White sharks have a well-established presence in the North Atlantic. However, it is not considered an abundant species and efforts to study the life history and ecology have been hampered by the shark’s elusiveness. Indeed, much of what has been known of this species in the North Atlantic comes from the analysis of distribution records, rare behavioral observations, and the opportunistic examination of dead specimens. Conversely, in the Pacific and Indian Oceans, white sharks are seen seasonally in high numbers near seal and sea lion colonies, allowing researchers to study white shark movements on a more in-depth scale. The only behavioral observations of white sharks in the North Atlantic have come from a single acoustic tracking study published in 1982.
It can be argued that the difficulty of studying white shark in the North Atlantic has been due to the historical lack of large seal colonies. Now, with the protection of marine mammals over the last 40 years, the western North Atlantic gray seal population has rebounded. There is strong evidence that white sharks are expanding their foraging strategies to include active predation on these animals. For the first time, we have predictable access to white sharks in the North Atlantic.
Over the last four summers (2009-2012), MarineFisheries staff have observed and tagged several white sharks in close proximity to high densities of gray seals off Cape Cod, MA. The objectives of this research are to examine fine- and broad-scale movements, habitat use, site fidelity, residency, and feeding behavior of white sharks in southern New England and along the east coast of the United States. This research is being conducted with the aid of multiple technologies including acoustic telemetry and satellite-based tagging. With these data, we hope to examine white shark behavior as it relates to prey species (e.g., seal colonies), environmental factors, essential habitat, and human activities.
Through 2012, a total of 34 individual white sharks were tagged off the eastern coast of Cape Cod, primarily in nearshore shallow waters from Orleans to the southern tip of Monomoy. These fish were tagged with one or more of the following tags: pop-up satellite archival transmitting (PSAT) tags, real-time satellite tags (SPOT), passive acoustic coded transmitters, autonomous underwater vehicle transponders, active acoustic transmitters, and NMFS conventional tags. The sharks ranged from 7.5 to 17 feet in total length with a mean of 13 feet. Of the 13 that could be sexed, 12 were females.
Satellite-based technology, including PSAT and SPOT tags, as well as passive acoustic tags has allowed us to look at broad-scale movements in white sharks. Light-based geoposition data from the PSAT tags indicate that most of the sharks have a coastal migratory pattern with movement from Cape Cod in the summer and fall to a broad region off the southeastern United States extending from South Carolina to northern Florida in the winter and early spring. During most observations, these coastal sharks remained on the continental shelf and moved daily between the surface and the seafloor, in waters less than 175 feet deep. We documented similar coastal movements in one of the white sharks carrying a SPOT tag and from acoustic detections of the majority of sharks tagged with transmitters. These results show that the shelf waters along the southeastern United States are important winter habitat for the white shark.
However, not all the tagged sharks remained in coastal waters. The largest tagged shark – a female 17 feet in length – moved from Cape Cod to the edge of the continental shelf in the fall and early winter before migrating into the Sargasso Sea for the late winter and early spring. While off the shelf, this shark descended daily from the surface to depths as great as half a mile. Similar behavior was demonstrated by a SPOT-tagged shark. Although this shark also exhibited coastal movement to the southeastern United States for the fall and early winter, she moved back into New England in late January before heading south to Bermuda and the Sargasso Sea for the remaining winter months. These movements show that large mature females exhibit different migratory behavior that may be linked to reproductive biology.
Fine-scale (local) Movements
To examine habitat use, site fidelity, residency, and local movements of white sharks off the coast of Massachusetts, we have tagged sharks with coded acoustic transmitters. Through 2012, there have been over 25,000 detections from 17 white sharks along the eastern seaboard from Halifax, Canada to Cape Canaveral, Florida. By far, most of these detections were off the coast of Cape Cod and, in particular, off Orleans and Chatham, although this could be a function of receiver coverage. For example, in 2012, five sharks, which were resident in Cape Cod waters for 39-124 days, accounted for 68% of detections in the area. Acoustic detection data continues to be collected end examined for behavioral patterns.
Two of these sharks, known as Mary Lee and Genie, were tagged in partnership with the non-profit organization, OCEARCH. These sharks can be followed, live, through OCEARCH’s interactive tracking website (http://sharks-ocearch.verite.com/).
In 2012, MarineFisheries partnered with Woods Hole Oceanographic Institution engineers and The Discovery Channel to modify autonomous underwater vehicle (AUV) technology for animal tracking. As a result of this collaboration, active acoustic tracks were conducted on four white sharks off the coast of Cape Cod in July 2012. Although track durations were relatively short (0.5-6.3 hours), these efforts represent the first marine animal tracking and filming by an AUV. The tracked sharks remained in nearshore waters less than 100 feet, frequently moving between the surface and seafloor, behavior that is likely predatory given the close proximity to gray seal haul outs in the area.
Data collected by the AUV as it tracked the vertical and horizontal movements of the sharks also included water temperature, current profiling, salinity, and conductivity. It is anticipated that the concurrent collection of these data will allow for fine-scale habitat modeling. In addition, the AUV was outfitted with high resolution video cameras, which allowed for the animals to be sexed and behavioral observations made.