So far as anyone knows, the first great white shark to arrive this year on Cape Cod is a white shark named Julia, whose presence was detected on June 13th, off Chatham. I say detected because no one has seen her. White sharks are nomadic and secretive and rare in the Atlantic. They have only been seen with any regularity off the Cape for the last decade or so, and almost only from the air. Julia has a tag, called an acoustic tag, which was implanted by a tuna harpooner, in 2010. The harpooner, named Bill Chaprales, was working for Dr. Gregory Skomal, who is an expert on sharks and works for the Massachusetts Department of Fisheries. An acoustic tag emits a signal that is received by receptors in the water from as far away as about a thousand feet. There are receptors in Chatham and in Truro, near the end of the Cape. Julia’s tag was detected in Chatham. Historically, she has preferred Truro, possibly because she is avoiding a white shark that Skomal named Large Marge, who appears to prefer Chatham. The preferences for both areas are the result of colonies of seals. Between Chatham and North Truro, which is a distance of about thirty miles, there are no seal colonies.

I wrote about all of this last September, but I might add that Skomal has put three kinds of tags on white sharks. Acoustic tags, such as the ones that Julia and Large Marge have; pop-up satellite tags, called PSAT tags; and SPOT tags, which stands for spot positioning or temperature transmitting. PSAT tags stay on the fish for a few months, collecting data about the depth at which the fish swims and the temperature of the water it swims through. Then the tags detach and float to the surface where they signal a satellite. Once they are retrieved, the information they contain is transferred to a computer. SPOT tags, which are fastened permanently with nylon bolts to a fish’s dorsal fin, signal a satellite each time that the fish’s fin breaks the surface. If the fin stays above the water for forty-five seconds, its location is registered.

Last summer, aboard a boat called the Ocearch, which belongs to a shark activist named Chris Fischer, I watched a SPOT tag being placed on a white shark that Fischer named Betsy. Sharks that Fischer has tagged can be tracked on the Ocearch Web site. A shark whose fin breaks the surface frequently is called a finner. I had planned to spend the winter keeping up with Betsy’s whereabouts, but she is not a finner. She was tagged off Chatham on August 15th; on August 17th she was detected a few miles east and then she disappeared. After five months she turned up, in December, in the mid-Atlantic. She disappeared for another five months, then turned up, in April, deep into the Gulf of Mexico, farther west than any shark that Skomal had tagged in the Atlantic had gone. Now she appears to be off the Gulf Coast of Florida. Being so far away means that she is not likely to show up off Cape Cod this summer. Since I spend a lot of time in Truro, I had been hoping I might see her. I don’t mean at close hand. I mean from a spotter plane, if I knew where she was.

Since 2012, Skomal and Fisher have tagged four white sharks off Cape Cod: Genie and Mary Lee, in 2012, and Betsy and Katherine, in 2013. Before these fish were tagged, fish scientists assumed that white sharks went north in the summer and south in the winter, like birds, a notion that turned out to be wrong. Genie was not much of a finner and went more or less south. Mary Lee, however, went on a joyride all over the western Atlantic. “Very dynamic in her movements,” Skomal told me the other day on the phone. “In a nutshell, she went south at first then moved up past New York City to Long Island, in January, which was mind blowing because it was cold, and white sharks were thought not to like cold water. She went back to New England then headed straight to Bermuda. She looped the island then headed for the Sargasso Sea and since the spring of 2013 has been pretty much associated with the southeastern U.S. Right now she is off Georgia and pretty much has been for a year.”

Genie, meanwhile, is off Virginia or maybe in the Chesapeake region, and seems to be making slow progress toward the Cape, although not necessarily. One of the things that Skomal has learned from watching these four fish is that white sharks, at least in the Atlantic, aside from being reclusive and hard to meet, are more independent than scientists understood. They appear to go anywhere they feel like, being, as Chaprales the harpooner told me, “the baddest fish in the ocean.”

The four sharks heading different ways spelled the demise of the north-south migration theory. Skomal wonders if there is still a migration pattern to be discovered, but one on a much longer scale than he or other scientists had guessed. Each fish is probably about twenty years old; white sharks live to be about seventy. The four sharks may be behaving as they are because of their age. Or, because white sharks have a two-year reproductive cycle, they may be influenced by whether they are pregnant or interested in breeding—no one knows where white sharks breed in the Atlantic or where their nursery area is. The fish may be moving in response to some instinct, or they may be being influenced by something they are learning while they travel. “My gut tells me there’s a pattern of some kind,” Skomal said. “We just haven’t figured it out. Maybe we’re only getting parts of the pattern. That’s probably an artifact of our sampling. We’re looking at where they’re going and when, and trying to force it into a time frame that may not make sense for the natural history of the animal. No one wants to draw conclusions from four fish. We have to be patient and see what emerges.” It’s too bad that Julia has only an acoustic tag, instead of a SPOT tag, which would track her whereabouts in real time, since she has returned to the Cape each summer for the last four years. It would be interesting to know where she has spent the rest of those years.

Instead of tagging much this summer, Skomal is going to try to determine how many white sharks visit the Cape. He plans to use a statistical survey model called mark-recapture population analysis. “Basically it’s a fairly simple technique,” Skomal said. “Go out twice a week in a spotter plane, four hours each day, find the shark then approach it in a smaller vessel and see if it’s got a tag and examine its markings. These fish have coloration patterns like fingerprints. On subsequent trips you re-identify them, and the survey becomes a relationship between the number of animals previously seen to the numbers you encounter later on. You end up with a catalog.”

“In the five years we’ve been tagging on the Cape, we’ve tagged thirty-eight sharks altogether, using all three kinds of tags,” he went on. “That begs the question, did we get all of them, and I can guarantee the answer is no. The question is what percentage did we tag. If we tagged ten per cent, that’s three hundred and eighty sharks. My guess, though, is that it’s on an order of dozens, not hundreds. We’ve gone out in spotter planes some summers very frequently and seen only one or two sharks a day, so over the course of sixty days we’re not talking about hundreds of animals.”

I said that I was hoping I might see Betsy again. “Being in the Gulf of Mexico in the spring, both her and Katherine, I’m not sold either of them’s going to be on the Cape in July and August,” Skomal said. “I think Genie’s going to come back, though. She’s close enough in the Chesapeake that she may at some point just beeline it here by next month. I’m watching her with bated breath.”

Photograph by Stephen Frink/Corbis.