Image: Whale shark
Koichi Kamoshida  /  Getty Images
This whale shark is the key attraction at an aquarium in Okinawa, Japan. The Gulf of Mexico is home to other whale sharks that begin their migration to northern Gulf waters in June.
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updated 5/19/2010 5:15:46 PM ET 2010-05-19T21:15:46

The world's biggest fish may be highly vulnerable to the Gulf of Mexico oil spill and one of its favorite feeding grounds is in the area of the spill.

Whale sharks feed on the surface, sucking plankton, fish eggs and small fish into their mouths. Surface oil could clog the cartilage filter pads that direct food to the back of their throat, and could coat their gills while they feed, said Eric Hoffmayer, a researcher at the University of Southern Mississippi who has been studying whale sharks in the northern Gulf since 2002.

"If it did get their gills coated, I can only imagine they would suffocate relatively quickly," he said.

More than one-third of all whale shark sightings in the northern Gulf since 2002 have been off the mouth of the Mississippi River, Hoffmayer said.

"The mouth of the river is the primary area where these things show up. Year after year after year," he said. "This is a prime feeding area for them."

Oil has been leaking from a well 40 miles southeast of the river's mouth since the BP Plc-operated rig Deepwater Horizon exploded and sank in April.

Whale sharks can grow up to about 40 feet in length and are known to dive more than a mile beneath the surface. They were added in 2008 to the World Conservation Union's list of threatened species.

So far, Hoffmayer said, no whale shark has been seen in the spill. The vast majority are seen in northern Gulf waters from June through October as they migrate from the Yucatan area.

"The best case scenario is they can detect the oil and will avoid it," he said. Other types of sharks have been spotted near the oil.

Nobody knows how many whale sharks there are worldwide. Bob Hueter, director of Mote's Center for Shark Research, said they may number 500,000, a small number for fish.

He said he has sent an alert to the up to 300 people who've reported whale shark sightings to him over time, asking them to be on the watch.

"It's just critical," he said. "We're in desperate need of knowing where the animals are at. Where are they in relation to the oil spill?"

The number of sightings rose sharply last year, to about 160 from the average of 80, as more people learned about Hoffmayer's research. It was unclear whether there are more whale sharks or if more people are looking for them.

Last year's reports included some a mile off the Florida panhandle and Alabama coast. "That's the first we'd heard of in 25 years from that region," Hoffmayer said.

He said most of the whale sharks are seen in at least 200 to 300 feet of water and 30 to 40 miles off the river's mouth.

"Most is in 200, 300 feet of water at the very least. Thirty to 40 miles off the mouth of the river."

Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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