“In many ways it feels like an encounter with something from prehistory—a giant from the past swimming towards a very uncertain future in today’s oceans.”
Whale sharks are a species under threat.
New research published last month in Frontiers in Marine Science using carbon data from Cold War-era nuclear bomb tests sheds new light on the ocean’s largest fish and how the endangered species lives.
Key information about how they age and grow was missing—a major gap in developing protection plans for whale sharks. When whale sharks age, growth bands appear on the animal’s vertebrae.
The new research, conceptualized by Australian Institute of Marine Science Principal Research Scientist Dr. Mark Meekan, ends a debate on the rate of the animal’s growth.
Getting access to sample vertebrae is difficult since researchers rely on the occasional stranding of animals or samples collected in fisheries. For Meekan’s study, researchers had access to vertebrae of a 10 meter-long whale shark that stranded in Pakistan and another (about 9.9-meters long) from a now-closed fishery in Taiwan.
The study showed that the whale shark collected in Pakistan was 50-years-old, whereas the slightly smaller shark from Taiwan was around 35-years-old.
“Until our results, much of what we know about the life span and much of the basic biology of the species has been based on assumptions and evidence from other species,” Meekan said in an interview with States of Life. “We thought that it was possible that they could reach ages of as much as 100 years, but we weren’t really sure as we had no validated data on age. Our study changes this. We still can’t say for certain if these sharks live to be 100 years old, but it now seems much more likely given that our largest shark was 50 years-old at 10 m in length and it is well documented that these sharks can get almost double this size, to around 18 m in length.”
Bomb carbon dating is a technique often used to age long-lived fishes and other animals.
Carbon-14 (C14) is a naturally-occurring radioactive element that is often used by archaeologists and historians to date ancient bones and artifacts, with the rate of decay constant and easily measurable for anything suspected of being over 300 years-old. The isotope is also a by-product of nuclear explosions.
Since the first nuclear test in July of 1945, at least eight countries have detonated 2,056 nuclear explosions at dozens of test sites around the world, according to the Arms Control Association (ACA).
In the fallout from the hundreds of nuclear tests carried out during the Cold War, the isotope gradually moved through food webs into every living thing on the planet, producing an elevated C14 label, or signature, which still persists today. The peak of C14 released by nuclear bombs came in the early 1960s at the height of the Cold War.
Meekan said the new data has “conclusively shown” that whale sharks are long-lived and slow growing animals and are likely to be very susceptible to threats such as fishing activity.
Whale sharks are also major target for ecotourism where snorkelers and divers swim with the animals in many countries across the tropics. The ecotourism industry has listed thousands out of poverty in countries including Indonesia and the Philippines, and prompted governments to legislate to protect whale sharks, something Meekan said is “truly a win-win situation for both people and whale sharks.”
“It is incumbent on us to make sure that these vulnerable sharks are still present in the future not just because they are an essential part of ecosystems, but also because they support the livelihoods of many thousands of people around the globe,” Meekan said.
Some of the most basic information about whale sharks, particularly data on reproduction, where adult female whale sharks go to pup, and the habitat of adults remains a mystery because the animal as large as a city bus primarily live in the open ocean, far from shore. In fact, most research on whale sharks come from the small portion of the population (juvenile males) that aggregate near coasts.
Going forward, Meekan said the new study will help recalculate growth curves and improve age and growth estimates for other whale shark populations.
“This is only a small part of our broader research program,” Meekan said. “We are examining the genetics of these sharks to see if we can use DNA to track their movements and to estimate numbers – essential information for an IUCN listed Threatened species. We are using small tissue samples to examine the diet and accumulation of toxins such as heavy metals in the bodies of these sharks and we are attaching cameras to log their behavior during the time they spend away from the surface at Ningaloo Reef.”
Meekan added, “Above all, we are trying to accumulate information that will help us manage these animals so that they are still present in the future, not just because they are an essential part of ecosystems, but also because they support the livelihoods of many thousands of people around the globe.”
Protecting the whale sharks future also allows others to have encounters similar to Meekan, who’s been fortunate enough to have had thousands of encounters with whale sharks across the Indian Ocean, the Red Sea and the Caribbean.
“Imagine yourself floating at the surface out the front of Ningaloo Reef in Western Australia,” Meekan said.
“The water is a deep dark blue beneath you and you can’t see the bottom. Looking out to the horizon you can see a large dark shape heading towards you that slowly begins to resolve itself into the shape of a giant shark.
As it comes alongside you, the patterns of white spots and stripes mingle with the surface refraction so the body decoration looks to shift and shimmer. It glides by with just a few broad sweeps of the tail, seeming to move without effort. But this is very deceptive, as when you try to keep pace you find that you have to swim very hard to keep up with the shark. Slowly you tire and the shark gradually leaves you behind with your last vision of the animal being its massive tail that calmly strokes onwards into the distance.”
Read Meekan’s account of the new research in the Conversation titled, “The mushroom cloud’s silver lining”: how the Cold War is helping the biggest fish in the sea.”