By Tanya Houppermans

Image by Tanya Houppermans

Although the image of the shark as a solitary predator may pervade the public consciousness, new research is showing that at least one shark species exhibits complex social behaviors normally associated with higher order mammals. A team of University of Delaware scientists led by PhD candidate Danielle Haulsee has been using technology to study sand tiger sharks (Carcharias taurus) along the east coast of the United States. Sand tigers are ferocious-looking, but quite docile, sharks that inhabit warm, coastal regions throughout the world (with the exception of the eastern Pacific). As a result of their study, Haulsee and her colleagues are showing that sand tigers may have a much more enriching social life than we ever could have imagined.

Sand tigers are known to make annual migrations up and down the eastern seaboard of the U.S., but the extent to which they form groups, and what the composition of those groups is (e.g., young, old, male, female, etc.), has remained largely unknown. Past research using sharks tagged with acoustic transmitters has only been able to show the geographic location of a shark once it is close enough to a receiver, which is typically mounted on a stationary buoy. But thanks to new technology, VEMCO Mobile Transmitters (VMTs) implanted in sharks and other marine species can now send and receive coded acoustic pings. The VMT is also capable of recording pings from animals outfitted with compatible acoustic transmitters (not just other animals with implanted VMTs).  This allows scientists to not only learn more about sand tiger sharks interacting with each other, but it also provides information about sand tiger proximity to other acoustically-tagged shark and fish species such as Atlantic sturgeon, sandbar sharks, great white sharks, and bull sharks. 

Image by Tanya Houppermans

For this particular study, 20 sexually mature sand tigers were implanted with VMTs in August 2012 after being caught on long lines in Delaware Bay in the northeastern U.S. After one year, two of those sand tigers, both male, were caught and their VMTs recovered. Upon analyzing the data, Haulsee discovered that during part of the year the two sharks would join larger groups of sand tigers, while at other times of the year they would break off into smaller groups and even travel solo (this behavior is known as fission-fusion). These movements appeared deliberate as opposed to just random wanderings, as the first sand tiger recorded 29,646 detection events from seven compatibly-tagged fish species, and the second sand tiger recorded 44,210 detection events from five fish species. The two sand tigers also encountered 52% and 62%, respectively, of the over 300 compatibly-tagged sand tiger sharks along the east coast, showing that some of the same sand tigers are encountering each other at various times throughout the year.

The two male sand tigers were recorded spending the summer in and around Delaware Bay before beginning their migration south in October and November. During this time the group dynamic changed from a mixed population of male and female sand tigers of various sizes and ages in Delaware Bay to a group of nearly all males of similar size during the fall migration. Females also migrate south during this timeframe, although the researchers theorize that they take a different path.

Image by Tanya Houppermans

Around December the male and female sand tigers regroup in the waters off the Carolinas to form large aggregations, perhaps looking for mates, in the vicinity of the hundreds of shipwrecks between Cape Hatteras, NC and Cape Lookout, NC. Haulsee has suggested that these wrecks may act as navigational aides, or that the sharks could be attracted to the myriad of marine life and the abundance of prey found in and around these artificial reefs.

By March the group of sand tigers began to disperse again, with the sharks entering a solitary phase where they encountered very few other sand tigers over the next two months. Even though they were in the same geographic region off the Carolinas, one of the tagged sand tigers went for several weeks without detecting another compatibly-tagged sand tiger. Between May and June the sharks began their northern migration, with the two tagged male sand tigers encountering each other for the first time in four months once back in the Delaware Bay.

Although Haulsee and the other researchers in the study know that sand tiger sharks are associating with each other, one of the most difficult questions to answer is why. More research is needed to determine whether internal forces, external forces, or a combination of both are driving the sand tigers’ behavior. Haulsee believes that the length of time the sharks in this study spent together is significant, and that the evidence points toward this being a deliberate choice rather than chance.

Image by Tanya Houppermans

Sand tiger sharks have been protected in U.S. waters since 1997, but they are still listed as vulnerable by the International Union for the Conservation of Nature due to being caught as bycatch in longlines, gillnets, and bottom trawls. Sand tigers also have the lowest reproductive rate of any shark species, with females reaching sexual maturity at 10 years of age, and only producing on average two pups every one to two years. As a result, sand tiger populations are very slow to recover from overfishing. By better understanding their movements and social behavior, conservation efforts and protection measures can be better tailored to ensure their future survival. 

Link to cited publication: http://www.nature.com/articles/srep34087