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International Wildlife

World’s Biggest Fish

World’s Biggest Fish – the great whale shark in Western Australia

Why tourists and researchers are flocking to Western Australia to swim with school-bus-sized whale sharks

My god, look at the size of that one,” shouts skipper Mark Small. An enormous gray shadow as big as a bus passes silently by the bow of our boat as we motor in the Indian Ocean two miles out from the coast of Western Australia.

Nervous anticipation runs through the boat’s passengers, a mix of tourists and scientists. We are about to jump overboard to swim with whatever is out there.

Small whirls the helm and calls out to a crew member to keep an eye on the marine leviathan. Even veteran shark-watchers such as marine biologist Dennyse Newbound are keyed up. “This is always the anxious moment for me,” says Newbound, a University of Western Australia graduate student. “Is it a whale shark or not?”

Hopes of seeing a whale shark have drawn all of us here to Ningaloo Reef, a marine park near the town of Exmouth. Despite its plankton diet, passive nature and enormous size (as much as 17 tons), the whale shark is not a species of whale. This cold-blooded giant is a shark–the world’s largest fish. And we want to meet it up close.

Some of us are here for the sheer thrill of the encounter; others, to learn more about this little-known creature. Even though they are big, whale sharks are elusive. Until the 1980s, few people had ever seen one. Then came the discovery that some of the fish gather yearly at Ningaloo, a finding that sparked the development of an ecotourism industry here.

Now, along with the adventuresome tourists, scientists such as Newbound are coming to Ningaloo to answer basic questions: How long do they live? How large do they get? How many of them are there? Most important, Newbound wants to know if these whale sharks stay near the Australian coast or migrate to other areas where they may be hunted.

For a few months every year, a group of these large filter feeders come to this reef 850 miles north of Perth. Here the sea bottom plunges from 30 to 600 feet; the continental shelf meets the deep ocean’s cold, nutrient-laden currents; and the color of the water changes rapidly from robin’s-egg blue to dark indigo. The whale sharks usually start arriving a few weeks after the first full moon in March, creating a predictable shark-watching season each austral fall. This year, however, the sharks are nearly two weeks late, worrying the 14 licensed tour-boat operators who charge more than $200 per person to snorkel with them.

Fortunately for us, the dry spell breaks. A search plane radios in the location of large, tadpole-shaped shadows, and we head for a site out in the open ocean. But before we reach it, the captain spots what appears to be another whale shark. The crew moves into action: One crew member (called a “spotter”) jumps overboard and swims over to identify and pinpoint the creature, while Small maneuvers us into its path.

As we prepare for the signal to dive, Newbound reaches for the sheath strapped to her shin and stows away her most crucial item: a pair of tweezers. She will use these to remove a sample of quarter-inch-long crustaceans called copepods from the skin of the whale shark. With the aid of these tiny parasites, she hopes to help solve at least one mystery about Ningaloo’s whale sharks: Where do the fish go when they leave this reef in late May?

Some researchers, such as shark biologist John Stevens of the Commonwealth Scientific and Industrial Research Organisation in Tasmania, suspect these sharks may travel to Christmas Island, south of Java, to feed during an annual crab spawning there. Stevens, who literally wrote the book on sharks in Australia, thinks that whale sharks are opportunists, taking advantage of mass spawnings to scoop up bumper crops of food from different locations.

Whale shark research is still in its infancy. Until three years ago, scientists did not even know if these sharks laid eggs or bore live young. (Whale sharks actually do a combination of both, creating eggs that hatch inside their body.) Researchers know that the sharks are also found in warm waters off North America, Africa, Southeast Asia and elsewhere, but no one knows their total population, or whether their numbers are increasing or decreasing. As a result, no one can say if they are endangered. But threats loom: In Taiwan, the Philippines, Thailand and Indonesia, fishermen have been killing several hundred of these animals per year. Middlemen sell the fins to Taiwanese restaurants for $100 per pound. (Whale shark hunting was recently outlawed in the Philippines, but some conservationists doubt this will stop the practice.) “A hundred whale sharks killed may not sound like a lot,” Newbound says, “but it could spell trouble if their numbers are already low.”

Before anything can be done to safeguard Australia’s whale sharks, biologists need to know where they travel. Perhaps the sharks drift just out of sight off the coast and then return to Ningaloo occasionally to gobble up plankton. Or they may migrate from west Australia to Christmas Island to Indonesia and the Philippines.

That’s where co-pepods come in, says Newbound. While our ship rides the swells, she describes how copepods, which resemble little horseshoe crabs, feed on the algae and bacteria growing on the whale shark’s skin. The particular species she’s studying cannot travel far unless they hitch a ride on their hosts. And since these copepods occur only on whale sharks, they serve as handy “biological tags.” If DNA studies of copepods here on Ningaloo show that they are identical to copepods in Indonesia, then they must be riding on whale sharks. In this indirect way, studies of copepods may show whether Australia’s whale sharks migrate.

Before I can ask her for more details, we are interrupted. The spotter has finally found the shark and is giving the signal. The captain yells, “Go, go, go,” and we plunge over the side.

Entering the water, I look directly downward and see nothing. I start to raise my head and then notice what looks like a submarine looming towards me. The 20-foot-long whale shark is only a few yards away. It slowly glides to my lower left with 6-foot-wide jaws open, vacuuming up plankton. An armada of smaller fish swim underneath, coasting on its bow wave. As the whale shark passes beneath me, I notice white spots on its dark gray upper half. This coloration may act as camouflage; when seen from above, the shark’s skin looks like the sun-dappled ocean floor.

I try to keep clear of the whale shark’s tail, which is taller than a man. That slow-moving tail fin is deceptive: Enormous energy is required to move such a large object under water, and the back half of a shark is practically all muscle. An accidental wallop from a tail like that broke someone’s ribs two weeks before. This latent power enables the shark to speed up easily. Nevertheless, we manage to keep it in sight until the leviathan tilts downward. The big fish submerges steadily, the blue of its body merging with the blue water until only its spots remain before fading into the background.

“That was amazing,” says Newbound after we climb back aboard. “My hands are still shaking; my heart’s still pounding.” She tries to explain the whale shark’s appeal, noting its differences from other sharks. Whale sharks aren’t aggressive like tiger sharks, for example. Nor are they timid, like gray reef sharks, which are known to bluff their way out of trouble. To Newbound, the calm demeanor and massive size of whale sharks make them seem above all that: “The mystique is part of what draws people to them.”

But what draws whale sharks to this isolated reef every year? At first the coral spawning was thought to be the key. For a short but intense period every year, coral produce hundreds of tons of protein-rich sperm and eggs. This food source attracts krill, crab larvae and mantis shrimp, which in turn attract bigger fish, including whale sharks–in theory.

The only problem is that the coral spawn is a very brief event that does not have time to work its way up the food chain, says Chris Simpson, the head of Western Australia’s Department of Conservation and Land Management. In addition, tour-boat operators have reported seeing whale sharks arrive long before or after the spawn occurs. This year, few sharks appeared until well after the coral spawn.

Simpson suspects many things are spawning simultaneously at that time, and that upwellings of deep ocean currents are bringing crucial nutrients to the surface at about the same moment, producing a nutrient-rich seafood soup for whale sharks every autumn. But no upwellings have yet been located on the Western Australia coast to help prove this theory.

Despite these unknowns, knowledge about whale sharks is gradually increasing, Stevens says. For instance, researchers have found that Ningaloo’s whale sharks tend to segregate themselves by age and gender. Almost all the ones off Ningaloo are juvenile males, staying in a loose group called an “aggregation.” This is similar to the behavior of many other sharks, which set aside areas known as “nurseries,” where their young stick together for mutual protection.

Scientists have also learned that a female whale shark can potentially give birth to a few hundred young, called “pups.” But researchers don’t know how fast the pups grow or exactly when they become sexually mature. If whale sharks reach sexual maturity late in life like other sharks, their population may not be able to grow quickly enough to recover from overfishing, experts say.

On the positive side, tourism does not appear to be harming whale sharks. Only 10 people are allowed in the water at a time with an individual whale shark; a total of 2,640 people swam with these sharks last year. Brad Norman, a marine biology graduate student at Murdoch University, has studied the interaction between whale sharks and humans since 1995. He has swum with the creatures hundreds of times and never seen them show irritated behavior. If anything, he says, these sharks are “too bloody docile.”

However docile they may be, trying to find out more about their possible migration is difficult, according to Stevens. He cites the attempt he and his colleagues made earlier this year to put radio-transmitting satellite tags on whale sharks. The scientists tried to attach the tags behind the shark’s dorsal fin with small darts. The darts just bounced off one shark’s skin, which is underlain by a blubberlike layer about 6 inches thick. Out of three successful taggings, only one stayed on for more than 10 days–too small a sample for statistical significance.

Although Newbound’s work doesn’t involve darts, it also can be a hit-or-miss proposition. On our next dive, we come across a whale shark that makes the previous one look like a guppy. Newbound submerges, checks its gender–another juvenile male–and, to her disappointment, finds that it has no parasites whatsoever. When she goes to the boat log to enter the size of this latest shark, she sparks a debate about the fish’s length. The crew agrees that it’s one of the biggest ever seen here, but disagrees on a figure. Estimates range from 29 feet to 36 feet; finally everyone agrees on 33 feet for the log entry.

The exchange illustrates a common problem in fieldwork: getting accurate data. Fishermen’s tales are notorious for exaggeration. While in Exmouth, I’d been told that wearing a gaudy wetsuit will attract a whale shark, that running a hose over the side of a boat will bring them over to investigate, and that if you scratch a whale shark on the chin, it will float vertically and rub up against you like a kitten.

More such stories emerge that evening, when Newbound and the crew relax at an outdoor cafe with bottles of Emu Bitter beer. As the Southern Cross constellation blazes above, tour-boat captains ask if anyone has seen the shark called “Stumpy,” a local favorite easily recognized by a stunted pectoral fin. Then someone relates a tale about the day a plane radioed down the coordinates of a large dark shape in the water and the spotter jumped in–only to find the giant fish was not a whale shark but an enormous, man-eating tiger shark. Luckily, it wasn’t hungry.

I wonder aloud if Newbound was referring to this case of mistaken identity when, on our first dive, she asked, “Is it a whale shark or not?”

The cafe owner responds reassuringly: “Don’t worry, we’ve got the most well-fed sharks in Australia.”

American science writer Dan Drollette spent a year in Australia on a Fulbright traveling scholarship.

COPYRIGHT 1999 National Wildlife Federation

COPYRIGHT 2000 Gale Group