Researchers Use New Methods to Track Sharks
Baltimore — When a shark tagged off the Eastern Shore as part of a marine-life tracking project took off on an unprecedented monthlong journey, researchers quite literally watched its every move.
Thanks to cutting-edge satellite tags, scientists at the Guy Harvey Research Institute plotted and analyzed the Mid-Atlantic odyssey of I-NSU, a juvenile shortfin mako shark that a GHRI team caught in May off the coast of Ocean City.
Since being tagged, I-NSU has traveled at least 8,000 miles, more than 1,000 of them in a surprisingly straight path over the course of about 30 days, researchers said. From late January to mid-February, the shark traveled almost exactly due south in the middle of the Atlantic Ocean, astounding scientists and helping upend conventional notions of the species’ migratory patterns.
“What was remarkable to us was this animal’s very directed movement, like it was going somewhere it wanted to get,” said Mahmood Shivji, director of the GHRI, based at the Nova Southeastern University Oceanographic Center in Fort Lauderdale, Fla. “It was not messing around.”
New Tools, New Discoveries
Before the advent of satellite-based tracking technology, scientists relied on simple ID tags to determine sharks’ patterns of movement. When anglers recaptured tagged sharks and reported the locations of their catches, researchers compiled the data points in an attempt to decipher the animals’ general migratory paths.
“So you catch a shark, put an ID tag on it, hope that somebody is going to catch it somewhere and report back to you,” Shivji said. “But, as you can imagine, when you put an ID tag on it and somebody recaptures it, all you have is a straight line connecting the two points.”
As a result, scientists long believed shortfin makos — typically tagged off the Mid-Atlantic coast — spent late spring and summer off the East Coast, Shivji said. As winter neared and waters cooled, scientists thought the sharks traveled toward the warmer Caribbean.
Now, however, SPOT tags (“smart position or temperature” tags) can send signals to satellites within range whenever a tagged shark surfaces, giving a clearer picture of its movements, Shivji said.
And, much to GHRI researchers’ surprise, I-NSU and six other shortfin makos tagged off the East Coast have been doing anything but adhering to traditionally observed migratory behavior.
“You can see that these animals are not just going up and down the U.S. East Coast,” Shivji said. “They’re going very far out east, into the open ocean.”
The GHRI tracking website, which provides satellite data for 13 shortfin makos tagged in the western Atlantic, displays a matrix of clustered dots that have charted the sharks’ paths for months.
Makos tagged in the Mid-Atlantic have swum farther north than scientists expected, Shivji said. One shark, GHOF4, traveled farther north than any shark on record, reaching waters off the northern tip of Newfoundland.
GHOF1’s current location interests researchers because the shark returned to the cooler Mid-Atlantic from the Bahamas earlier than expected.
I-NSU’s behavior, on the other hand, proved wholly unexpected, Shivji said. After meandering off the coast of Nova Scotia — presumably feeding, Shivji said — the shark began its southward journey in December.
“We thought it would sort of show the same pattern as the other two (sharks), where you would sort of come down a little south and then either go into the Caribbean or start turning back toward the U.S. East Coast,” he said. “But this shark didn’t do that, and it’s also a juvenile, small animal, which to me is just remarkable what this animal is doing, how far it’s going.”
Since then, I-NSU has appeared to follow the Mid-Atlantic Ridge, an underwater mountain range that marks the boundary between tectonic plates, skirting the ridge for about two months before turning east, away from the South American coast, in March. The shark since has begun traveling north back up along the Mid-Atlantic Ridge.
“Our bets were on this shark not going toward the east, but actually going toward the west - turning right and going into the Caribbean,” Shivji said. “Now here’s the question: Where is it going?”
Above all, the information GHRI researchers have collected to date indicates that the institute will need to track many more sharks to gain a comprehensive understanding of the species’ migratory patterns, Shivji said.
“A couple of animals are pretty consistent in what they’re doing, but this animal, I-NSU, is doing something completely unexpected and totally different,” he said. “To derive a general pattern of mako shark movements in this part of the Atlantic, we’re going to have to track a lot more animals.”
That’s where Capt. Mark Sampson comes in. As the charter fishing captain of Fish Finder Adventures, Sampson runs trips out of both Ocean City and the Florida Keys and specializes in sharks. Last May, he helped the GHRI team tag five makos off Ocean City, and this May, he and the team will have 10 days at the end of the month to tag 10 more sharks.
“We are a recreational sport-fishing charter business and guide service. However, sharks have always been a specialty of ours,” Sampson said. “We’ve always done it with an (emphasis on) conservation and education, that is, the conservation of sharks and education to the people about sharks. As long as I’ve been doing it, we’ve been involved in tagging of some sort.”
Tagging shortfin makos typically entails standard rod-and-reel fishing, Shivji said. The team will attach a tail whip to a shark after reeling it in, ensuring that it won’t break free.