This article originally appeared in The Shark Tagger-1998 Annual Summary
Brad Wetherbee joined the APP as a National Research Council post-doctoral associate in the spring of 1998. He is currently heading up the tracking component of the nursery ground studies. Brad received his Ph.D. from the University of Hawaii, where he studied biochemical and physiological buoyancy adaptations in sharks. This study integrated the disciplines of biochemistry and physiology to investigate aspects of the ecology of sharks, particularly those that inhabit deep water. He found that there is a tremendous difference in the mechanisms involved in buoyancy control between shallow water and deep-sea sharks, and regulation of buoyancy is an extremely dynamic process throughout the life of a shark, resulting in striking differences in buoyancy characteristics even within species. While in Hawaii, Brad also became involved in studies of the movement patterns of fish through the use of telemetry and worked on tracking projects ranging from activity patterns of small reef fish in a conservation zone to long distance movements of large tiger sharks in coastal and pelagic waters. Prior to his work in Hawaii, Brad received his Masters degree at the University of Miami's Rosenstiel School of Marine and Atmospheric Science where he studied the digestive physiology of sharks. Brad is at the APP studying the movement patterns of juvenile sandbar sharks in the Delaware Bay nursery area.
Members of the APP have recently undertaken studies to examine the use of nurseries by sandbar sharks, including quantification of their activity patterns in Delaware Bay through ultrasonic telemetry. Telemetry essentially involves attaching a transmitter to a shark and following the movements of the shark for several days. Sharks can be followed by researchers in a small boat equipped with a hydrophone and receiver as long as the boat stays within range of the signal emitted by the transmitter.
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Receiver and Computer |
During the summer of 1998, 12 juvenile sandbar sharks were tracked in Delaware Bay for periods of up to 70 hours. The majority of sharks tracked showed a high degree of site specificity, remaining in the same area during the entire track with several sharks detected in the same area for up to five days following release. These juvenile sharks essentially patrolled the coastline near Lewes, DE, primarily restricting their movements to water less than 3 fm deep, remaining within 1.6 nm of shore and moving with the tides. There was also a high degree of overlap in activity space utilized by different individuals. Nine of the 12 sharks tracked shared at least some common area and several sharks exhibited extremely similar movement patterns.
Two sharks that were tracked late in the summer moved long distances within the bay. Both individuals were captured and released nearshore, but one moved to the central and deepest part of the bay (>16 fm deep, 11 nm from shore), where it spent several days. During a subsequent track the next week, this shark was detected back near its release site in water <3 fm deep and within 1 nm of shore. The second far-ranging shark traveled from nearshore Delaware across the entire bay to New Jersey (>16 nm) in a matter of 18 hours. Both sharks were tracked in late September, which is just prior to the departure of most sharks from the Bay as they move to their wintering areas. It is possible that differences between the long distance movements of these sharks and the home ranging behavior of the other ten tracked sharks is related to an increased range of movement exhibited as the sharks prepare to leave the Bay at the end of their summer residence.
The results of this study indicate that relatively large numbers of sharks share common areas within a restricted portion of Delaware Bay and that this behavior may provide an opportunity to protect large numbers of juveniles through the setting aside of limited areas within the Bay.
