Whale Sharks Track Tiny Prey

August 17, 2007

Release from: Mark Davis
Atlanta Journal-Constitution

They came hundreds, maybe thousands, of miles to the Gulf of Mexico.

The whale sharks began arriving in late April and early May, long shadows in blue depths off the coast of Isla Holbox, Mexico. The world's largest fish comes for some of the smallest food imaginable.

"They gather there for one reason," said Mike Maslanka, a Georgia Aquarium nutritionist who visited Holbox Aug. 3-9 to study the fishes' eating habits. "And that reason is the plankton."

Plankton is the name given to a variety of small organisms that drift in the ocean. The plankton pushed into the Gulf of Mexico from the Caribbean teems with tiny shrimp, equally small worms and creatures called copepods, so miniscule they resemble reddish grains of sand to the naked eye.

The plankton lure the whale sharks from across the gulf and Caribbean; and, for the past few years, the Rhincodon typus have brought scientists to Holbox, a town with sandy streets and palms that rattle in the wind.

Maslanka was part of a group of about eight scientists and others who spent a week collecting plankton, tagging whale sharks and watching their movements. Maslanka, who oversees the feeding of the Georgia Aquarium's four whale sharks, came back with plankton he's studying to learn more about what whale sharks eat. He collected the plankton with a net whose mesh is finer than the cotton in a dress shirt. Some of the samples are in his office in containers the same size as One-A-Day Vitamin bottles; others are in petri dishes.

Whale sharks are found across the world, from Asia to Australia to Holbox, which may contain the planet's greatest concentration of whale sharks. They can grow up to 40 feet or longer by eating a steady diet of plankton. Scientists estimate that between 500-1,500 whale sharks gather off the Mexican coast from spring through September to feed.

The plankton off Holbox, scientists discovered, has a high concentration of minute, reddish shrimp called amphipods. In some places, said shark specialist Robert Hueter, the water contained so many of the tiny organisms that it had turned red. He joined Maslanka in the daily trips 20-25 miles off the Mexican coast to study the sharks.

"It was almost like a shrimp soup, if you will," said Hueter, who heads the shark research program at the Mote Marine Laboratory in Sarasota.

The water was so dense with plankton, he said, that sharks sometimes disappeared in the murk, only to reappear on the surface to eat.

"It was exciting to have sharks all around you," he said.

Maslanka, who has made two trips to Holbox, agreed. On the first day he ventured into the gulf, the water was choppy, the plankton, thick: At first, Maslanka said, he and others didn't see any sharks.

But they noticed the plankton, and relaxed.

"We knew they were out there," he said.

The aquarium is still compiling information which will be published in scientific journals. It also plans to send another official to Holbox late next week for an international whale shark conference.