ORONO — Discovery’s Shark Week began Sunday. And right on cue, a nearly 14-foot great white shark is making headlines.
Contender, the largest male white shark tagged in the western North Atlantic, was recently detected near North Carolina’s Outer Banks. OCEARCH tagged him off Florida and Georgia in January 2025.
Contender is not one of Walter Golet’s tagged sharks. But the University of Maine associate professor of marine sciences has tagged more than 100 sharks in the Gulf of Maine through his work with UMaine’s Pelagic Fisheries Lab and the Maine Department of Marine Resources.
Golet said shark tracking is widely misunderstood.
Fish tags are not exactly like the GPS devices most people are familiar with. Scientists cannot continuously follow sharks in real time because they don’t spend all their time at the surface. Each point on a tracking map might represent locations a few minutes apart, or even weeks or months apart. What happened between those points is often unknown, so researchers never really know all of an animal’s movements.
Do shark tags use GPS?
The answer is somewhat nuanced. The original SPOT tags, short for Smart Position and Temperature, relied on the Argos satellite system. They calculated positions using the Doppler shift of radio signals transmitted when an animal broke the water’s surface.
More advanced SPOT tags can now take advantage of GPS technology. However, in both cases, radio signals do not work underwater, Golet said. Because sharks can remain below the surface for long periods, their tags cannot continuously report their locations as they could on land.
Many larger sharks are capable of carrying SPOT tags on their dorsal fins. The tag can, Golet said, communicate with a satellite only when the fin breaks the surface and its antenna is exposed to the air.
“When that dorsal fin of a shark breaks the surface, it can make contact with the satellite, and it can say, ‘Hey, here I am. I’m right here,’” he said.
That location can be quite precise, but researchers may wait days, weeks or even months for the next one.
“We don’t have our finger on the button and know exactly where these sharks are all the time,” Golet said. “There are limitations.”
How is Contender being tracked?
According to OCEARCH, Contender carries one of these SPOT tags on his dorsal fin.
The dots on his tracking map represent individual satellite detections. The lines connecting those dots do not necessarily show the exact route he followed.
“Those straight lines are really simplifications of what’s likely a complex series of movements,” Golet said. “Those types of maps seldom give you a complete picture of what the shark did.”
Depending on the type of tag used, Golet said, researchers can use swimming speeds, water temperatures and other ocean conditions to estimate where a shark may have traveled. Its exact path between detections, however, is generally unknown.
Why do scientists tag sharks?
Even without showing a continuous route, tags provide very valuable data that can reveal where sharks travel, which habitats they use and where they may feed or reproduce, Golet said.
The information can guide decisions about fishing, conservation, public safety and other uses of the ocean. In the Gulf of Maine, movement data could help determine whether sharks regularly use areas being considered for offshore development and how that development might affect their life histories.
Tags can also show whether a shark survives after being caught and released, Golet said.
“A mortality is a mortality,” he said. “It doesn’t matter if the fish dies because you harvested it and you’re going to eat it, or if it dies once you release it. They all need to be accounted for. These mortality tags can also help us develop best practices for handling sharks to ensure they survive once they are released.”
How did shark tagging begin?
Early tagging programs used numbered pieces of nylon tubing attached to a dart or anchor, Golet said.
Scientists inserted the tag into a fish’s muscle and released it. They then relied on another angler to catch the animal and report the tag.
The method showed where a fish was tagged and where it was recaptured. It was the first method to describe the wide-ranging habits of these fish, but it did not reveal what happened between those points.
A fish tagged in the Gulf of Maine one August and recaptured there the following August could have crossed the Atlantic and returned, but this type of tag would not reveal that, Golet said.
Electronic tags now provide orders of magnitude more information, but no single tag offers a complete picture.
How do acoustic tags work?
Acoustic tags emit coded ultrasonic signals and can last up to 10 years, Golet said.
Underwater receivers called hydrophones detect the signal when a tagged shark swims close enough.
“You catch the shark, you tag it, you let it go, and that tag is just pinging away the entire time it’s on the shark,” he said.
According to Golet, traditional receivers store the information until scientists retrieve them and manually download the data. Newer buoys can transmit a detection immediately, but are more expensive, harder to maintain and generally used on a limited basis.
“If one of these real-time detections alerts us that there is a shark nearby, it doesn’t mean everybody has to panic,” Golet said. “It just means that, ‘Hey, there’s an animal here.’”
If the shark swims beyond the network of receivers, however, its signal will not be detected.
What do archival tags record?
Archival tags can record depth, water temperature and light levels. Scientists use that information to estimate where a shark traveled and what habitat they occupied.
Golet said some archival tags collect information for up to four years, but researchers must recover the device to access the full record.
“It’s like you and I,” Golet said. “I tag a shark, but you would have to catch it again for me to get the data back, or I’d have to catch it.”
According to Golet, pop-up satellite archival tags release from the shark after a set period, float to the surface and transmit their stored information to a satellite constellation.
Scientists then reconstruct the shark’s likely route using the data from the tags and other environmental data collected by other programs around the globe. The result is an estimate of where researchers believe the shark went, not a continuous series of exact locations, Golet said.
What else can tagging reveal?
Long-term records can show whether sharks are appearing in different places or at different times.
“You always get these things, especially in the media, where people say, ‘Oh my gosh, we’re seeing so many more sharks now than we did before,’” Golet said. “Well, the question is, are you?”
For example, Golet said white sharks have been present along the Maine coast for millennia. People often judge their abundance based only on what they have personally experienced, but white shark sightings along the Maine coast have certainly increased in recent years.
“All of us are victims of our own timelines,” he said.
Tagging data across species allow scientists to compare current observations with earlier years and identify feeding areas, reproductive grounds, different shark populations and the structures within those populations. Scientists use this information to learn more about the animals and, more importantly, to determine how to effectively manage and conserve the species.
Can the public help researchers?
The Maine Department of Marine Resources asks members of the public to report possible shark sightings through its website.
Some reported white sharks turn out to be basking sharks or ocean sunfish. Still, Golet said uncertain reports can help researchers document where animals are being observed and help members of the public confirm their observations.
“Citizen science does play an important role,” he said. “The public has many, many more sets of eyes out there and can cover a whole lot more coastline than the scientists can themselves. I would encourage people to report their sightings.”
Contender’s map may look like a continuous record of his journey along the East Coast. It is actually a collection of isolated moments when his tag connected with a satellite.
Most of his journey remains hidden beneath the water.
“The marine environment is a harsh one to work in,” Golet said. “Salt water is not forgiving, and neither is pressure, so engineering tags to perform in the marine environment is challenging. But tagging is a critical technology scientists need to use.”