Waiting on a Whale Shark

Finally armed with an old enough specimen, researchers use biological markers from atomic bomb testing to answer questions about how long these elusive, endangered creatures live.

In 1947, adrift on the Pacific Ocean in a balsa wood raft known as the Kon-Tiki, Norwegian adventurer Thor Heyerdahl saw a “sea monster.” The four-foot-wide mouth, the head “broad and flat” with two small eyes, the “enormous body” that could have been as large as the 30-foot raft itself, dotted with bright spots, and the tall fin marked it not as a whale but something else entirely. He took a moment to study the “zebra-striped pilot fish” that swam companionably in front of the “gaping jaw,” and the remora fish that stuck to side and fin. For Heyerdahl, the leviathan became instead “a curious zoological collection crowded round something that resembled a floating deep-water reef.” He had seen a whale shark, the largest fish in the sea.

photo of whale shark
Researchers believe whale sharks might live to be 100 years, subsisting solely on plankton and small fish. Photo by Simon Pierce.

Little did Heyerdahl know scientists would one day use similar methods to determine the ages of whale sharks and the coral reefs they reminded him of with the unlikely help of biological markers left by atomic bomb testing during the Cold War. In fact, just the year before Heyerdahl set his raft adrift on the open ocean, the United States began a series of bomb tests, the radioactive residue of which would create a measurable spike in the carbon in the atmosphere on the land and in the water, that would concentrate in somewhat predictable levels in the bodies of some ocean dwellers, if scientists knew where to look.

For a species that has been around for at least 30 million years and that was first described by scientists in 1828, whale sharks remain a mystery, as they reside mostly in the open ocean. Off the Galapagos Islands, near Darwin’s Arch, full-grown female whale sharks pass on a migratory path witnessed only occasionally by those who don scuba gear, dive into the fast-moving current, and hold onto a rock. Off the coast of western Australia near Ningaloo Reef, divers wait on boats for a spotter plane to radio in the location of a whale shark, typically one of the immature male whale sharks that flank the shore. But almost never are males and females seen together. Scientists have never witnessed mating. They don’t know where the newly born and young hang out. When whale sharks die, most drift to the bottom of the ocean, where they decompose and form the basis of an ecosystem. It’s not “whale fall,” but elasmobranch fall. (Whale sharks are part of the elasmobranch family, the name derived from the Latin words for “beaten metal” and “gills.”)

Until recently, scientists didn’t have access to the carcass of a whale shark old enough to conduct radiocarbon dating research. But in 2012, a group of fishermen off the coast of Pakistan in the Arabian Sea, floated a 32-foot whale shark to shore, which may have died as a result of being caught in fish nets. It took two large cranes to lift the fish out of the water. A circus of sorts followed as possession of the whale shark moved from a fish dealer charging spectators to enter an unrefrigerated tent to see the decomposing creature, to government officials who seized it, and finally to the Pakistan Museum of Natural History. The last relocation occurred after the vertebrae were removed by a team of scientists when they conducted a necropsy, says L. Paul Fanning, who at the time was the Chief Technical Advisor on a project run by the Food and Agriculture Organization of the United Nations with the Marine Fisheries Department of Pakistan.

Fanning, who would become a contributor to this study, contacted whale shark specialist and former colleague, Steven Campana, then at the Bedford Institute of Oceanography in Canada, and arranged for a section of vertebrae to be set aside for Campana. For some time, Campana and Mark Meekan, a research scientist with the Australian Institute of Marine Science, had been thinking about how to solve mysteries around whale shark growth and longevity, since these factors often drive conservation work. Do whale sharks live 50 or 100 years? Do they grow up to 12 or 20 meters? Knowing how quickly a whale shark grows can help conservationists estimate the ages of those they encounter in the wild. And this information helps scientists plan for species recovery efforts, for how early and how often a species multiplies can determine how long it might take to replenish a stock that is endangered. (In 2016, whale sharks were added to the IUCN Red List of Endangered Species after it was estimated that the global population of whale sharks had decreased by over 50 percent in the last 75 years.)

photo of whale shark
Research scientist Mark Meekan swims with a whale shark off the coast of Western Australia at Ningaloo Reef. Photo by Wayne Osborn.

Sections of the vertebrae of the whale shark from Pakistan were also sent to Taiwan Ocean University and stored along with samples from 19 other whale sharks, most of which were much smaller and younger at the time of death. In this giant freezer, a suitable amount of material for study awaited, declared Campana and Meekan. The pair came up with a two-pronged approach: first, cross-section whale vertebrae to look at the growth rings (one potential method of determining age in a whale shark); second, use radiocarbon dating with sample rings from the two oldest whale sharks to verify when bands were created. They wanted to confirm if bands were formed yearly or twice a year, so that they could better understand how long whale sharks live.

Meekan asked Joyce Ong, a former student and a fish ecologist and doctoral associate at Rutgers University, to join the research team, and work with Hua Hsun Hsu, then a postdoctoral researcher at Taiwan Ocean University who had used many of the samples for a previous study. Ong, who was lead author on the study, sectioned each vertebra with a diamond-bladed saw, photographed for contrast, and then peered closely at the images in order to accurately identify and count the sometimes miniscule bands. The bands themselves give the spine of the whale shark flexibility and strength, making possible the characteristic side-to-side thrash of the tale that sharks use to propel their bodies forward. To Ong, who learned to slice the vertebrae into slim segments, each 10-to-30-centimeter piece looked something like an hourglass.

Campana drew samples from the growth bands with a hand-held drill in preparation for bomb radiocarbon assays — in effect, isolating the tightly bound bands to measure the radiocarbon level with accelerator mass spectrometry and thus illuminate the mystery of when each growth band formed. The accelerator mass spectrometer works, Campana told me, by “spinning electrons in a magnetic tube around to speeds that are phenomenal, close to the speed of light.” From this, isotopes are created that can be measured. Allen Andrews, an affiliated faculty member at the University of Hawaii who did not work on this study, sees this process as a way of accessing “time stored.” To get a year of formation, Campana compared the radiocarbon levels in the vertebrae to bands layered in coral already correlated to specific years along with known quantities of carbon in the surface water at that time.

The first accelerator mass spectrometer was developed in 1939 by two physicists who would later go on to contribute to the Manhattan Project and the Los Alamos Laboratory, developing the very same atomic weapons that would double the level of radioactive carbon in the atmosphere, the oceans, and finally, in the cartilage of whale sharks and the bony structures of coral.

Campana’s team determined that growth bands form once a year — evidence that whale sharks age more slowly and might live longer than formerly thought, which makes them more vulnerable than fish that reproduce earlier and more often. They concluded that the whale shark landed in Pakistan could have been up to 50 years old. That meant it wasn’t the oldest whale shark they could imagine, as whale sharks might live 100 years. But it was the oldest one officially recorded.

The team admits, their research only supports the idea that growth bands form once a year up until the age of 50, given that they haven’t studied an older whale shark specimen, and the bomb radiocarbon dating was only done with two mature whale sharks. Perhaps as whale sharks age, bands form more slowly, or not at all; more older and younger whale sharks are needed to support additional research.

Andrews, who has used bomb radiocarbon dating to understand how coral, sharks, and other long-living fish age, says the results of this new study should be taken with caution. “Band counting in sharks,” Andrews says, “has been shown to be highly error prone.” And yet, Andrews adds, “this study sets the baseline for starting to compare more whale sharks from other areas” where radiocarbon dating has been performed.

While there’s further research to be done, this study is proof that scientists are willing to get pretty creative when it comes to finding out more about whale sharks that swim the deep seas, wild and most often out-of-reach.

Correction: An earlier version of this article stated that whale sharks have been around for at least 100 million years. They have been around for at least 30 million years.

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