WASHINGTON (Reuters) - Scientists have figured out how to calculate the age of whale sharks - Earth’s largest fish - with some guidance from the radioactive fallout spawned by Cold War-era atomic bomb testing.

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By measuring levels of carbon-14, a naturally occurring radioactive element that also is a by-product of nuclear explosions, the researchers determined that distinct bands present inside the shark’s cartilaginous vertebrae are formed annually, like a tree’s growth rings.

It was already known that these bands existed and increased in number as a shark aged. But it was unclear whether new rings appeared yearly or every six months.

The researchers compared carbon-14 levels in the rings to data on fluctuations in its global presence during the busy years of atmospheric nuclear tests in the 1950s and 1960s.

“These elevated levels of carbon-14 first saturated the atmosphere, then oceans and moved through food webs into animals, producing elevated levels in structures such as the vertebrae of whale sharks,” said marine ecologist Joyce Ong of Rutgers University in New Jersey, lead author of the study published this week in the journal Frontiers in Marine Science.

Scientists now will be able to calculate a whale shark’s age after its death - one ring equals one year. But just as importantly the study established that these endangered marine giants possess a very slow growth rate.

“For the management of any marine species, knowledge of growth rate is critical as it determines the resilience of populations to threats such as fishing. Fast-growing species have fast rates of replacement and can withstand relatively high losses, whereas slow-growing species have low rates of replacement and are much less resilient,” said marine biologist and study co-author Mark Meekan of the Australian Institute of Marine Science in Perth.

Whale sharks are filter feeders, swimming great distances through the world’s tropical oceans to find enough plankton to sustain themselves. They have a brownish-grayish color on the back and sides with white spots, with a white underside.

The researchers tested carbon-14 levels in long-dead whale sharks whose remains were stored in laboratories. The oldest one tested, stored in Pakistan, had lived 50 years.

“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,” Meekan said. “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 meters (33 feet) in length and it is well documented that these sharks can get almost double this size, to around 18 meters (59 feet) in length.”