Nine-year Study Reveals Whale Shark Mysteries

After a nine-year study, researchers have a better understanding of whale shark migration patterns and are a few steps closer to reaching what one scientist calls the "holy grail" of whale shark biology.

Whale sharks are the largest fish ever known to have lived. The gentle giants of the sea are not well understood, but as a result of the study by Mote Marine Lab and Mexico's National Commission of Natural Protected Areas, researchers now have a better understanding of where the fish travel as well as a good idea of where the 10-ton, school bus-sized fish go to give birth.

The study area centered on an area of open water off the coast the state of Quintana Roo, which is northeast on Mexico's Yucatan Peninsula. There, more than 800 whale sharks congregate for summer feeding on the rich concentrations of plankton in the water. Many of the whale sharks in the study travel huge distances each year to get to the feeding grounds.

Robert Hueter, director of the Center for Shark Research at Mote Marine Laboratory in Sarasota, Fla., organized a project to tag the sharks in the feeding ground. After tagging hundreds of sharks across the nine-year study, Hueter and his research colleagues found that many of the sharks return to the feeding ground annually, but then swim off in random directions after having their fill of plankton.

"From this one feeding area, these animals spread out over vast parts of the region -- throughout the Gulf of Mexico, down into the Caribbean Sea, through the Straits of Florida up into the open Atlantic Ocean," Hueter told National Geographic. "We found animals coming back for as many as six years at a time. Clearly they are returning to this site to fuel up on the rich food that's there to carry them through much of the rest of the year."

One noticeable characteristic of the sharks off the coast of Quintana Roo is that they are 70 percent male, which the researchers say is odd.

"You can't have a stable population with that many males. You don't see that in nature," Hueter said.

But one particular female whale shark (which the team named Rio Lady) gorged on plankton then swam out to the middle of the Atlantic Ocean between Brazil and Africa where she seemingly loitered around for a noticeable amount of time.

"We think that Rio Lady has led us to the place where this particular species gives birth," Hueter told NPR's Christopher Joyce.

"The females have to be somewhere, and we hypothesize that mature, pregnant females undergo long migrations to the middle of the ocean, near seamounts or remote islands ... and that's where they give birth," Hueter told National Geographic.

Mike Maslanka, a whale shark researcher at the Smithsonian's National Zoo in Washington, DC, told National Geographic discovering where whale sharks give birth is "the holy grail of whale shark biology."

"For most sharks, that's oftentimes the million-dollar question ... where they give birth," Demian Campbell, a shark researcher Stony Brook University in New York, told NPR. "Nursery areas are places usually close to the coast, so giving birth in the middle of the ocean is a fairly unusual thing for sharks to do."

The results of the whale shark study are published in the journal PLOS One.