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.