Giant sharks swarm in waters off Mexico

By Jim Tharpe
Special to The Washington Post
Monday, March 7, 2011; 6:37 PM

ISLA MUJERES, MEXICO - U.S. and Mexican scientists believe they are close to solving one of the shark world's great mysteries.

They want to know why whale sharks, the largest shark species, gather each year by the hundreds in the teal-blue waters off this Yucatan Peninsula barrier island.

"It's like a fishbowl full of whale sharks," said Robert Hueter, director of the shark research center at the Mote Marine Laboratory in Sarasota, Fla. "We are witnessing a spectacle of nature down there that we don't fully understand."

Hueter, Mexican biologist Rafael de la Parra and a group of other marine scientists are trying to make sense of the big shark reunion, an event dubbed the "afuera." The word means "outside" in Spanish, and it was the name de la Parra initially used to describe to the phenomenon.

"They are showing up in an area outside the area the Mexican government set up for their protection and outside the area we had normally studied them," de la Parra said.

As research coordinator for Mexico's Domino Project (Mexican fishermen call whale sharks dominoes because of their polka-dot spots), de la Parra has been studying whale sharks off the Yucatan for eight years. "These will be some of the most studied sharks in the world," he said.

Scientists have collected data on water temperature, salinity, oxygen content and types of fish eggs and plankton found at the afuera site. The researchers have documented the length and sex of as many of the sharks as possible and have taken genetic samples and photographs to identify individual sharks. They have also attached visual and satellite tags to many of them, the latter being used to electronically track the animals after they leave the afuera site and head for open ocean.

They are just beginning to explain the mystery of the huge sharks, often described as gentle giants. If toothy predators such as great white sharks (Carcharodon carcharias) are the pit bulls of the shark world, whale sharks (Rhincodon typus) have the temperament of Labrador retrievers. They are harmless to humans. Ironically, the biggest shark in the sea dines on some of the ocean's tiniest food offerings, plankton, small crustaceans and fish eggs, sieving them out of the water through devices near their gills.

Whale sharks are pelagic (ocean roaming) and spend most of their lives swimming alone, traversing thousands of miles of open water in search of food and mates. They are found in most of the world's temperate oceans and occasionally show up in large numbers at events known as "feeding aggregations." They have spawned thriving eco-tourism businesses in places as far-flung as Australia, Africa, the Philippines and Belize.

A slow-moving frenzy

Hundreds of whale sharks appear off the Yucatan barrier island of Holbox each summer to feast in a large swath of ocean turned pea green by a massive plankton bloom. But the Holbox sharks are spread over dozens and even hundreds of square miles in often murky waters.

The afuera is extraordinarily different. Here the giant sharks are concentrated in clear water in an area that can be as large as a few square miles or as small as a football field. Visibility in the blue water is often 100 feet. During the afuera, the huge sharks swarm at the surface, their massive mouths agape, vacuuming up a feast of tiny fish eggs in a chaotic, slow-moving feeding frenzy.

"It's easily the most amazing biological phenomenon I've ever seen," said biologist Al Dove, senior scientist at the Georgia Aquarium in Atlanta. "Even as a scientist, it takes you back to an almost childlike state of wonder, a state of absolute awe."

Scientists know that the phenomenon has taken place near Isla Mujeres with some regularity during the past decade: It occurred during the summers of 2005, 2006, 2008, 2009 and 2010, de la Parra said. "I have photos [of an afuera] from 1991, but we thought it was very sporadic," de la Parra said.

Some scientists believe the afuera is directly tied to another spectacular event, one that goes unseen by humans. Every year, huge schools of little tunny, an abundant member of the tuna family, arrive to spawn in the warm Yucatan waters. The countless tiny, clear eggs produced by the little tunny drift on the currents and are sometimes concentrated in a relatively small patch of ocean. This is the spot, scientists believe, where the afuera occurs.

"The spawning probably happens at night," Dove said. "It has to be a huge aggregation of fish because it can support a massive aggregation of whale sharks for a few months."

Finding the afuera can be a daunting challenge. It takes place in open ocean, well out of sight of land. Conditions have to be perfect for humans to witness the spectacle, and that might happen only a few times each summer.

But on a flawless day in late August last summer, the afuera blossomed in its full glory. Gener Coral, the captain of our boat, throttled back his craft's engines after a 45-minute sprint through light swell to a spot about 10 miles offshore.

'They are here'

"There," he said, pointing through the glare of the midmorning sun as a half-dozen large dorsal fins split the water ahead. "They are here."

Our 30-foot craft slid silently into a swirling mass of massive sharks. On this day, the afuera was contained to about a half-square-mile of ocean. Aerial counts later revealed there were about 180 whale sharks on the surface competing for food at this spot.

A few dozen boats eventually arrived with eco-tourists, scientists and locals who ventured out to witness the spectacle. The sharks seemed oblivious to the human commotion around them. They swam right up to our boat, slowly submerging to clear the craft and then resurfacing on the other side. You could literally reach out and touch them if you wished, though scientists strongly discourage any direct contact with the big fish.

"It's like you could walk across them," Coral said. Most of the Yucatan sharks are adolescent males, and most range in size from 15 to 25 feet. The occasional sighting of a 35-footer brings a special sense of awe. A grown man could curl up and lie down in the mouth of the largest whale sharks, which can grow to more than 40 feet and weigh 40 tons. The shark species had the misnomer "whale" attached to its name by the 19th-century British doctor who identified it as a separate species.

Human fascination with whale sharks and with aggregations such as the afuera has some scientists worried. About 250 boats based in tourist towns along the Yucatan have obtained permits to ferry out eco-tourists to get a closer look. Though regulations require the boats to keep a distance and swimmers to stay several feet from the sharks, our captain pointed out a 20-foot-long shark whose dorsal fin had been lacerated, apparently by a boat propeller.

"We are very lucky, and yet we have a huge responsibility," de la Parra said.

"We have to make sure we don't love these animals to death," Dove said. "They are something to be marveled at and then protected."

Tharpe is an editor at the Atlanta Journal-Constitution.

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