Featured Story

April 2006

The Biggest Fish

Unraveling the mysteries of the whale shark

By Steven G. Wilson

A whale shark cruises waters off Australia's outback coast. Photos by Steven G. Wilson

Finally the radio crackled to life. “I’ve got two sharks, about a mile off Tantabiddi Passage!” The vessel suddenly transformed as everyone scrambled to gather and don masks and fins. As we skimmed over the water, I struggled to attach a dart and tag to my Hawaiian-sling polespear—a sport-fishing spear powered by a thick rubber band. Five minutes later, two large, dark shadows were looming beneath the ocean’s surface, about a hundred feet off the bow.

I plunged in, but once in the water I had to ask the boat crew for directions to the unseen giants. I swam toward where they signaled. A cobia came into view, a game fish that often accompanies whale sharks, and I knew I was close. Then, slowly, the outline of a gaping oval mouth and, behind it, an upright tail fin resolved from the featureless blue background. As I swam closer, the shark’s tapered body and distinctive checkerboard markings also came into focus.

My attention, though, was drawn to the first of its two dorsal fins. The base of the fin was the target for the dart and tag. My colleagues in the water measured the animal (fifteen feet long) and determined its sex (female), while I positioned myself along its side. When they were finished, I cocked the polespear and released it. The dart penetrated the whale shark’s tough hide, but not as deeply as I had hoped. Unless adjusted, it would pull out in a matter of days. With a quick shove of the polespear, I pushed the dart deeper. The shark reacted with a flick of its tail, then dove.

I watched with satisfaction as the tagged shark sank slowly into the depths. I felt a jolt to my lower back, and suddenly found myself being propelled through the water. All I could see was a whirl of spots. It took me a moment to comprehend that another, much larger whale shark had struck me with its dorsal fin and was pushing me forward. With all the excitement, I had completely forgotten about the second shark! Stunned but unhurt, I dislodged myself and swam back to the boat to reload my polespear. By the time I returned, the second whale shark was about twenty-five feet beneath the sea surface. I filled my lungs with air, then dove after the thirty-five-foot leviathan. This time the dart penetrated with ease, and the shark showed no reaction. My job completed, I swam back to the vessel and learned that the plane had located more sharks. In spite of my little fright, there would be no time to dwell on it that day. Whale sharks were popping up everywhere.

Fortunately, accidental clobbering is the only danger whale sharks (Rhincodon typus) present to people. Unlike their toothier, more aggressive relatives, whale sharks have such gentle dispositions that the chance to swim with them has spawned lucrative ecotourism industries in several places where they gather. Ningaloo Reef, the Philippines, Belize, and the Baja Peninsula of Mexico have all benefited from whale shark tourism. But the same lumbering slowness and tendency to swim near the surface that make whale sharks a favorite with snorkelers also make them easy targets for fishermen, and frequent victims of collisions with ships. Before the mid-1980s, only a few hundred whale shark sightings had been reported worldwide; in the past two decades, human interaction with them has grown substantially.

	A spotter pilot searches for whale sharks along the front of Ningaloo Reef. Once located, tourist or research vessels are directed to position their divers ahead of the approaching shark.

Yet despite the increasingly frequent contact between people and whale sharks, and despite their presence throughout the world’s tropical and temperate seas, including the waters of some 125 nations, surprisingly little is known about them. Marine biologists don’t know much, for instance, about how whale sharks reproduce: no one has ever observed their courtship, mating, or birth. How they interact socially is anyone’s guess. What they do on the prolonged, deep dives they make is yet another mystery. No one knows how many there are, or whether their populations are rising, stable, or declining. Given their largely unregulated harvest and vulnerability to capture, however, decline seems most likely.

To a small troop of biologists—myself included—those gaps in knowledge present a challenge. Our recent research into the whale shark’s feeding habits, diving behavior, and migrations is slowly giving us a better understanding of its role in the marine environment. Our hope is that we will be able to use this knowledge to help ensure the species’ survival, before it becomes another casualty of the changing world.

At least some facts about whale sharks are clear. First, the name “whale shark” is somewhat misleading: the animals are indeed sharks, but they are “whales” only by virtue of their size. They grow more than forty feet long (the length of a luxury motor home), and there are unsubstantiated reports of a sixty-five-footer that weighed thirty-seven tons. Unlike most sharks, though, whale sharks are filter feeders. They share that behavior, fittingly, with the world’s biggest animal, the blue whale. Whale sharks suck dense concentrations of minute prey, such as krill and other zooplankton, fish spawn, and small fishes, into their enormous mouths. To collect the prey, they filter out the accompanying water through sievelike gill plates, and then expel it through their gill slits.

Whale sharks often feed passively by swimming slowly with their mouths agape. They can also assume a head-up, tail-down feeding posture, sometimes bobbing up and down near the surface to pump prey-filled water over their gills [see illustration below]. Oddly, they are not closely related to the other two filter-feeding sharks, the basking shark and the megamouth shark. Instead, their closest relative is the nurse shark, a bottom-dwelling predator. In spite of their filter-feeding ways, whale sharks possess some 27,000 minute teeth, similar to teeth in the fossil record that date to about 55 million years ago. Little else is known of their evolutionary history.

Scientific knowledge of whale shark reproduction is based on a single female, harpooned off Taiwan in 1995, that carried 301 embryos in various stages of development.

Whale shark feeds passively on small prey by swimming with its mouth open. A snorkeler watches from above. Whale shark is a “suction filter-feeder” on dense congregations of minute prey such as krill. It strains mouthfuls of food-filled seawater (brownish-blue arrows) through porous gill plates and consumes the prey that remains in its mouth. Channels behind its gill plates direct the filtered water over its gill filaments, which extract oxygen for respiration. The filtered water (blue arrows) is then released through the whale shark’s gill slits. The animal can generate suction to draw in its meals, perhaps by expanding its oral cavity and depressing its basihyal, a tonguelike structure on the floor of its mouth. Illustration © Emily S. Damstra (www.emilydamstra.com); used with permission of the artist

Biologists know from that catch that the pups are born alive when they are about two feet long. (The eggs hatch inside the mother.) Studies of growth rings in vertebrae suggest that whale sharks reach sexual maturity when they are between twenty and thirty years old, and may live for several decades more. Young whale sharks less than ten feet long are rarely seen, leading some investigators to speculate that they occupy deep, offshore habitats during that most vulnerable stage in their lives. Newborns have been recovered from the stomachs of a blue shark and a blue marlin. The adults likely have few natural predators, except perhaps great white sharks and killer whales.

The whale shark’s most prominent feature—other than its sheer magnitude—is its distinctive markings. Pale spots speckle a grid of bars and stripes atop the shark’s blue, gray, or brownish back and flanks (its belly is white). The markings probably act as camouflage, mimicking wave-dappled sunlight in the water or perhaps a school of small fish. If so, an important function of the markings may be to conceal juvenile sharks from predators.

Individual sharks have unique markings. Recently a team led by Bradley M. Norman, a marine biologist with the marine conservation group Ecocean in Perth, Western Australia, adapted a computer algorithm, originally devised for mapping stars, to identify individual whale sharks from photographs of their spots.

Ningaloo Reef lies along a lonely 160-mile stretch of outback coast and is Western Australia’s answer to the Great Barrier Reef. Although smaller and less well known than its east-coast counterpart, Ningaloo is famed for the large marine animals—humpback whales, manta rays, whale sharks, and others—that gather seasonally in its waters. I first visited the reef in 1997 and spent nearly every day of my two-week stay snorkeling with whale sharks and photographing them. By the time I left, I’d become curious to know much more about these gentle giants, and surprised by how little science could tell me. Nine months later, I enrolled in a Ph.D. program at the University of Western Australia in Perth to study the species at Ningaloo. I wanted to know why the whale sharks gather on the reef each year, and why more sharks come in some years than they do in others.

	Map of the Ningaloo Reef region showing geographical landmarks and the current regime during the seasonal whale shark aggregation Map courtesy of Steven G. Wilson

Whale sharks, like many other shark species, segregate themselves by sex. At Ningaloo Reef most whale sharks are immature males, suggesting that they come to feed, not to mate. The region’s oceanography may explain why they—and perhaps some of the other large creatures—visit the reef. Flowing southward along the continental shelf, the Leeuwin Current dominates the area. But a smaller countercurrent called the Ningaloo Current flows northward between the Leeuwin Current and the reef. Each fall, the two currents join to form a gyre, which may keep nutrients and prey in the area rather than flush them out [see map right]. It hardly seems coincidental that that’s when the whale sharks arrive on the reef.

Still, whale shark abundance varies widely from year to year. Some years, as many as a few hundred sharks come to Ningaloo Reef; in other years, the numbers are much lower. To determine why, I began by looking at patterns in whale shark abundance derived from records of shark interactions that commercial tour-boat operators must keep as part of their permit requirements. Many years of such data show that fewer sharks are present in El Niño years than during La Niña years.

In La Niña years, ocean temperatures and sea levels in the western tropical Pacific are relatively high; during El Niño episodes, water temperatures and sea levels are lower. Both patterns have long-range effects on climate and currents from Australia to South America, as well as in many other parts of the world. I began to suspect that the El Niño phenomenon somehow negatively affects the whale sharks’ food supply at Ningaloo Reef.

To confirm my suspicions, I first had to determine what the whale sharks eat along the reef. They were already known to feed on schools of a tropical species of krill, Pseudeuphausia latifrons, but no one could say whether it is their primary food. To answer that question, I examined fecal samples from whale sharks, which divers had collected at the reef over several years. All the samples included crustacean remains that resembled krill, and a genetic analysis later confirmed that the species was indeed P. latifrons. Surveys using sonar to look for krill while whale sharks were congregating on the reef also turned up plenty of krill, forming schools about the size of a football field and some ninety feet deep. Most tellingly, when my colleagues and I came across schools of krill, we almost always found whale sharks feeding on them.

A curious young whale shark approaches a tourist boat.

Do krill populations fluctuate with El Niño, too? As part of a study on fish larvae, biologists from the Australian Institute of Marine Science in Townsville set traps each month for two consecutive summers. I was able to study the krill and other zooplankton they caught. As luck would have it, the first year had strong El Niño conditions and the following year strong La Niña conditions. In line with my hypothesis, krill abundance proved to be much higher during the La Niña year. Two years of data is not proof, but it does offer a good lead.

How does El Niño influence the production of krill? The Leeuwin Current that dominates the reef is stronger in La Niña years than it is in El Niño years. Paradoxically, however, the stronger current suppresses nutrient upwelling, and that leads to lower chlorophyll concentrations and a diminished supply of most kinds of zooplankton in La Niña years. So what accounts for the high krill abundance we discovered in a La Niña year? For now, at least, that remains a mystery.

Although I finished my doctoral studies in 2001, I still return to Ningaloo Reef each whale shark season. In 2002 I collected tissue samples for a genetics study by a graduate student at the University of South Florida in Tampa, comparing DNA from whale sharks in the Atlantic, Indian, and Pacific oceans. Once finished, the study will show how much genetic mixing takes place between whale sharks in the three ocean basins. That should shed some light on how much impact regional fisheries may have on the global abundance of whale sharks, and thus guide efforts to manage and conserve them.

My surprise encounter with the dorsal fin of a whale shark resulted from an effort to answer another basic question: Where do the Ningaloo Reef whale sharks go in the winter, spring, and summer? In 2003 and 2004, I joined three fellow marine biologists—Brent S. Stewart of Hubbs–SeaWorld Research Institute in San Diego, Jeff J. Polovina of the U.S. National Marine Fisheries Service in Honolulu, and Mark G. Meekan of the Australian Institute of Marine Science in Darwin—in attaching pop-up archival tags to nineteen sharks. The tags record data about the light level, depth, and temperature of the tagged fish’s environment until a preprogrammed date. Then the tags detach, float to the surface, and transmit their archived information to satellites. From those data, the sharks’ movements can be reconstructed to within about a hundred miles.

	An adult whale shark passively feeds in coastal waters off Ningaloo Reef.

We recovered several months’ worth of data from each of six tags. All six sharks had moved northeast after leaving Ningaloo Reef, and several individuals had approached the Indonesian coast, where, we feared, they risked becoming fishermen’s quarry. Our depth and temperature data also showed that whale sharks inhabit a more extensive niche than anyone had suspected. The animals spent most of their time in surface waters, but they also dove occasionally to depths of more than 3,200 feet, where temperatures drop as low as forty degrees Fahrenheit—a big change from the balmy eighty-four-degree waters at the surface. Why do they dive? Perhaps the sharks need to cool off, or perhaps they are feeding on some unknown, deepwater prey.

Since they spend so much time near the surface, though, we realized we might track them much more precisely with a different kind of tag: a satellite-linked radio transmitter. With such a transmitter, an animal’s position can be determined to within a mile anytime the transmitter’s antenna is above the sea surface. In 2005, with the help of John D. Stevens, a shark biologist, and Matthew G. Horsham, a mechanical engineer, both at Australia’s Commonwealth Scientific and Industrial Research Organisation in Hobart, we attached these instruments to several whale sharks at Ningaloo Reef. Some tagged sharks moved northeast toward Indonesia, and some moved northwest.

Tags of various kinds have been attached to whale sharks in the waters of several other countries as well, including Belize, Honduras, Japan, Mexico, the Seychelles, and Taiwan. Most of those tagging studies are not yet published, but preliminary data suggest that whale sharks migrate long distances. One shark traveled from Mexico’s Sea of Cortez across the Pacific, a distance of more than 8,000 miles.

A pop-up archival tag is attached at the base of a whale shark’s first dorsal fin. Deployed for periods of up to one year, these tags record the horizontal and vertical movements of sharks and later transmit their data to satellites.

Whale sharks have long been hunted at many of their seasonal gathering sites, typically by artisanal fishermen using harpoons. In some places the catches have been quite high: fishermen in Gujarat, India, for instance, took 591 whale sharks in 1999 and 2000, before whale shark hunting was banned nationally in 2001. Whale sharks often wind up in Asian markets, particularly in Taiwan, where they are known as “tofu sharks,” for their soft, white flesh. There, the meat and fins fetch the highest price of any fish.

Signs of overfishing have already begun to appear. Whale shark catches have declined in several places that have been fished intensively. Meekan recently suggested that the Ningaloo Reef whale sharks are smaller by about six feet, on average, than they were a decade ago. Because they grow so slowly, reproduce so late, and congregate in small, migratory populations, whale sharks are particularly vulnerable to overfishing. Indeed, the World Conservation Union, a Switzerland-based environmental group, has listed them since 2000 as vulnerable to extinction.

Yet there are some hopeful signs, too. In the past decade several nations have banned whale shark hunting––though opportunistic capture appears to continue in some of those nations and elsewhere. Taiwan’s fishery, perhaps the largest, persists with an official quota of sixty-five whale sharks per year. Still, it seems likely that the whale shark catch is lower than it was in the unregulated past, and several countries, such as the Philippines, have converted whale shark fishing centers into tourism destinations. Beginning in 2003, the Convention on International Trade in Endangered Species of Wild Fauna and Flora (also known as CITES) imposed permit requirements on trade in whale shark products.

Just as encouraging has been the recent surge in scientific attention. Western Australia hosted the first International Whale Shark Conference in May 2005, bringing together scientists, resource managers, and conservationists from more than twenty nations. Perhaps the most valuable outcome of the meeting is still being played out, in the relationships and collaborations it nurtured among interested parties from around the world. If science can improve human understanding of the whale shark, future generations will be able to indulge the simple pleasure I have enjoyed—the chance to swim with the biggest fish in the sea.

Steven G. Wilson earned his doctorate from the University of Western Australia in 2001 for studies on the physical and biological factors that affect the aggregation of whale sharks at Ningaloo Reef, off the coast of Western Australia. Wilson had earlier worked as a high school biology teacher, dive-boat captain, and pearl diver. His research focuses on the migratory movements and vertical behavior of large pelagic fishes: tunas, billfishes, and sharks. He holds an appointment as a postdoctoral research scientist at Hubbs–SeaWorld Research Institute in San Diego, California. This May, Wilson will return to Ningaloo Reef to continue his studies of the whale shark.

Copyright © Natural History Magazine, Inc., 2006

Return to Web Site Archive