Demon Fish: Travels Through the Hidden World of Sharks - Juliet Eilperin (2011)
Chapter 6. SHARK TRACKERS
Right out there, it’s the Serengeti. We just can’t see it.
—Barbara Block, Stanford University marine biologist
It wasn’t until a few decades ago that serious scientists began focusing on sharks at all. The renowned biologist E. O. Wilson became fascinated by them at age nine, just like many other young boys who imagine how they would fare against the wildest beasts on earth. “The great white and I go way back” is how he likes to phrase it. Seven decades later Wilson—who combines the courtliness of a southern gentleman with the patrician New England air of his longtime adopted home, Cambridge, Massachusetts—calls himself an expert in “selachology.” That’s a word he made up himself (though it takes its roots from the Greek word Aristotle favored, selache, or sea fish) and translates roughly into “the study of sharks.” Wilson is a rumpled academic whose hearing gives him trouble at times, but having devoted his life to unlocking the mystery of the natural world, he is extraordinarily skilled in relating his findings to people who are less brainy than he is. Sharks are not just an obsession but a pedagogical tool that helps the professor explain a universe to which most people are oblivious.
Wilson has taught an introductory biology class to Harvard undergraduates for decades, and when he needs to convey the concept of “adaptive radiation”—how species evolve in different ways depending on the part of the world in which they live—he always uses sharks as his example because they boast such a wide array of physical forms across the globe. Their lifestyles vary too: Some move from deep water to breed in shallow water, lagoons, and estuaries; they can spawn between one and a few hundred offspring at a time. While they traditionally swim in salt water, some survive in freshwater. They live in the tropics and at the poles, in both warm and cold water. They began as what researchers at Dalhousie and Florida International universities, led by Francesco Ferretti, called “small coastal consumers” in one journal article, but over time evolutionary forces favored larger species that continued to grow and reached sexual maturity later, so they could “colonize deeper oceanic waters.”1 One group, known as sleeper sharks, live at incredible depths, like the Portuguese shark, which survives at twelve thousand feet below the sea’s surface. These deepwater sharks are some of the longest-living sharks on earth: Greenland sharks live more than an entire century.
In this way, sharks serve as a lens into an array of worlds that have historically lain beyond our grasp. Researchers are still investigating why hammerhead sharks have such odd, flattened heads, but this form clearly gives them exceptional peripheral vision that helps them hunt. The fact that sharks’ litter sizes vary so enormously reflects the wide range of habitats that the ocean offers: coastal sharks produce larger litters because their offspring enjoy more abundant resources yet at the same time face plenty of predators; open-ocean sharks produce a small number of babies that will have to work harder to find food but are less likely to be eaten. Coastal sharks can grow to be fearsome animals, but at their early stages of development they are prey to an array of species, such as rays and other carnivorous fish.
The range of shark litter sizes is enormous: the pelagic thresher and bigeye thresher tend to produce two offspring at a time, while the great hammerhead shark can produce several dozen young at once, and a tiger shark can produce more than a hundred.2 Whale sharks are the most fecund sharks on earth, capable of producing three hundred young at a time. These sharks carry eggs at different stages of development, which could be, in the words of the Australian biologist Brad Norman, “a survival strategy” in which the mother births her pups when the external conditions are best, or it could just reflect how much space the shark has in its twin uteruses.
The variety of tails among shark species also testifies to their adaptability. Threshers boast the longest tail of any shark: its upper lobe alone is as long as its body, ranging between five and eight feet, and serves as a powerful hunting weapon. By contrast, nurse sharks, which stick close to the bottom and swim more slowly, have relatively weak tails. The fast-swimming sharks, including the great white and mako, have tails with upper and lower lobes that measure almost the same length. This particular design, called the thunniform fin, gives it more thrust per stroke. By using a larger surface area to push aside a greater amount of water with each tail movement, a great white or mako can turn forcefully or push itself forward, positioning itself better to attack its prey.3 Tuna and other sharks in the family Lamnidae also boast this feature, allowing them to swim at high speeds and traverse long distances.
The fact that sharks have so many ways of existing in the world underscores how much they convey about the planet. Sharks have developed unique ways to eat, swim, and reproduce because they have survived such vastly different circumstances. Few other animals provide as clear a lens into the natural world as sharks, yet we are just beginning to crack this code. We would do well to learn from them, for the sake of our own survival.
Sharks—along with skates, rays, and the nearly extinct chimaeras—are all elasmobranchs. All of these fish have five to seven paired gill openings on the sides of their head. Some rays, like guitarfish and sawfish, look like sharks that have been flattened, which makes them an alluring fishing target because their large fins are ideal for shark’s fin soup. However, they are not actually sharks. The fossil record boasts more than three thousand species of elasmobranchs, a hardened testimony to the diversity of sharks, skates, and rays that once roamed the seas. But this figure has narrowed over time. (Since their cartilaginous skeletons are seldom well preserved, it’s sharks’ spines and teeth, along with impressions of sharks in rocks, that constitute the species’ fossil record.)
There are roughly five hundred known species of sharks, and this number is constantly inching higher: sometimes in tiny increments, other times in a massive leap. In any given year a scientist might find distinct species either through genetic analysis or by exploring remote waters. But in some instances the number can change on a larger scale, as when CSIRO Marine and Atmospheric Research’s Peter Last, William White, and their team used DNA to identify a total of forty-six new sharks in the course of a year and a half. To put the current global shark count in perspective, it is more than twice as many species as scientists knew about in 1971 when the respected naturalist Peter Matthiessen published his book Blue Meridian: The Search for the Great White Shark. However, this does not mean sharks are doing better than they were several decades ago: it suggests some of these populations are more vulnerable, because they are smaller than previously thought. For example, the CSIRO team identified the northern river shark, which is unique to Australia, grows to be nearly six feet long, and ranks as one of the country’s largest freshwater animals. Until recently, it was confused with another Australian freshwater species in an adjacent region, but now researchers understand it is distinct in its own right.
Great whites now rank as the world’s most terrifying shark—E. O. Wilson calls them “one of the four or five last great predators of humanity.” But these fish appear to be pikers compared with their ancestors, Carcharodon megalodon, which researchers estimate were double the size of great whites and boasted teeth twice the size of a human hand. Living between 50 and 4.5 million years ago, Carcharodon megalodon may have stretched as long as fifty feet and weighed twenty tons, equal to five elephants piled on top of one another.4 Using three-dimensional computer modeling techniques, a group of researchers from Australia and California have calculated the bite force of both modern and ancient great whites: the largest great whites living today have a force of up to 1.8 metric tons of pressure, compared with the 18.2 tons a Carcharodon megalodon could exert. (For comparison, a large, modern African lion can produce roughly 560 kilograms of bite force and a Tyrannosaurus rex would have boasted a force of 3.1 metric tons—a sixth that of the ancient great white.5) In the history of the earth, virtually nothing has roamed the sea that is as scary as these animals.
Some living sharks are massive, of course: whale sharks are the largest fish in the world, stretching up to fifty feet long, and basking sharks grow up to forty feet long. But others are tiny—the spined pygmy shark, called Squaliolus laticaudus, is six inches long, and lantern sharks span twelve inches. Three feet represents the median length for sharks, with half of all species measuring less than that. Despite all the hype that surrounds sharks and their killing capacity, most of them aren’t nearly as fearsome as their reputation suggests.
The sharks that swim in our waters today evolved about 100 million years ago, with a more powerful, mobile jaw that allowed them to target prey more effectively than their ancestors. All sharks have multiple rows of teeth on their upper and lower jaws, and as these teeth break or become worn, spare teeth lying just behind take their place in a sort of conveyor-belt fashion.
While sharks share a similar jaw structure across different species, modern sharks each take on their own bite sizes—and shapes. A shark’s choppers depend both on the species and the age of the animal, since different types of teeth are effective for various types of prey. Saw-edged teeth, like the kind great whites have, come in handy when biting big chunks out of marine mammals. Baby great whites, by contrast, have pointed teeth that let them grab and swallow smaller prey. Fangs or spearlike teeth are better for gripping squid.
The cookie-cutter shark, Isistius brasiliensis—a dwarf shark that swims in schools and is bioluminescent, meaning it gives off a greenish glow in the water—extracts a perfectly circular chunk of flesh out of large fish for its meals, and for years researchers were at a loss as to how it managed to attack larger predatory fish such as tuna, swordfish, and even porpoises. In 1998, after making a series of close observations, the Harbor Branch Oceanographic Institute scientist Edith A. Widder figured out how they do it. The cookie-cutter shark derives its bioluminescence from thousands of very small photospheres around the edges of its scales. There is one part of the shark’s body that lacks this phosphorescent gleam: a collar around its throat that is darkly pigmented. As the light streams into the water from above, this pattern provides a silhouette that acts as a lure to larger predators, which mistake the cookie-cutter shark for a small fish.
While many larger sharks travel as loners, these pygmy sharks congregate instead, which gives them an even greater advantage against their opponents. “Schooling may also explain how these very small sharks avoid a counterattack from the very large predators such as swordfish and porpoises on which their crater wounds are commonly found,” Widder writes. “The damage these sharks inflict would make their company as appealing as a swarm of wasps.”6
Other sharks adopt a more obvious line of attack: the thresher shark wreaks havoc within schools of fish by whacking them with its exceedingly long tail and then returning to consume the ones that have been the most immobilized.7 Flattened sharks such as wobbegongs lie pressed against the ocean floor and then swallow fish as they swim by. A nurse shark has a set of feelers, or barbels, on its nose so it can ferret out small prey in the sand below it.
Sharks’ jaws don’t just tell us about their evolution: they reveal details about our own. For years scientists have studied vertebrates’ transition from jawless animals such as lampreys to ones with jaws, because it was such a massive step forward in evolution. But they’ve been hampered by the inadequate fossil record dating from the Devonian period, which occurred sometime between 412 and 354 million years ago. For literally a hundred years, researchers focused on a 370-million-year-old shark named Cladoselache, because they had several good fossils to examine. But then a decade ago scientists found a shark relative from Bolivia called Pucapampella that predates Cladoselache by 30 million years.
John Maisey, a curator in the American Museum of Natural History’s Division of Paleontology, determined that Pucapampella had a jaw that was attached to its braincase in a way that was more like a bony fish, or osteichthyan, than a chondrichthyan. The finding Maisey made along with Philippe Janvier at the National Museum of Natural History, Paris, has been bolstered by the unearthing of another early Devonian chondrichthyan fossil, Doliodus problematicus, discovered in New Brunswick, Canada.8 The nearly 409-million-year-old Doliodus has sharklike jaws and rows of sharklike teeth, whereas Pucapampella has what Maisey calls “a very odd single row of teeth.” Scientists are still puzzling over the fact that while the two specimens are very early chondrichthyans and date from roughly the same time, they are anatomically very different. They are also exploring additional clues—including osteichthyan fossils from the end of the preceding Silurian period, between 423 and 416 million years ago, and sharklike skin denticles from Silurian-period rocks—which indicate jawed vertebrates might have evolved earlier than previously thought.9
This research is significant because it suggests—for the first time—that modern shark jaws are a more advanced characteristic than the jaws of bony fish. It also indicates that an essential part of the human anatomy originated in fish. As Maisey said, “The psychology of evolution is interesting. People don’t mind being called a primate or a mammal, but they don’t like being called a fish.”10
In his excellent book, Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body, Neil Shubin details the many evolutionary debts we owe to sharks. This includes not only the bones in our inner ear but also the lever system we use to bite. (The muscles and cranial nerves that enable us to swallow and talk are the same ones that move the gills of sharks.)11 Not every aspect of our shark inheritance is a plus, at least if you’re a man. Sharks’ gonads are nestled near the heart. But in human males, gonads are positioned within the scrotum in order to keep sperm at the proper temperature. This has created a weak spot in the body wall, which in turn accounts for why men experience hernias. As Shubin writes, “Men’s tendency to develop hernias is a trade-off between our fish ancestry and our mammal present.”12
Our common ancestry with sharks extends to the genetic level as well. In December 2006 researchers revealed that the genome of the elephant shark, which is native to waters off New Zealand and southern Australia, features a large number of ancient DNA fragments held in the human genome. These fragments regulate genes that produce proteins integral to human development and physiology. The team from A*STAR’s Institute of Molecular and Cell Biology in Singapore and the U.S.-based J. Craig Venter Institute described the discovery as a major development that could help scientists understand how our genes are regulated, and unlock the origin of several human diseases.13
Other aspects of shark biology, such as their electroreception, are totally alien to humans. Many shark species have a row of small holes that run from head to tail, which picks up weak vibrations. This network, along with tiny, fluid-filled sacs in their snouts and chins known as ampullae of Lorenzini, helps sharks find fish buried in the sand because they can detect the electromagnetic fields generated by a fish’s beating heart or gills. Other fish have a lateral line to sense movement, but they do not have the gelatinous material that serves as a conductor for electric vibrations, radiating these signals out to a shark’s nervous system.
This sensing ability, a critical asset in the wild, can prove a liability in captivity because the electrical signals emitted by an aquarium’s lights, pumps, and metal can confuse the animals. But scientists across the United States are exploring whether they can capitalize on sharks’ unique voltage-charged gel for more practical purposes. University of San Francisco physics professor Brandon R. Brown has extracted the material from dead sharks to gauge its thermal sensitivity, while Case Western Reserve University nanoengineering professor Alexis Abramson is leading an effort to develop a synthetic gel with similar thermoelectric properties that could be used to convert waste heat, from devices such as car engines, into usable electricity.
Sharks’ extraordinary ability to hear low-pitched frequencies also helps them identify weakened fish: in the early 1960s, the University of Miami researcher Don Nelson theorized the spasms of a dying fish produce water movement sharks can detect through their lateral line and a staccato sound they can pick up through their inner ear.14
Sharks’ ability to pick up vibrations also helps them migrate across ocean basins, because they can orient themselves within the earth’s magnetic field. These magnetic particles, which became embedded in the basalt in the aftermath of volcanic eruptions, provide a path for sharks to follow. The UC Davis marine biologist Peter Klimley describes it as a series of underwater highways as elaborate as anything surrounding a major American city. “You end up having these magnetic roads,” Klimley says. The seafloor boasts a pin-striped pattern of strong and weak magnetic fields, which only sharks know how to navigate. Periodically, the earth’s magnetic field reverses, a shift that sharks can detect without a problem.
Klimley views sharks’ multiple senses as their greatest asset. Having studied hammerheads swimming around Pacific seamounts, he’s documented that each night as they forage for food, the sharks swim more than twelve miles in one direction and then return along the precise same path, in the dead of night.
When you add in sharks’ sense of smell, which can detect a scent from miles away or help them find a mate, the species boasts a total of six senses, outpacing humans. Their acute sense of smell stems from a series of nasal flaps that lie in front of a shark’s mouth. When seawater passes over these flaps, they guide it to delicate membranes that can detect small substances in the ocean. As they glide through the ocean, sharks can determine where the scent is coming from by comparing what they perceive from one nostril with the other. This question of timing is crucial: sensing small differences in when the scent hits each nostril helps sharks steer the most efficient path toward their prey, as they follow an odor plume to its source.15 Some scientists estimate that more than a fourth of a shark’s brain is dedicated to its sense of smell. Their eyesight isn’t shabby either: while it’s not as keen as humans’, sharks have a reflective layer over their eyes, called tapetum lucidum, which makes it easier to see in the dark.16 They can also see in color and have several mechanisms to protect their eyes when they attack prey: many species have a nictitating membrane that closes when they strike, and some roll their eyes back in their heads so their victims cannot scratch or poke their eyes out.17 Every sense they possess works to their predatory advantage.
But it is the way sharks resemble humans—and differ from most fish—that helps account for why they are now in trouble. While most people think of fish as spawning millions of eggs, only a fraction of which will survive, sharks generally take the opposite approach. They take years to mature sexually—sometimes more than a decade—and only then do they produce a small number of young, which stand a good chance of making it to adulthood. This is why sharks are so vulnerable to human predation: while they are adept at devising different ways to produce their offspring, they simply don’t generate enough young on a regular basis to withstand a sustained assault from fishing.
There’s no single way sharks produce their offspring. Some species give birth to live pups; others lay eggs encased in yolk in leathery cases, which can sustain the embryo through its gestation. Others disperse eggs at the bottom of the sea. The number of young that sharks produce varies just as widely. While some sharks can carry dozens or even hundreds of eggs at a time, a spiny dogfish takes nearly two years to produce a single pup. The spiny dogfish’s pregnancy ranks as one of the longest gestation periods recorded in the animal kingdom, rivaling that of the elephant, which leaves it particularly vulnerable to overfishing.
Of the roughly 40 percent of shark species that reproduce by laying eggs, known as oviparity, many do this shortly after fertilization. The mothers anchor these eggs, which have a tough protective layer covering them, in the seabed so they will hatch in the sharks’ nursery grounds. Horn sharks produce a spiral-shaped egg case that can be wedged into rocks, while cat sharks anchor their egg cases to whatever is growing on the seabed. Over the course of their gestation these shark fetuses will receive sustenance by absorbing the yolk contained in the egg. Some species, however, choose to lay their eggs only a matter of weeks before they hatch in order to ensure that other ocean predators don’t have a chance to eat their young.
Even the sharks that engage in live births do it in very different forms. Both whale and spiny dogfish sharks keep their eggs internally, allowing the young to consume the egg yolk and hatch inside of their mothers, before releasing them into the wild. Other species don’t rely exclusively on the yolk sac, and instead produce infertile eggs they feed to their fetuses to keep them growing. This hybrid form of reproduction, called ovoviviparity, takes places in about a quarter of shark species.
Still other sharks engage in the more sophisticated form of reproduction, placental viviparity, in which they produce a placenta and engage in live births. This form of birth, which takes place in 10 percent of sharks, most closely resembles humans’: the fertilized egg develops into a placenta, which is connected by a cord to the uterine wall so the mother can feed her pup. In the case of hammerheads, which can produce a few dozen young in a single litter, each pup is connected to its mother through an umbilical cord. The similarity between humans and sharks ends after labor, however, because once a shark gives birth, her kids are on their own. Right after birth lemon sharks will rest for a short period, still attached by an umbilical cord, before breaking it by swimming away. At that point the babies must depend on mangroves and other natural fortifications, rather than their parents, to protect them.18
In many ways, the mother shark is doing her offspring a favor: in the immediate aftermath of birthing, female sharks experience a rush of hormones that minimizes their natural instinct to attack their children. But after a period of time those hormones dissipate and the mother takes off, leaving her children—which are born fully independent—to fend for themselves.
I got to witness this phenomenon myself when I waded through Bimini’s mangroves with Sonny Gruber and two other shark researchers, Ellen Pikitch and Elizabeth Babcock. After we trekked to one of the island’s most reliable lemon shark nursery grounds and threw a bit of fish into the water, a parade of juveniles came flocking over. The whole experience amused me: Gruber has developed plenty of shark tricks over his years in the field, and one of them is that he can hypnotize a shark by flipping it over. It’s an impressive yet comical feat: while the shark first struggles and flaps about as Gruber repositions it, within moments it lies harmless and still, in a trance. Soon I was holding my own small lemon shark, suspended upside down. I felt the rough, scratchy surface of its skin, and then, having put it right side up, I released it back into the wild to join its companions.
By this point the water was teeming with teenage sharks—not exactly in a school, but rather a disorganized pack, roaming together for a finite period before each one broke off to pursue its own target. There was something a little sad about it, this group that had no real bond. There are instances when sharks travel in schools out of self-interest, such as when female hammerheads are seeking mates or when smaller sharks need a form of defense against larger predators. Whitetip reef sharks gather together for a very specific purpose: to seek out and consume their prime targets, bony fish. These fish are out and about on the reef during the day, but they hide in the cracks and crannies of the reef once night falls. That’s when the whitetips emerge, traveling in groups in order to more effectively sniff out the fish they plan to tear to pieces.
“They are the most vicious predators; they hunt in packs, like wolves,” relates Elliott Norse, who heads the Marine Conservation Biology Institute in Bellevue, Washington. “They put their faces in every crack of the reef—they are social predators. I don’t know of any animal who has that strategy in the ocean.”
Most large shark species, like tiger sharks and the lemon sharks that circled before me, have little need to travel with their kind once they become adults, because other sharks represent potential competitors for food, rather than allies. As I looked at them, I reminded myself that sharks aren’t about bonding or establishing elaborate social structures. They’re about surviving and dominating everything that comes in their path.
Pikitch, who directs the Institute for Ocean Conservation Science at Stony Brook University, has devoted a significant portion of her career to tracking sharks in locales such as Glover’s Reef, an atoll by Belize in the western Caribbean. I journeyed with her there to get a sense of the mechanics of how such tracking works, since no matter how high-tech the devices scientists use, attaching them to sharks can involve backbreaking work. Most of the time researchers employ traditional fishing methods, which may involve a baited line, spear, or lasso. Hovering over the water in Glover’s Reef, I learn how addictive lassoing sharks can be.
To figure out what shark lassoing entails, I must hang over the side of a motorboat that Ellen Pikitch has commandeered for research. Pikitch started out as a mathematician in college but became drawn to studying fish during grad school, even though she pursued her doctorate in landlocked Indiana. She speaks in the often flat tones of a Brooklynite and approaches her work with clinical precision, hoping to discern the sorts of mathematical patterns she studied earlier in her career. But Pikitch is passionate about two fish in particular—sturgeons and sharks—and her insatiable curiosity has prompted her to spend more than a decade conducting the longest-running shark survey in the world. To survey sharks properly, Pikitch and her team must tag them. And in order to tag sharks methodically, lassoing is required.
Naturally, one must perform this rodeo maneuver with some degree of skill, to guarantee that the subject in question doesn’t bite your hand off. There’s a straightforward way to ensure this doesn’t happen: first and foremost, make sure the shark has gotten hooked on a longline before you begin to lasso it. Many fishing vessels use lines that stretch for miles, but Pikitch and her colleagues rely on a line that measures a few dozen yards instead. They suspend fishing line baited with at least fifty hooks between two buoys, and then let it sit for a couple of hours to maximize the number of sharks that get caught in the line. Establishing a longline with baited hooks is the best way to catch sharks, as opposed to dragging a large net through the water that sharks could tear apart with their teeth. After the sharks are caught on the hooks, the lassoing can begin.
Once we’ve pulled up a line with a shark hooked on it, one of Pikitch’s colleagues holds the fishing line near the shark’s snout while the lassoer (me, in this case) grabs a rope that’s been knotted to form a noose. At this point the shark is floating in the water, parallel to the boat, so the entire procedure can transpire with the least amount of damage to the fish. With my left hand, I grab the dorsal fin to steady the shark while slipping the noose around the shark’s tail. Then I rapidly bring my left hand over to anchor the knot while pulling the rope tight, and, voilà, the shark has been properly lassoed.
This is anything but easy: during the entire procedure, the shark is focused on escaping my grasp. After taking over lasso duty on Pikitch’s boat, I am faced with the undeniable fact that not only are sharks not as dumb as they’ve been historically portrayed but even the reassuringly named nurse shark is a fighter.
Shark wrangling gives you an acute sense of these animals’ muscular bodies and the extent to which they can maneuver themselves in the water to elude any potential foe. On top of all that, grabbing a shark tail that’s swaying back and forth and covered in denticles is akin to having a baseball bat covered in sandpaper rub up and down your hand.
There’s a simple reason why we’re lassoing sharks today: it’s the easiest way to keep a shark pinned down if one is hoping to implant a radio transmitter in its body, which is what Pikitch and other researchers are planning to do. The fact that they need to resort to such elaborate tactics suggests how difficult it is to study sharks in the first place. They spend their lives underwater, where it’s hard to spot them, and they move fast. If one gets close to them, they might attack. None of these attributes make them nearly as easy to examine as, say, lab rats.
Pikitch is the kind of woman who actually craves run-ins with sharks. She divides her time between the Upper East Side and Stony Brook, but spends much of her work life in habitats such as Glover’s Reef, which she first began visiting in the mid-1990s. At the time, Pikitch was starting up the Wildlife Conservation Society’s marine program, and she was investigating the idea of establishing a research station on an island on the atoll called Middle Key, which a British noblewoman named Claude Kinnoull had left to the nonprofit. One day she was hanging out on the dock, chatting with Archie “Chuck” Carr III, a fellow WCS scientist, when she spotted two baby lemon sharks darting in and out under the pier.
“You don’t have baby sharks unless there’s a nursery,” Pikitch thought to herself. She soon set out to answer a basic question: What kinds of sharks, and how many, spend their days navigating the more than 750 patch reefs that make up Glover’s Reef, and is this pattern changing over time? This straightforward query, unfortunately, can only be answered if you can deploy enough technological gizmos on fish with sharp teeth.
The methodology of tagging—which has been applied to both terrestrial and marine creatures in recent years—has dramatically expanded scientists’ understanding of sharks. These devices can range from the mundane—plastic spaghetti strips with numbers, which simply let researchers know if they’ve recaptured an animal—to the high-tech, $3,500 satellite tags that can track where an animal has traveled, how deep it has gone diving, and how much energy it has expended in the process. Quite simply, these devices allow scientists to track marine life once it disappears below the surface, which amounts to a revolutionary advance.
This sort of information is so valuable in part because it gives scientists a sense of what parts of the ocean need to be sheltered more than others. Faced with the enormity of the sea, and the many different demands on it, conservationists are trying to identify the most ecologically valuable regions so they can establish marine protected areas. That way they can strike a deal with competing groups such as commercial and recreational fishing interests: leave these parts of the ocean alone, and you can exploit the rest.
Pikitch and her top assistants, Demian Chapman and Elizabeth Babcock, have been tracking their sharks with fairly straightforward, inexpensive monitoring equipment, spaghetti tags and radio transmitters, both of which can be applied to a shark once it’s lassoed. The spaghetti tags help researchers identify individual animals once they’re recaught, while the acoustic tags send radio signals that provide scientists with a sense of where a shark has traveled over time.
Once I’ve conducted the lasso operation with Chapman’s assistance, the scientists first record some basic information about the shark we’ve secured to the side of the boat. Using a simple band of measuring tape, they record the shark’s species and gender along with three separate lengths for the shark, because different researchers rely on different measurements. The first measurement captures the distance between a shark’s snout and the upper part of its tail, the second goes from the snout to the fork in its tail, and the third reaches from the snout to the tail’s end. Then it’s time for a quick surgical operation.
While keeping the shark secured in the water, Chapman makes a surgical incision about an inch long, right next to the dorsal fin. This allows him to insert an acoustic tag that will send a unique signal every time the shark swims within 1,640 feet of the nearly two dozen radio transmitters the team has stationed throughout the reef. With a few swift motions Chapman stitches up the incision, and we prepare to release the shark, taking care to make sure it doesn’t savage us in the process. I loosen the noose, he clips the hook with a pair of pliers, and I yank the rope back, sliding it easily over the shark’s tail. Then it’s off, thrashing furiously as it leaves behind the researchers who detained it from its usual cruising activity.
One of the oddest aspects of surveying sharks is that, in the end, it’s still fishing: you never know what you’ll wind up getting. In order to get a sizable variety of sharks, Pikitch and her team fish at night as well as during the day, since different species are active at particular times of day.
Because Glover’s Reef is full of shallow patch reefs, negotiating the waters at night without wrecking the boat amounts to a feat in itself. Only one man seems equipped to do this: Norlan Lamb, a Belizean fisherman who has worked on Pikitch’s project for nearly a decade. While Lamb uses GPS maps to get a general idea of how to negotiate the reefs, he relies mainly on instinct to avoid a shipwreck. He sometimes wears his sunglasses at night—it’s sort of a Zen approach, from what I could glean from our conversations.
Once Lamb has zipped across the reef to the study site, the researchers begin to check the line they set earlier in the day for sharks. The first shark that appears is a pregnant sharpnose: when it makes it onto the deck, its babies are squirming visibly inside, their small bodies forming clear and shifting outlines on her stomach’s surface. The shark’s eyes sparkle in the reflection of the assorted headlamps pointed in her direction; after Chapman and Babcock place a simple spaghetti tag on her, they let her go. Then they turn their attention to two male Caribbean reef sharks caught on the line; since both of them have been tagged in the past, they just jot down the tag numbers and identifying characteristics before releasing them. Still, even this basic summary requires the whole lasso routine, which at times doesn’t go so smoothly.
Once the scientists manage to tag their random collection of sharks, it’s up to the fish to let them know what’s happening underwater. The twenty-two monitors sit at the bottom of the sea for six months at a time, and before long these relays begin to blend into their surroundings in the way that a shipwreck does. The team refers to one of its relays as a “lobster farm” because, in Chapman’s words, “it’s got little wee lobsters all over it.” Another is so covered with algae it looks more like the outcropping of a reef than a piece of electronic equipment. But how they look is, ultimately, irrelevant. All the researchers care about is what signals they receive and transmit back to the research station.
After a decade of surveying Glover’s Reef and acoustically tagging more than fifty sharks, Pikitch and her colleagues can draw some basic conclusions. The sharks they find here stay in the atoll year-round: unlike some pelagic species, which roam far and wide, these animals display a remarkable fidelity to these waters. But they defy expectations in some respects. As Chapman observes, “These sharks are thought of generally as shallow, coral reef sharks.” But the acoustic tags testify to something different. One of the sharks they tracked dived down twelve hundred feet into water that was just thirty-nine degrees Fahrenheit. “You throw some technology at it, and find they live way down deep,” he says, adding that technology has “almost completely rewritten our understanding of their life history and biology. If you read shark books from a decade ago, large tracts of them are incorrect.”
Researchers used to think great white sharks were largely coastal dwellers; now they know these creatures are pelagic as well, capable of crossing vast stretches of ocean. They had believed sharks of all species could conceive only through copulation; Chapman has proved them wrong. And now that scientists are capable of attaching sensors to sharks that track their dives below the surface, researchers are beginning to understand how low the sharks go. A decade ago conventional wisdom held that Caribbean reef sharks live almost exclusively up on top of the reef, but tagging now proves they regularly dive anywhere from four hundred to a thousand feet below the surface.
The sharks in Bimini confounded the scientists’ expectations in a number of ways: nurse sharks have the reputation of being “couch potatoes,” in Pikitch’s words—they’re the sharks you’re most apt to run into if you’re snorkeling in a reef, since they often just rest at the ocean’s bottom—but according to radio tracking they’re fairly active. In July 2004 the team tagged a young female they describe as “Nurse Shark 3333”: this shark circumnavigated the entire reef, which spans 116 square miles, several times within a 150-day period. Caribbean Shark 3348, an adult male tagged in May 2004, proved to be a little more adventurous. At times during his 150-day tracking period this shark left Glover’s Reef entirely: at one point he swam nearly nineteen miles to Lighthouse Reef, where a separate set of researchers picked up his acoustic signal on their equipment. These results suggest that if Belizean authorities established stringent marine reserves in Glover’s Reef that were off-limits to fishing, they would protect the sharks from harm, since the fish tend to remain in the same general area. More broadly, now that it’s clear some pelagic sharks can cross ocean basins, international policy makers need to start thinking of ways to regulate fishing activities on the high seas.
Glover’s Reef, one of just four coral atolls in the Western Hemisphere, already enjoys some shelter from exploitation as a UNESCO World Heritage site. At the moment 30 percent of the reef experiences no fishing at all, and authorities prohibit gillnetting and long-lining—the more damaging fishing techniques—throughout the rest of the reef. Gill nets—massive mesh operations anchored by a lead line below and a float line on the sea’s surface—catch anything and everything that swims into their path, earning them the nickname “wall of death.” As a way of making peace with local fishing interests, the researchers have worked to construct a sort of shark park: part of it is off-limits to fishing, while a “general use zone” allows only artisanal fisheries, which generally inflict less damage than large, industrial-scale fishing operations. When local authorities initially set the rules, they were more focused on banning industrialized fleets than protecting Pikitch’s study subjects. “It just happens it’s worked bloody well for sharks,” Chapman observes. But the scientists are now pushing for even more restrictions, and local government officials have indicated they’re willing to expand upon the current regime. In 2007, the local fishery management council proposed banning shark fishing entirely in Glover’s Reef. Rachel Graham, a Wildlife Conservation Society scientist who lives full-time in Belize and has surveyed a broader range of sharks there, has spent years working with authorities there in an effort to ban shark finning nationwide.
“They still have a chance here in Belize,” Pikitch insists, even though she knows there’s a limited amount of time for activists to protect the reef. The top predatory shark populations within Glover’s Reef have remained stable between 2000 and 2007, according to her survey, which is no small achievement given the rise in shark catches elsewhere in Belize. But other reefs don’t enjoy the same level of protection, and they’re the ones that are coming under increasingly intense fishing pressure.
When Pikitch first came to the country in the mid-1990s, she rarely saw sharks at the market, which she regularly visits to buy bait. Belizeans don’t particularly crave shark: the fish only started showing up for sale once the shark fin market began to boom in the late 1990s. In 2005, Pikitch was concerned to learn that a nurse shark—which has relatively small fins, and therefore fetches just a modest price—was for sale at the market. As the demand for fins has risen, fishermen are going to any lengths possible to bring sharks to market to take advantage of the high prices before they disappear.
“I just gasped, because that is really scary,” she says, as we prepare to head out on our fish shopping trip in Belize City. “The sharks are starting to go.”
While Pikitch and her team can handle most of the sharks in Glover’s Reef with relative ease, not all sharks can be monitored by swinging a lasso around them and surgically implanting an acoustic tag. Given that whale sharks are roughly the same size as a school bus, it’s not easy to haul one of those fish over the side of their boat and cut it open. This is where the slingshot comes in.
One of the best places in the world to spot whale sharks is off Isla Holbox, site of a former pirates’ cove off the Yucatán, lying roughly ninety miles northwest of Cancún. At this point Isla Holbox, a sleepy tourist town whose only motorized vehicles are golf carts, has become a critical research location where American and Mexican scientists are seeking to determine the migration patterns of sharks that cross national borders regularly as they traverse the Mesoamerican reef.
Robert Hueter, from Mote Marine Laboratory, spotted his first whale shark in 1975, when he was a graduate student at the University of Miami and a whale shark managed to make its way up Florida Bay. The whale shark eventually disappeared, and a week later it was found dead. Hueter didn’t spot another for a quarter century.
He started coming down to Isla Holbox back in 1994. At the time, he was researching blacktip sharks, which gave birth in one of the island’s secluded lagoons. After nearly a decade some of the locals informed Hueter that in late May and early June—just after he would leave town to return to Florida—a posse of whale sharks would come into the region. Curious, he helped convene a group of fishermen, activists, and Mexican and American scientists to determine what was happening.
Like Pikitch, the survey Mote scientists have constructed with researchers from Proyecto Dominó, a Mexican-based conservation group, seeks what amounts to bare-bones information about the sharks that arrive at the start of each summer and depart by the time fall arrives. What does this animal population look like, why do they come here, and where do they go when they leave? Rafael de la Parra, who heads Proyecto Dominó (the locals coined the nickname dominó for whale sharks because of their many spots), says he and his colleagues can’t expect to protect the sharks if they don’t have a clue about how they operate. While shark researchers are using some of the most sophisticated scientific techniques that now exist for tracking their subjects’ movements, they are starting from a base of knowledge that lags decades behind their terrestrial counterparts. All that, and in order to get any work done, they have to aim a fancy slingshot at a beast that could easily crush them.
At first glance, Rafael de la Parra doesn’t look like much of a spear carrier. De la Parra—who generally goes by Rafi—is a somewhat portly, middle-aged Mexican in a black Speedo and snorkeling mask, holding a tagging spear. He boasts a beatific smile nearly all the time, except when a given tour operator pisses him off. Then he glowers.
But right now he is smiling, because we have spotted a congregation of whale sharks, and he’s ready to dive in. He has a clear goal in mind: jump in the water and deploy the elastic band on his metal pole as soon as he gets within striking distance of the shark, which is about three feet away. At that moment, once he’s angled it properly, he releases the elastic with his thumb so that the tag will shoot forward and attach itself to the side just below the fish’s front dorsal fin. De la Parra can accomplish this feat in a matter of seconds, with the ease of an expert javelin thrower.
With little warning, de la Parra slips off the boat and heads toward the massive polka-dotted animal that’s swimming alongside our vessel. I scramble in after him and manage to get close enough to see him fire the tag into the shark’s body. De la Parra is within reach to determine the fish’s sex: as he pops up on the surface, he shouts, “Macho!” before submerging once again.
De la Parra has done this dozens of times: at this point researchers have tagged more than seven hundred whale sharks in the region since they started studying them in earnest in 2003. In fact, tagging whale sharks is the least difficult part of their job: Hueter, who has personally tagged at least three thousand sharks over the past thirty years, says whale sharks are easy targets. “In a thirty-foot long shark, they don’t even flinch.” But once they’ve gotten an animal tagged—especially if they’re using a satellite or acoustic tag—the challenge begins.
On this particular afternoon, for example, I’ve joined de la Parra, Hueter, and another Mote senior biologist, John Tyminski, as they track the path of the shark to which de la Parra has affixed an acoustic tag. Standing by the tracking equipment, Tyminski recaps what we’ve done today when it comes to placing the tag on the shark. “At 13:20, we put it in a mature male seven miles north of Cabo Catoche, ten nautical miles north of Cabo Catoche point.” The biologist can also record water the shark started cruising in once it was tagged—it’s 78.96 degrees Fahrenheit, composed of 14.4 percent dissolved oxygen. But where’s the shark now?
Nowhere to be found. Apparently, something has gone wrong. We circle the water in broad loops for hours, hoping to pick up a signal from the tag, which conveys a high-frequency beep every second to a hydrophone the scientists are monitoring on the boat. If the animal is within thirty-seven miles of the tracking equipment, the researchers should be able to use the hydrophone to determine which direction the sound is coming from, and thereby keep track of the shark. But in this case, either the shark is an extremely fast swimmer or the equipment—a $650 acoustic tag with a depth sensor—has failed. We are practically, as Hueter explains, “looking for a needle in a haystack.”
“Active tracking has never been one of my favorite things to do,” he says, as the afternoon wears on and the sun continues to beat down on us. “Even when it works, it’s very tedious.” And, in this case, it’s not working.
This is the fundamental problem with shark tracking: it’s costly, complicated, and unreliable, even though it’s essential to understanding these ocean predators. Rachel Graham of the Wildlife Conservation Society uses three different methods to monitor the whale sharks she tracks: photo identification (each whale shark has a unique spotted pattern), satellite tags, and acoustic tags. Back in 2000, Graham placed the first pop-up satellite archival tag on a whale shark. These instruments contain a miniature computer that measures pressure, light, and temperature. This combination of measurements gives scientists a precise sense of where the sharks are traveling, in terms of not just latitude and longitude but also depth. Researchers like Graham program the tags for a set period of time, and when they pop off the animal, they transmit streaming bytes of data to an earth-orbiting satellite, providing a snapshot of what the shark was doing at a given point in time.
These tags can provide a treasure trove of information. Once Graham, diving with her husband, Dan, spotted an archival tag she needed to recover. “Get that tag!” she screamed. (They were underwater, but he got the point.) The tag offered up 206 days of information on what a whale shark had been doing—including diving to unprecedented depths of nearly five thousand feet.
And while Hueter and his colleagues have lost tags on many occasions, the ones that have stayed on have provided promising clues to where whale sharks travel when no one can see them. For years, scientists have suspected the animals give birth in remote areas of the ocean, because no one has spotted their young off the Atlantic coast. In 2007 his team attached a satellite tag to a twenty-five-foot-long female with a rotund belly they nicknamed Rio Lady. Over the following 150-day period, the whale shark traveled nearly five thousand miles from the Yucatán Peninsula through the Caribbean Sea to south of the equator between Brazil and Africa. The area where she ended up, north of Ascension Island and south of St. Peter and St. Paul Rocks, is remote but full of marine life, including sea turtles, billfish, and other sharks. Hueter believes he’s found one of the whale sharks’ elusive pupping grounds, though it will take more research to verify whether that’s true.
Whether it’s satellite and radio tagging or genetic sampling, shark tracking has produced several of the most significant advances in how we understand sharks’ movements and their evolution. It is how we have come to know that great white sharks off the coast of California hover much closer to us than we previously thought.
To discover this, however, researchers such as the Stanford University graduate student Chris Perle had to spend plenty of time at sea, trying to retrieve the very tags they attached to sharks in the first place. Perle was out on the water on a sunny mid-October afternoon in 2006, while I was interviewing his adviser, the Stanford University marine sciences professor Barbara Block.
Block, who works out of the Hopkins Marine Station in Monterey, is the unquestioned queen of shark tagging. She has worked with a group of collaborators to pioneer the Tagging of Pacific Predators program, which has tagged more 4,300 predators from twenty-three species since 1999. That includes great white, salmon, thresher, blue, and mako sharks, along with a wide assortment of seals, whales, seabirds, and tuna. Tagging a great white shark involves an elaborate procedure featuring a seal decoy, which researchers use to lure the shark close to their boat. As Block puts it, “It takes incredible man and woman hours to do that kind of work.”
Even though she’s been doing this work for several years, Block can’t help marveling at how the high-tech tags help her keep track of elusive sea creatures. “You can, in real time, see where a shark is on the blue planet,” she explains as she points out the tracks the great white and salmon sharks have made according to satellite data. “I wake up every day, get my cup of coffee, and see where my sharks are.”
The promise of that modern marvel is what had Perle searching for a small, titanium-encased, seawater-resistant item awash in the Pacific Ocean. The day before the tag had popped off a female great white shark’s fin, to which it had been attached for three hundred days. Despite having its rough coordinates and $7,000 worth of monitoring equipment in hand, Perle and his colleagues couldn’t locate it. So he called Block for advice.
Block checked her computer, rattled off a few coordinates to Perle via cell phone, and then explained she and her researchers were “throwing everything we’ve got at this,” even though it amounted to “a small thing in a big ocean.” And, as she admits, most of the time the scientists are making it up as they go along. No one has ever tried to track animals, when they are not visible, on this global scale. Block describes it as “constructing mission control” for the sea.
Sometimes the researchers at Hopkins Marine Station get lucky, since if ordinary citizens stumble upon a tag they often return it in exchange for a $500 reward. One woman recovered one of Block’s pop-up tags in Hawaii, while another time a five-year-old named Calvin Wisner discovered one while walking on the beach with his parents near San Francisco just after Christmas in 2005.
For Perle, no such luck. It turned out the radio signals from the tag were bouncing off local cliffs, making it impossible for him to pinpoint exactly where it was, and on top of that it drifted twenty-five miles down the California coast in the course of a week. Still, Perle persisted, walking along the cliffs himself in order to scan the shore and the sea.
He found the tag, but got such a serious case of poison oak from his beach walk—“the worst case of poison oak in my life”—that he landed in the emergency room, and it took him a month to recover.
Block has not only managed to tag dozens of great whites off the Pacific coast; she’s helped establish an elaborate acoustic receiver system that lets her and other researchers know where the sharks are migrating along an aquatic corridor between California and Hawaii. This project has provided Block with one of her most astounding finds: white sharks stay much closer to U.S. shores for a longer period of time than anyone realized, and in greater numbers.
Block, working with researchers such as Stanford University’s Salvador J. Jorgensen, used either satellite, acoustic, or mitochondrial DNA tags from a total of 179 white sharks over the course of eight years to prove these creatures were not wandering aimlessly in the open ocean. Deploying a decoy made out of carpet that resembled a seal’s silhouette, they attracted great whites to their boat and inserted the tags with the aid of a 2.3-inch titanium dart and a lance. Many of these tags managed to track a shark’s movements for an average of six and a half months; one did it for just over two years.
Since great whites are capable of traversing vast ocean basins, scientists had thought they would explore large swaths of the sea rather than stick to a single pattern. But to their surprise, they discovered that they migrated in the same sort of predictable, long-distance route year after year, like pronghorn antelope on land and purple martin songbirds in the air. Each winter the animals left the central coast of California and headed between 1,240 and 3,100 miles offshore, along the Hawaiian archipelago. By August, they had returned. While foraging off the California coast, the sharks tended to congregate around certain “hub spots,” including the entrance to San Francisco Bay and off Carmel Point, a popular beach.19
These migrations are so regular, in fact, that the white sharks of the northeast Pacific have become genetically distinct from two sets of counterparts on the other side of the ocean, one close to Australia and New Zealand and another off South Africa’s coast. There are no visible differences between the whites swimming off California’s coasts and those on the opposite side of the Pacific: Carol A. Reeb, a research associate in Block’s lab, was able to make the determination by examining differences in mitochondrial DNA, which mothers pass directly to their offspring through the egg. Reeb estimates that the great whites circling close to San Francisco likely descended from migrants that came from the other side of the Pacific during the late Pleistocene, between 150,000 and 200,000 years ago. And Mahmood Shivji, who works for the Save Our Seas Shark Centre in addition to the Guy Harvey Research Institute, has used DNA analysis to determine that almost all species with global distributions have distinct genetic populations within individual ocean basins.
Scientists do not yet understand why this happened, but it highlights an incongruous fact: the predator Americans have fixated on for years has been much closer to us than we have realized. And beyond the question of great whites’ lineage and migration patterns, the work of these scientists suggests that sharks may not interbreed with their own kind across ocean basins, which means they may be more vulnerable than previously thought.
Block still doesn’t know how the white sharks managed their travels with such exactness. “Are you born with this node in your head?” she wonders. “These sharks are coming back with such precision to these areas. They do it with whatever they have in their brains.”
After making this discovery, Block and several of her colleagues have pushed their research even further by counting the number of individual white sharks that spend their time in the northeast Pacific. While these animals are notoriously elusive, the UC Davis marine biologist Taylor K. Chapple and six other researchers had two things working in their favor as they embarked on this daunting task of identification: The trailing edge of a white shark’s dorsal fin, like a human fingerprint or a humpback whale’s fluke pattern, has unique ridges and indentations that become worn over time and can be used as a form of identification. These white sharks spend a good amount of time at the surface investigating prey, allowing the researchers to photograph and identify them. By feeding this information into a sequential Bayesian mark-recapture algorithm, researchers have been able to estimate white shark abundance off central California.
The results are stunning: scientists identified 129 whites by taking 321 photos, and concluded there are roughly 217 individual great whites swimming off central California. It is, in the words of Chapple and his co-authors, “an order of magnitude smaller than populations of other large marine and terrestrial predators currently protected internationally.”20 Until scientists learned how to count great whites, they had no idea how their ranks had dwindled.
On one level, the new research suggests this population of white sharks may be more vulnerable than previously thought. On the other hand, the fact that more than two hundred top predators are thriving off the West Coast provides further proof of what Block had suspected for years, and shared with me as we sat in her office, looking out over the water: “Right out there, it’s the Serengeti. We just can’t see it.”
Even less stunning areas than California’s central coast boast a bevy of sharks. New York’s waters, for example, serve as home to at least two dozen species. They range from some of the most fierce—great white, bull, and tiger sharks—to lesser-known ones, like finetooth, chain dogfish, and silky sharks.
It is hard to appreciate something invisible to the human eye. But if we could see the ocean for what it is—a vast expanse of wilderness, filled with even more extraordinary creatures than those roaming the African plains—we might begin to value it for its true worth.