Atom bomb isotopes reveal true age of whale sharks for the first time

A key part of wildlife conservation is establishing a clear picture of how animals age and how long they can be expected to live. For biologists studying the world’s largest fish, this vital information hasn’t been so forthcoming. A new study, however, has demonstrated how the atomic bomb tests from the Cold War era could help fill in some of the blanks, with scientists using nuclear isotopes to measure the age of a whale shark for the first time.

Carbon-14 is a radioactive element that occurs naturally in the atmosphere and is absorbed by every living creature on Earth. The atom bomb tests that took place in the 1950s and 60s caused a big surge carbon-14, temporarily doubling the amount of the element in the atmosphere and therefore driving up its concentration in all organic material to have formed since.

In this way, measuring carbon-14 levels in tissue samples can give a good snapshot of what the atmospheric levels were at a certain point time. And because the radioactive isotope decays at a regular, predictable rate, calibrating it alongside other dating methods, like tree rings for example, can also give a good indication of the rate something is aging.

Instead of tree rings, the international team of scientists behind the new study turned to whale shark vertebrae, which feature visible bands that increase in number as they grow older. There has been some conjecture over the rate of growth of these rings, with some studies concluding that a new one forms annually, and other studies arguing they form as frequently as every six months.

A whale shark vertebra with markers indicating growth rings

A whale shark vertebra with markers indicating growth rings

The researchers analyzed the growth rings in two dead whale sharks to assess the carbon-14 levels throughout. The varying levels of the radioisotope in the growth rings then offered them a clear picture of how often they were forming, and by extension enabled them to calculate the age of the animal.

“We found that one growth ring was definitely deposited every year,” says study author Mark Meekan from the Australian Institute of Marine Science. “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.”

While some studies have suggested whale sharks may live as long as 100 years, this method allowed the team to unambiguously verify one’s age for the first time, with one of the specimens calculated to be 50 years old.

“Although our understanding of the movements, behaviour, connectivity and distribution of whale sharks have improved dramatically over the last 10 years, basic life history traits such as age, longevity and mortality remain largely unknown,” says Meekan said. “Now we have another piece of the jigsaw added.”

The research was published in the journal Frontiers in Marine Science.

Source: Australian Institute of Marine Science

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