Examining the social networks of sharks

New research conducted at the University of Delaware found that sharks prefer to travel in groups of various sizes and spend a surprising amount of time together. Some individuals even spent up to 95 consecutive hours together over the course of the year.

Researchers gathered this data by using a novel tagging procedure to collect tens of thousands of interactions between 300 or so tagged sand tiger sharks along the Eastern Seaboard. They also documented interactions between sand tigers and seven other fish species.

The findings, published in the Nature journal Scientific Reports, could help researchers in future efforts to improve the life cycle of sand tiger sharks.

Documenting shark encounters in the open ocean

The project began in summer 2012, when UD researchers tagged 20 sand tiger sharks with implanted acoustic transceivers to study their interactions and movements. When the scientists recovered the tags from two male sand tiger sharks in 2013, the downloaded data told an unexpected story.

"Based on previous work, we knew that during the summer the sand tiger population formed groups in the Delaware Bay, with male and female adults and juveniles all together in the same places, sometimes very close together," said Danielle Haulsee, the paper's lead author and a doctoral student in the College of Earth, Ocean, and Environment's School of Marine Science and Policy.

But where the sharks went during the other six to eight months of the year, or with whom, was mostly a mystery.

Sand tiger sharks are a vulnerable species due to their inherently low reproduction rates (they produce only 1-2 pups every other year) and historic overfishing.

The acoustic transceiver's transmitter feature tracked the tagged sharks' movements throughout the year, eliminating gaps in the researchers' detection record, while the receiver component detected and recorded other animals carrying transmitters, such as Atlantic sturgeon, that came within a 1,000-foot radius of the tagged sharks.

The researchers learned that though the two male sharks didn't always travel together, they reconnected throughout the year and spent time with other shark and fish species.

Overall, the two individually tagged males recorded 29,646 and 44,210 detection events from a total of seven fish species. In addition to encountering over 50 percent of all the other tagged sand tiger sharks on the East Coast, the sand tigers came in contact with Atlantic sturgeons, white sharks, sand bar and spiny dogfish sharks, and even southern species like the lemon shark and bull shark.

"While our study suggests that sand tigers may be more social than previously thought, confirming this requires more detailed observations of individual shark behavior. One of my future goals is to try to tease out if individual sharks are associating with one another," said Haulsee.

Seasonal behavior

The researchers also discovered that the sand tiger species' grouping behavior changed drastically throughout the year. After summering in the Delaware Bay, the two males migrated south in the fall and were around other males of about their same size, leading the researchers to theorize that the females took a different migration route in October-November. Off the coast of Cape Hatteras, North Carolina, the adult males and females and the juveniles all came back together around December.

As the continental shelf broadened again, the researchers noted that the sharks seemed to go solo for a while, generally from March to May. Whether the group dispersed for a specific reason, whether individuals went into a solitary feeding phase or maybe searched for mates, or whether the animals were in company of sharks or fish species without tags, is unknown.

But as the tagged males began to migrate north in March, the group came together again near Cape Hatteras, and headed up the coast to Delaware Bay.

"It's a big ocean and the continental shelf is pretty wide, so it was surprising how connected the sand tiger group was throughout most of the year," she said. An eye toward future research

For now, as oceanographers consider whether behavioral models used for terrestrial species apply under the ocean, Haulsee said that acoustic transceiver tags provide scientists a way to explore how an individual marine population works.

"If we only studied them in the Delaware Bay, we would think the sand tiger shark population is always mixed, with male and female juveniles and adults together throughout the year. Our work has shown that, seasonally, they are changing the size and composition of their groups," Haulsee said. "Now that we know this, we can start to identify places where human impacts may be affecting one portion of the sand tiger life cycle disproportionally."

The data also may contribute to scientific understanding of the species' genetic diversity, how they find and choose a mate, how diseases are transmitted, and may inform conservation or monitoring strategies.