waters, and a recent stock assessment suggested these trawlers could be contributing to heavy pressure on the bonnethead.

Throughout the first two research chapters, we have identified that migration is common in marine animals. Bonnetheads from the North Edisto River were shown to migrate over 1,000 km and successfully return 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 controversial. 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 southern field within their range in the Gulf of Mexico oriented homeward, whereas orientation was random when exposed to a northern field outside of their range (in the continental U.S.A.) and in a field that controlled for nonspecific effects of the testing procedure. Given that sharks could have experienced more southern magnetic fields than the test site, but never 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.

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