Beautifully patterned with white spots and stripes, the 60-foot-long whale shark is the largest — and one of the most striking fish in the sea. Though it’s beloved by ecotourists and native to temperate oceans the world over, very little is known about these behemoths — including how long they live.
Recent investigations into other shark species have revealed astounding life-spans: The Greenland shark, for example, can live nearly 300 years, longer than any other vertebrate on Earth. Many more sharks, such as the great white, near the 100-year mark.
Those discoveries are largely because of advanced methods for determining a shark’s age, such as tracing carbon-14, a rare type of radioactive isotope that is a byproduct of Cold War-era bomb detonations, in shark skeletons.
Measuring amounts of this element can tell scientists a shark’s age more accurately than the previous approach, counting tree-like growth rings on whale shark vertebrae. That’s because how much time each ring represents has long been a subject of dispute.
By comparing the amount of carbon-14 in the oceans during certain years with the amount of the isotope captured in successive vertebral growth bands, the researchers could discern a shark’s age.
From 1955 to 1963, atomic bomb testing in the United States and other countries doubled the amount of carbon-14 naturally in Earth’s atmosphere. That excess was absorbed into the ocean and taken up by everything in the food web — including cartilaginous whale shark skeletons.
The whale shark’s longevity makes the species as a whole more vulnerable to threats such as legal and illegal fishing, warming ocean temperatures and ship strikes.
Conservationists need to know the growth rate of a species because a slower-growing species is more susceptible to extinction than one that reproduces quickly. The whale shark’s global population has fallen by more than half over the past 75 years, according to the IUCN.