When most people think about sharks, they worry about the biting or chomping that might occur should they meet one. Marine biologists aren't so self-centered. They consider deeper mysteries, like: Are sharks social?
University of Delaware scientists recently used a radio tracking technology called acoustic tagging to reveal how sharks interact. The technique has long been used to track where animals are, not how they interact with other animals. The way to do that is to simply tag more sharks, and see how all of those radio signals plot on a map. But because sharks range so far and wide, humans don’t have gills, and sharks are sharks, a large tagging effort operation is pretty difficult.
But understanding how sharks congregate is important. For one, it would answer longstanding questions about whether they really get together and form vegetarian support groups. Also, conservation. Despite recent rebounds, shark populations around the world remain threatened by culling, finning, overfishing, by-catch, coastal development, pollution, and habitat loss. And the more sharks swim in shark unfriendly areas, the more sharks are vulnerable.
"It's a misconception that sharks are loners," says Andrew Nosal, a marine biologist at the Scripps Institution of Oceanography at UC San Diego. Some species have been observed as semi-social—they swim in groups, but might not interact socially like apes or other land animals that seem to enjoy being around one another. These include scalloped hammerheads, leopard, white-tips, and whale sharks---species that might simply like the same water temperature or eating the same prey. But there are over 400 different shark species (only a tiny fraction of which are scary apex predators). And nobody knows much about how members of any species interact with one another over long periods of time.
Technology is the biggest limitation. Scientists using standard acoustic tagging systems attach a transmitter to a shark. This sends out a unique sound pulse, kind of like a sonic bar code, that bounces off receivers on anchors or buoys. But that means the researchers can only collect data on sharks that deign to swim within tracking distance of the moored receivers. “With stationary tags, we’re limited to putting them in places that are easily accessible, because you have to return to the mooring to collect the data with a Bluetooth device," says Nosal.
The University of Delaware scientists are using a hack to work around this. Rather than outfitting their target fishes---sand tiger sharks---with receivers, the team used transceiver tags that combine receivers and transmitters. Over time, they collected information on which individual sharks were detected within range of one another. It’s sort of like a social network---Finbook for sharks, says Tristan Guttridge, director of the Bimini Sharklab in the Bahamas.
By using fish to narc out other fish, scientists can cover parts of the ocean that stationary receivers can’t reach. And that’s kind of a big deal—not just for understanding shark sociability, but also because animals that form groups can be more vulnerable to big disturbances in their species. “When you put a transceiver in a shark, you’re turning the shark into a mobile platform that will go to ecologically important areas where other sharks or fish will likely go as well,” says Danielle Haulsee, project lead for the study. "This means we could potentially be more likely to get detections of sharks when they are in areas that we either don't know about, or can't feasibly monitor because of logistics.”
The method needs work, though. The few shark studies that have used this technology before created transceivers that reported movement within a limited range, usually 200 to 300 meters. The Delaware team increased that to 500 meters---larger, but not large enough to scan the whole shark-filled ocean.
And simple proximity doesn't always indicate social behavior—ever ridden public transportation? "We have to be careful when we're using the word social, because it implies that there's a mutual attraction for individuals—they are perhaps communicating, or interacting with each other in some meaningful way," says Nosal. Again, are the sharks really hanging out, or simply drawn to the same site for mating, food, water temperature or anti-predatory reasons?
So, scientists use the tracking data to supplement their observations of shark behavior---like the case of two adult male sand tiger sharks in the Delaware Bay, who were revealed to have a far-ranging bromance. Considering that the sharks could go anywhere in the whole ocean, that's pretty interesting.
