Whale shark sightings in the Gulf of Mexico baffle scientists

Jim Tharpe, The Atlanta Journal-Constitution 15 Aug 09;

Scientists say they are baffled by the soaring number of whale shark sightings this summer in the northern Gulf of Mexico, an area where the huge polka-dotted fish have been a rare spectacle in the past.

“The sheer number of anecdotal reports from the public is amazing,” said Sarasota-based shark scientist Bob Hueter. “There’s obviously something going on.”

Whale sharks usually gather in large numbers during the summer in plankton-rich waters off the Yucatan Peninsula. Even those concentrations are larger and more dense than usual this year — hundreds have been spotted in a single location. However, multiple whale shark sightings in the eastern and northern Gulf near the Florida coast is unprecedented.

Fishermen, helicopter pilots, divers and tourists have reported seeing groups of the bus-sized sharks from Clearwater to the Florida Panhandle and along the Alabama, Mississippi and Louisiana coasts.

Hueter, whose whale shark research is partially funded by Atlanta’s Georgia Aquarium, said a massive “loop current” in the Gulf changed course a bit this year. There is also a stronger than usual upwelling of cold, nutrient-rich water off the Yucatan this year, he said.

“That changed the oceanography a bit, and it could have driven some of these animals up into the northern Gulf,” Hueter said.

Whale sharks, which can grow to more than 40 feet, are solitary filter-feeders that occasionally gather in large numbers to feast on seasonal plankton blooms. Little is known about the gentle giants, four of which are housed at the Georgia Aquarium.

Eric Hoffmayer, a biologist with the Gulf Coast Research Laboratory on the Mississippi coast, said his lab has gotten reports of 30 sightings in just the last two weeks. Hueter said sightings in the Florida Gulf are running four times the normal rate.

“We don’t know what’s going on,” Hoffmayer said. “We’ve had reports of 15 to 20 whale sharks in one area.”

On Aug. 1, Hoffmayer’s lab got a reported sighting of more than 100 whale sharks congregating about 60 miles off the Louisiana coast. Many of the sighting are occurring on the full moon, he said, suggesting the sharks there are massing to feed on fish eggs.

Hueter said the increased northern Gulf sightings correspond to this summer’s unusually large whale shark gathering — called an aggregation — near Isla Mujeres off the Yucatan near Cancun. The Yucatan aggregation is an annual phenomenon, though it usually occurs near Isla Holbox.

An aerial survey last week found more than 400 whale sharks in a relatively small area near the Isla Mujeres.

“They have been packed in there like sardines, 25-foot-long sardines, and they are staying there the entire summer,” Hueter said.

Hueter said records at Mote Marine Laboratory, where he heads the Shark Research Center, indicate there were only three whale shark sightings in the Florida Gulf in 2005, two in 2006, five in 2007 and three last year. But there have already been 12 just from July to mid-August of this year.

A few weeks ago, five whale sharks appeared just offshore along Grayton Beach near Destin, where thousands of metro Atlantans vacation every summer. The sharks apparently remained in local waters for two days before departing for points unknown.