Gray Seal Tracking

Hunters wiped out gray seals from the Cape Cod region in the 1800s. They've returned with federal protection, and now biologists are using high-tech tracking devices to study gray seals and how they interact with their environment, so they can be better protected.

BEAUFORT - North Carolina’s beachgoers shared the sand with an unexpected visitor early this summer. Back in May, a juvenile gray seal waddled ashore on Carolina Beach. It wasn’t injured, it apparently just wanted to spend some quality time on a beach in the TarHeel state. That’s never happened this far south. Gray seals normally range from the Canadian Maritime Provinces south to New Jersey.

While tourists and town officials were surprised and delighted with this unique site, Duke University Marine Biologist Dr. David Johnston wasn’t that surprised.

“Based on what we now know about gray seals, their range, and their ability to travel incredibly long distances, a trip that far south is unusual but not out of the question,” said Johnston. “We’ve observed seal behavior becoming more unique over the past decade or so and our new insights reveal that in even more detail.”

That new insight into seal behavior comes from groundbreaking new research utilizing a high-tech monitor that is glued to the seal and is able to record the animal’s activity for almost nine months.

“In some ways, seals are easier to study because they separate their time on land and in the water, so they are visible when they come onshore to rest, molt or mate, but still, that is only a portion of their lives,” adds Johnston. “So telemetry devices like this allow us to get a glimpse of what they do when they are not at the shore or at the surface, and study their behavior.”

The waterproof device looks like a cell phone in a thick plastic case, with a short, stubby, black antenna sticking out. There’s a good reason for that, because that’s essentially what it is. The device records location, the depth of seal dives and water temperature. Then, it transmits the data using cell phone towers whenever the animal comes to the surface for a period of time. It can be used because cell coverage is so complete in the Cape Cod region, and because it is cheaper than traditional satellite tracking, more animals can be tagged for the study.

“We call it bio-logging, and it is a very low power, efficient system for being able to track animals,” Johnston says smiling. And by looking at his computer screen, it appears the system has worked.

"It’s better than we ever expected,” says Johnston. “What we found was fascinating!”

The Google Earth map of the Cape Cod region shows the area north up to the Bay of Fundy and south into Georges Bank. Bright yellow lines scatter everywhere, with heavy concentrations close to Cape Cod, Martha’s Vineyard and Nantucket, but then individual lines almost to Canada and far out into the Atlantic. The lines showed the travels of Bronx, a young, 179-pound gray seal.

“You can see that a lot of the time, Bronx stayed close to shore, swimming at the surface and then diving, sometimes up to 200 feet deep," says Johnston, pointing to the various lines on the map. “A lot of the time the animal spends at sea is probably foraging. We don’t know much about the social behavior at sea, but they probably do spend some time interacting with other animals. However, the majority of what we detect is the animal at work and how it is making a living and that means foraging and diving down after any number of species.”

But it also showed Bronx traveled long distances; at one time spending 17 straight days at sea. The voyage took him 156 miles, which is half way to Canada and back. At another point, the seal recorded a dive of almost 900 feet deep.

“The distances and the depths are amazing,” Johnston says excitedly, pointing to the various points on the map where the events happened. “First of all, seeing how it makes different decisions is really interesting. It is not always staying in the same place, it forages in different locations.”

Researchers hope their new insight into gray seal behavior will provide a better understanding of how the seal fits into the region’s ecosystem.

That’s a good thing because gray seals have had a rough time in the area. Hunters killed the entire population of seals in the 1800s. Gray seals have rebounded under federal protection since 1972 and they are returning to areas where they once thrived. But not everyone is happy about those large numbers. Fishermen lose valuable catches to the animals and seals crowd onto and spoil beaches. 

“Essentially, I’m interested in how humans interact with those animals and how that affects their sustainability over the long term,” says Johnston.

Bronx will be able to share his travels with scientists for about nine months. The batteries on the cellular tags should last about that long. The device will come off when the animal molts or sheds it’s fur. Johnson admits he’ll miss his updates on the adventures of Bronx the gray seal but he’ll be studying those adventures for years.

Plans call for nine additional seals to be tagged in the research project.

“I’m hoping the data will help us to be able to understand spatially how the animals fit into the marine environment,” Johnson adds, looking at the computer screen of Bronx’s travels and then at a photo of Bronx. “And that’s really important for the seal population because they interact with humans, they interact with the equipment in fisheries and with the fish that are part of the commercial enterprise. So we need to know more about seals so we can protect them as well as the fisheries.”

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