Whither whale sharks? What 9-year study reveals so far about mammoth fish.
One tidbit is that the plankton-eating whale sharks like to feed in waters northeast of the Yucatán Peninsula. The study's aim is to learn the species' migration patterns in the western Atlantic region.
Hundreds of whale sharks can't be wrong: Dine off the northeast tip of the Yucatán Peninsula.
In what may be the largest study of its kind globally, and certainly the largest for the western North Atlantic region, researchers have tracked the travels and travails of these enormous sharks to and from feeding grounds off Mexico's state of Quintana Roo – a whale-shark hot spot that the scientists describe as one of the most important population centers in the world for the species.
Over nine years, scientists tracked the sharks as they spread throughout the Gulf of Mexico, the Caribbean, and out through the Florida Strait after the Quintana Roo's offshore diner shuts down at summer's end. One adult female, dubbed Rio Lady, ventured as far as the central South Atlantic off the coast of Brazil. In 150 days, she covered just shy of 4,500 miles as the crow flies – but far more as the fish swims – before she shed her tracking tag. There, the researchers suggest, she may have given birth to pups.
The scientists also note that a significant number of whale sharks bear scars from collisions with boats, raising concerns that collisions with larger vessels that ply the shipping lanes off the coast may be killing some of these creatures.
Even the 2010 Deepwater Horizon blowout and subsequent oil spill may have affected these plankton-eaters.
Early in the summer of 2010, a "pulse" of sightings was reported along Florida's Gulf Coast, indicating that the whales were traveling much closer to shore than they typically do, notes the team, led by Robert Hueter, a marine biologist at the Mote Marine Laboratory in Sarasota, Fla. It remains unclear if the whales were avoiding oil and dispersants, or merely were attracted to waters rich in plankton, one of the whales' staple foods.
The study's immediate aim is to unravel the mysteries behind whale-shark migration in the western Atlantic region. This study and comparable ones around the globe are aimed at preventing the sharks from slipping into further trouble on the International Union for Conservation of Nature's conservation-status list. Whale sharks are listed as vulnerable – the lowest of three levels within the IUCN's "endangered" category.
Until about 10 years ago, whale sharks were targets for commercial fishermen in parts of Asia. They reportedly are still hunted in the region, although much less intensively.
Concerns over the sharks' future stem from the length of time it takes for them to reach reproductive maturity, about 30 years. They also stem from increasing coastal development, which can introduce pollutants to the whales' feeding grounds.
Even ecotourism, a key reason for conserving the fish, represents a mixed blessing. Snorkeling tourists visiting whale-shark feeding grounds can help researchers by photographing the fish or noting the information on tracking tags and passing any photos or other information back to scientists. But the boat traffic can get so heavy that it may affect whale-shark behavior.
Whale sharks appeared in the fossil record about 60 million years ago. They are the largest fish in the oceans and the largest animals, outside of whales, in the world. Adult whale sharks typically reach 30 feet long from tip of snout to tip of tail, and weigh upwards of 10 tons. Unlike their smaller, carnivorous cousins, they feed on plankton and fish eggs.
To work among them is "absolutely breathtaking," says John Tyminski, another Mote Marine Laboratory scientist who took part in the study.
Recalling his first encounter with a whale shark in 2003 as the project was getting under way, Dr. Tyminski says, "It's like swimming with something the size of a school bus next to you." One can sense its enormous swimming power. And its eyes clearly track alien visitors, so "it knows you're there, but it ignores you. It's spectacular."
Throughout the nine-year period, the team tagged 812 of these giants with markers that serve as a kind of luggage tag, complete with a tag number and a phone number to call if you spot it. Later, the team used satellite tags on another 35 whale sharks, which allow more consistent tracking until the fish sheds its high-tech passenger.
In addition, the team, which includes Rafael de la Parra, a marine biologist in Mexico who focuses on whale-shark conservation, used photos of 914 whale sharks as a means of identifying and tracking individuals. The sharks sport arrays of white dots on their bodies in patterns unique to each individual.
The scientists found that not only are summertime waters off of Quintana Roo packed with hundreds of feeding whale sharks, but also that many are repeat customers from one season to the next. Some individuals visited the site annually for at least six years. The menu: plankton, and eggs and fry from a species of tuna known as little tunney, which also use the area as spawning grounds. The eggs are the whale-shark equivalent of caviar, Tyminski quips.
While whale sharks of all ages were dining, about 72 percent of individuals feeding in the area were male.
The researchers were producing so much information as the project progressed that in 2009 the Mexican government moved to protect the area, according tho Dr. Hueter.
From a conservation standpoint, feeding grounds are one set of areas to protect. But so are the marine maternity wards where the females give birth to pups. So far, woefully little is known about this aspect of the whale-shark life cycle, researchers say.
One hint comes from previous research that involved the dissection of a pregnant female who had died, Tyminski says. Scientists found that she was carrying more than 300 pups in various stages of development. This, combined with Rio Lady's excellent adventure in the southern Atlantic, prompted the team to speculate that she headed for deep, open waters to give birth – broadcast-spreader-style. Whale sharks don't drop all of the pups at once, but may drop them one by one over distances spanning tens to hundreds of miles.
Pups range in size from 15 to 24 inches at birth and grow quickly. But they also are vulnerable to predators.
If "you're not dropping all of them at one point, the odds are probably in their favor that some of them are going to be able to grow quickly and survive" before they become another fish's meal, Tyminski says.
Indeed, the team is hoping to focus on this aspect of whale-shark behavior next – driven in no small part by sightings of young whale sharks in the open Atlantic, as well as sightings of pregnant females in remote places such as the island of St. Helena. Presumably, if the island is remote enough to have kept Napoleon Bonaparte from causing any more trouble in Europe, it's safe enough for dropping whale-shark pups.
A formal report of the latest results have been published in the online journal PLOS One.