Cold War N-test reveals whale shark age

Measuring has been difficult because they lack bony structures

Melbourne, April 6: Scientists have used data from atomic bomb tests conducted during the Cold War to correctly determine the age of whale sharks for the first time.

The research, published in the journal Frontiers in Marine Science, will help ensure the survival of the species which is classified as endangered. Measuring the age of whale sharks (Rhincodon typus) has been difficult because, like all sharks and rays, they lack bony structures called otoliths that are used to assess the age of other fish. Whale shark vertebrae feature distinct bands — a little like the rings of a tree trunk — and it was known that these increased in number as the animal grew older. However, some studies suggested that a new ring was formed every year, while others concluded that it happened every six months.

The researchers, including Mark Meekan from the Australian Institute of Marine Science, turned to the radioactive legacy of the Cold War’s nuclear arms race. During the 1950s and 1960s, the US, Soviet Union, Great Britain, France and China conducted tests of nuclear weapons. Many of these were explosions detonated several kilometres in the air. One powerful result of the blasts was temporary atmospheric doubling of an isotope called carbon-14, a naturally occurring radioactive element that is often used by archaeologists and historians to date ancient bones and artefacts.

Its rate of decay is constant and easily measured, making it ideal for providing age estimates for anything over 300 years old. Using bomb radiocarbon data, the researchers set about testing the carbon14 levels in the growth rings of two long-dead whale sharks stored in Pakistan and Taiwan.

Measuring the radioisotope levels in successive growth rings allowed a clear determination of how often they were created — and thus the age of the animal.

“We found that one growth ring was definitely deposited every year,” Meekan said. “This is very important, because if you over- or under-estimate growth rates you will inevitably end up with a management strategy that doesn’t work, and you’ll see the population crash,” he said.

WE FOUND that one growth ring was definitely deposited every year. This is very important, because if you over- or under-estimate growth rates you will inevitably end up with a management strategy that doesn’t work, and you’ll see the population crash. — MARK MEEKAN Australian Institute of Marine Science