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Giant sharks swarm in waters off Mexico

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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