Figuring out how, when and where great white sharks are likely to feed on seals close to the shore is the goal of a study now underway by Dr. Greg Skomal, shark scientist with the Massachusetts Division of Marine Fisheries. Skomal's work, using different tags that collect a variety of information, will paint a broad picture of the behavior. Bryan Legare, a biologist at the Center for Coastal Studies, hopes that his concurrent project will fill in some of the fine-scale details.
Legare, who is working with Skomal and Atlantic White Shark Conservancy scientist Megan Winton, has set out two arrays of buoys equipped with acoustic receivers that register signals from the nearly 200 tags Skomal has affixed to great white sharks in Cape waters. Located at off Head of the Meadow Beach in Truro and just south of the swimming area at Nauset Beach, each array consists of three buoys about 200 to 300 meters apart. When a shark “pings” within its range, the arrays allow a shark's movement to be triangulated, providing more data than just a location, as with a single receiver.
Because the areas where the arrays are located have been mapped using sidescan radar, the resulting data allows scientists to see how the sharks are moving in relation to the topography of the ocean floor.
“I'm looking at the fine scale, particularly how [sharks] are using the near-shore topography of the ocean to feed and hunt,” Legare said. That's critical, because those are the areas where seals and people often overlap.
A pilot project off Head of the Meadow over a three-week period last fall saw 31 tagged sharks travel through the array. On 21 sharks, accurate positions were recorded, a total of 1,900 of them, Legare said.
Head of the Meadow was chosen because it is near a dynamic seal haul-out. “Often these search patterns are, anecdotally, generally in front of the seal haul out,” he said. “Looking at the data, you can easily say don't swim near seals.” The data showed that sharks sometimes moved quickly through the area and at other times appeared to be using a search pattern.
Legare, a doctoral candidate at the University of Massachusetts at Amherst, worked with Skomal on his master's degree and had been discussing a project like this for a couple of years.
“Nobody else has done this with white sharks,” Legare said. “And nobody has done anything like this on the back side of the Cape, in an environment like this.”
That environment is incredibly dynamic, both in terms of shifting sands both on and off shore as well as currents and wave action. One of the tools Legare is using is an acoustic doppler current profiler that measures the currents through the water column, including speed and direction, allowing him to determine if a shark is swimming in or out of the current. If a shark is choosing to be in the water when it's rough, a “washing machine,” as Legare put it, “there's got to be a really good reason.”
By putting all the data together, scientists can start to better understand shark behavior, how they search and cruise a given area, he said. While he's eager to learn the answers to these questions from a biological perspective, he sees the information being put to work by beach managers. “They can look at it and understand it and say, this is what I do to solve a problem,” Legare said.
“We're really building a data set that is either going to improve or strengthen recommendations for people using the shoreline,” he said.
Legare said he expects to have preliminary results from this summer's project by the fall or winter.