The world's biggest fish are hungry migrators on a mission, according to a tracking study that mapped whale sharks' long journeys around the Gulf of Mexico and Caribbean to a favourite feeding hot spot off the Yucatan Peninsula.
And one whale shark's incredible 7,200-kilometre swim could even help solve the long-standing mystery of where whale sharks give birth—an event no scientist has ever seen.
The largest-ever study of whale shark migrations, nine years in the making, shows that the hundreds of school bus-sized animals that feed in a plankton-saturated stretch off the Mexican coast come from far and wide.
The gentle giants—which can reach up to 12 metres or longer in length, and weigh an average of 5 tons—use mouth filters to feed on the tiny plankton and small fish or eggs.
Whale sharks are known to gather at a dozen major feeding locations around the world, from Western Australia and Indonesia to Belize. But between May and September, the waters of Mexico's Quintana Roo state, on the northeastern Yucatan Peninsula, draw far more animals than other spots and attract an estimated 800 or more in a given season.
"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," said study co-author Robert Hueter, director of the Centre for Shark Research at Mote Marine Laboratory.
"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."
A free-diving photographer encounters the world's largest fish, a giant whale shark, off of the coast of Mexico [Image: Mauricio handler, National Geographic Stock]
Analysing the Data
The reliable numbers and accessibility of whale sharks at the site prompted Hueter in 2003 to begin accumulating the nine years of tagging and satellite tracking data that formed the backbone of the recent study by the Mote Marine Lab and Mexico's National Commission of Natural Protected Areas.
The amount of time invested, and data collected, by the study's authors is nothing short of phenomenal, said whale shark researcher Mike Maslanka, the Head of Department of Nutrition Science at Smithsonian's National Zoo in Washington, D.C.
"The summer work we do [at feeding aggregations] is just a tiny snapshot in the life of a whale shark," he said. "These tagging efforts allow us to discover more about what happens when they aren't gathering to feed in the summer. Without the tagging we wouldn't even have a glimpse into that part of their lives. That's the really cool part of this study."
Maslanka added, "These things are so big, to think that they 'disappear' is pretty amazing. It's the largest fish in the ocean and we don't know where it goes for six months of the year."
(The whale shark research was partially funded by a grant from the National Geographic Society.)
Birthing Mystery Solved?
Among more than 800 individuals studied, one animal stood out.
A mature and presumably pregnant female called Rio Lady was tagged and then tracked along an odyssey of some 7,800 kilometres, which ended when her tag came off after five months of observation.
"She just kept going," Hueter said. "She swam out between Brazil and Africa until she passed the Equator, and that's where her tag came off."
But her journey, and other whale shark sightings in the remote region, could help answer a question that has plagued whale shark researchers for years: Where are all the females? Quintana Roo is more than 70 percent male, and other global aggregations show the same gender imbalance.
"You can't have a stable population with that many males. You don't see that in nature," Hueter said.
"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 explained. "In coastal zones where the feeding aggregations are, their young-which are less than two feet long at birth—might be subject to higher predation."
He added, "We feel good about the hypothesis, but it's out there to be tested. So now we'll have to see if it's proven right in the years to come."
Few very young whale sharks have been seen in nature. And discovering where the animals give birth, Maslanka said, is "the holy grail of whale shark biology."
But the story isn't as simple as finding out what area or areas they use to pup. The find would lead to greater understanding of basic whale shark biology, much of which is still lacking because so much of the animals' lives are lived out of our sight.
"And from the perspective of ecosystem management, we'd want to make sure that area was protected over time so they could continue to pup in an unmolested state," he said.
Protecting the Gentle Giants
Using this study and others to determine where the animals travel, feed, and reproduce is key to protection of a species that is becoming increasingly beloved by ecotourists and others.
"It's the largest fish in the ocean, and it's a real representative of healthy marine ecosystems," Maslanka said. "It can be a real flagship species for protecting the oceans, especially in the band that stretches around the Equator."
But as Hueter's group makes clear, conservation of the far-roaming animals will take international cooperation because whales spotted in one area may depend, in other seasons, on resources located many hundreds of miles away.
And while mating remains a mystery, whale shark genetics suggest that animals swap genes among far-flung geographic locales, and that only two large metapopulations exist—one in the Atlantic and another in the Indo-Pacific.
Each population requires management on a broad scale. The species as a whole is currently listed as vulnerable on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species and is still hunted for fins and oil in some Asian waters.
Hueter said he's encouraged that whale sharks can be protected by the process that's already begun, notably in his study area, with the Mexican government's designation of a Whale Shark Biosphere Reserve in the feeding aggregation grounds.
There's more work to be done, he cautioned, but the species definitely warrants the effort.
"This is the largest fish that has ever lived, and it's charismatic," Hueter said. "It poses no danger to people who love to see it and swim with it in the wild. It might be the largest animal on the planet that you can be close to in it's natural environment and not be in any danger whatsoever."