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February 26, 2016

Sharks Have Complex Social Networks, Study Shows

New research reveals that sand tiger sharks show complex social behavior of the kind more commonly observed in mammals.
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With their snaggletooth gape and pointed stare, sand tiger sharks don’t exactly scream “social butterfly.” Sharks aren’t generally known for their social tendencies, but there is increasing scientific evidence that this view is incorrect.

In fact, research presented this week at the 2016 Ocean Sciences conference in New Orleans reveals that sand tiger sharks (Carcharias taurus) form networks of up to 200 “friends” that they associate with in certain months. They even have “best friends,” or individual sand tigers they encountered more than 20 times over nearly a year.

Sand tigers sharks along the eastern seaboard group together in late spring through early winter, and disband in late winter and early spring. This “fission-fusion” behavior is commonly seen in mammals, but rarely documented in fish.

This suggests that sharks, long characterized as mindless machines, may be performing a social cost-benefit analysis, weighing the protection or mating opportunities that come with group living against the intensified competition for food.

“Our research shows that it is important for the scientific community to not rule out these types of behaviors in non-mammalian species,” said Danielle Haulsee, a Ph.D. candidate in oceanography at the University of Delaware in Lewes, in a statement. “Behavior can often give us insight on how species interact with their ecosystems and how resources that humans depend on are distributed around the world.”

Haulsee and colleagues tagged more than 300 sand tiger sharks as part of a broader study tracking these top predators throughout the year. Of those, 20 sharks got special acoustic tags that recorded the other sharks and fish they encountered.

So far, the researchers have recaptured two of the 20 sharks with the special tags. In addition to “friends” from their own species, the two sand tigers interacted with an assortment of other shark and fish “acquaintances”, including white sharks and Atlantic sturgeon.

Sand tiger sharks can grow up to 10.5 feet long but they pose little threat to humans — instead, the sharks have to watch out for us.

On the east coast of the United States, their numbers have plunged 90 percent since the onset of fishing. Their sociability has been part of their downfall, making them easy targets to catch en masse. Globally, sand sharks are vulnerable to extinction, with some populations considered critically endangered.

Though it’s been nearly twenty years since sand tiger fishing was banned in the US, the species has struggled to recover. They grow and reproduce slowly, and, as with many other shark species, often fall prey to fishing lines and nets set for other fish.

The researchers hope that a better understanding of shark social hubs can help curb human pressure in these ecologically sensitive areas. “If we know there are certain times and places where breeding females, or even more importantly the pregnant females, are aggregated together,” said Haulsee, “we can devote resources into those areas to protect those sharks.”

Oceana works to protect sharks and other marine wildlife from indiscriminate fishing gear such as long lines and gillnets. Learn more about our campaign to reduce bycatch, or support our work today.