OFF EXMOUTH, AUSTRALIA — At the cry of ''Shark!'' we got ready to jump in the water. No, we had not been in the tropical Australian sun too long. My wife and I had joined eight other people in pursuit of whale sharks - the largest fish in the world.
We could all confidently jump in the water because the whale shark is not a predator but feeds on plankton. With filters in its giant mouth, the whale shark meanders along with mouth open, straining out the microscopic organisms.
Once a year these fish, which can grow as long as 36 feet, arrive outside Ningaloo Reef on Australia's far west coast. Marine biologists believe the sharks come to mate and feast on the coral blooms that usually spawn around the beginning of April. The timing, however, is something only known to the coral and the sharks since it depends on the moon, the tides, and the whim of whale sharks.
Once we've jumped in the warm waters, the captain of the boat, Steve, points toward the whale shark, which he can see from the flying bridge 20 feet above us. Wearing flippers and snorkels, whale-shark viewers quickly intersect with whale shark.
Suddenly, in the blue water of the Indian Ocean, there's a giant mouth moving from side to side. From the front of the shark, one can see the giant tail slowly fanning the water. We are so close you can see gill slits open and close.
The first fish is young - perhaps only 18 to 20 feet. The skin looks like an aboriginal painting: The gray background is covered with yellow dots, swirls, and lines. The color seems unique to the juveniles. The ''artwork'' on the adults is white on gray. At some point these sharks may have needed the dots and swirls as camouflage against predators.
After a few vigorous minutes of swimming with the shark, it rolls on its side - a rare occurrence that day - flicks its tail, and quickly demonstrates that whale sharks can swim faster than humans.
Before the shark leaves, however, the gaggle of snorkelers look down on a sea turtle six feet across, swimming directly under the shark. Underwater cameras click. My wife quickly tells me, ''I'm hooked.''
Spotting the sharks is relatively easy. The captains of the dozen boats that left from Exmouth had pooled their resources to hire a search plane. In long circles, the plane cruises over the outer reef - about two miles offshore. The whale sharks are about 1,000 feet off the reef in water clear enough to see the bottom 100 feet below.
The rest of the morning is spent pursuing sharks. Some are spooked when divers sink below them and send silver bubbles past them. Where does a scared shark go? Into deeper water and out of sight.
Other whale sharks seem ready for Hollywood. They practically swim over and say ''G'day.'' We swim with one such shark for nearly an hour, since it stayed close to the surface.
On our boat are Ron and Vallerie Taylor, Australian divers who make a living off their knowledge and curiosity about sharks. Ron recognizes some of the whale sharks from his trip to Ningaloo Reef the prior year. As we tread water, I ask Ron about the life span of whale sharks. He thinks they might live 50 to 100 years, but concludes: ''No one knows,'' because ''we haven't been aware of them that long.''
We quickly note that the whale sharks do not travel through life alone. Darting in and out of their mouths are black-and-white-striped pilot fish. A bright yellow fish looks for plankton on its way into one black mouth. Other sharks carry remoras, who hitch rides on the bottom of the fish. Still others have iridescent blue wrasse that flit in and out of the whale's gills, cleaning bacteria off them as they open and close.
Although the whale sharks are harmless, snorkelers on one of the other boats get a glimpse of a 14-foot long tiger shark swimming along the ocean floor. We are told that there are plenty of other predator sharks here, since there are plenty of fish for them to feed on. On the other side of the peninsula is Exmouth Gulf, with its spectacular coral reefs. Locals claim it is not unusual to see orcas and humpback whales and their calves in the bay.
WE had originally planned to only go as far as Coral Bay, about 130 miles north of Carnarvon, which is about 540 miles north of Perth. But as we were eating dinner one night in isolated Coral Bay, one of the locals asked if were planning to go to Exmouth. ''Why should we?'' I asked. ''Well, the whale sharks are in,'' he answered.
Without knowing much about whale sharks, we booked a room in Exmouth. The next day we left early for the 90-mile drive up the Northwest Cape.
Typical of this part of Australia, there aren't many people on the road. But, there is lots to see along the ''bitumen,'' as Australians call the asphalt highway. Termite mounds, five-feet high, look like chimneys sprouting from the earth.
Exmouth itself is used to Americans since the US Navy still uses a base there for its communications with submarines. Once we had checked into our motel, we walked to the nearest dive shop. I had already called from Coral Bay to book us for a snorkel trip to the gulf side. However, the owner told us the trips out to the whale sharks were sold out for several days.
As we mulled this over, the phone rang. One of the locals, scheduled for a whale shark trip, was willing to postpone her trip for a few days. ''Turn around,'' he ordered my wife.'' As she complied, he said, ''I just want to see the size of your wallet.''
No question about it, a whale shark expedition is expensive at about $300 per person. But, we reasoned, how often do you get to see the world's largest fish?
The next day we snorkeled in the bay. Although it was a bit choppy, the coral and fish were dazzling. Because the water was deeper, the fish were much larger than the ones we had seen off Coral Bay. The highlight of that trip was following a two-foot turtle as it foraged among the corals. It whetted our appetite for the whale sharks.
The next day a van picked us up early in the morning. As we drove to the Indian Ocean side of the peninsula we saw some wild emus on the side of the road. Shortly afterward, we saw camels, introduced to Australia from Afghanistan decades ago.
An underinflated rubber boat ferried people and equipment to the boat, about a 30-footer out on its mooring. Although it was windy and choppy, the captain announced conditions could be different on the other side of the reef.
He was right. The wind died down and a big swell rolled over a glassy sea. And then we heard the cry, ''Shark!''