Tiger Sharks Along East Coast Are More Social Than Once Thought

Some sharks are friendlier than we give them credit for—at least among themselves. Although scientists have long considered sharks to be loners, new research shows at least one species of shark to be not so solitary. Sand tiger sharks even exhibit behaviors typically seen in mammals and only rarely observed in fish.

Sand tiger sharks are important predators in waters off the East Coast where they are counted on to help regulate the marine food web. But they are historically understudied and their populations are shrinking fast, so scientists from the University of Delaware decided to look into how these animals behave in the wild before it was too late. Their presentation on February 22 at the Ocean Sciences Meeting focused on how the study's findings could influence conservation efforts.

The sharks were already known to congregate during summer in the shallow waters of Delaware Bay, but little was understood about how they interact out in the open ocean. It's common to study shark interactions within pens or laboratories, but this is the first study to evaluate how free-swimming sharks get along in the wild over the course of almost an entire year.

Starting in 2007, researchers used acoustic tracking devices to trace the movements of 300 individual sharks in the open ocean from off Cape Canaveral, Florida, to Long Island, New York. During the summer of 2012, 20 sand tigers were implanted with mobile transceivers capable of both transmitting and receiving coded acoustic pings. Since then, two of the 20 sharks have been recaptured and their transceivers recovered. The devices showed that those two sharks had encountered 200 other individual sharks representing eight different species. The team reconstructed the two sharks' approximate locations throughout the year by analyzing this social networking behavior.

Location and time of year played a part in how the sharks socialized, with the number of individuals in each given group changing as the months went by, a pattern scientists call "fission-fusion behavior." The exception occurred in late winter and early spring, when the sharks suddenly entered a phase in which they encountered very few other sharks. Danielle Haulsee, a Ph.D. candidate in oceanography at the University of Delaware in Lewes, and lead author on the study. suspects this is because they mate and search for food during those periods, which suggests they could be performing a type of social cost-benefit analysis.

"If you're living with a group, there could be some kind of protection or information sharing that comes with being in that group," Haulsee says. "But if there's a lot of competition for food resources or mating resources, then it's not beneficial anymore to be in a group, and you might swim away from your group and go off on your own."

The study found the sand tigers to be rare among fish in how they associate with other sharks, whether of their own or different species. "Higher-order decision-making processes are often associated with mammals or species that we think of as really smart—dolphins, elephants or chimpanzees," says Haulsee.

This marks the first time scientists have documented a change in group composition among individual sharks, according to Haulsee. The University of Delaware-led research team hopes that identifying where sharks congregate will give conservationists the data they need to devise plans better capable of protecting this species.

"If we know where and when the population is grouped, we can focus on limiting human-induced disturbances in those times and places," Haulsee says. "For example, if we know there are certain times and places where breeding females, or even more importantly the pregnant females, are aggregated together, we can devote resources into those areas to protect those sharks."