The radioactive legacy of the arms race solves a mystery about the world’slargest fish.

Atomic bomb tests conducted during the Cold War have helped scientists for thefirst time correctly determine the age of whale sharks.

The discovery, published in the journal Frontiers in Marine Science , willhelp ensure the survival of the species – the largest fish in the world –which is classified as endangered.

Measuring the age of whale sharks ( Rhincodon typus ) has been difficultbecause, like all sharks and rays, they lack bony structures called otolithsthat are used to assess the age of other fish.

Whale shark vertebrae feature distinct bands – a little like the rings of atree trunk – and it was known that these increased in number as the animalgrew older. However, some studies suggested that a new ring was formed everyyear, while others concluded that it happened every six months.

To resolve the question, researchers led by researchers led by Joyce Ong fromRutgers University in New Jersey, USA, Steven Campana from the University ofIceland, and Mark Meekan from the Australian Institute of Marine Science inPerth, Western Australia, turned to the radioactive legacy of the Cold War’snuclear arms race.

During the 1950s and 1960s, the USA, Soviet Union, Great Britain, France andChina conducted tests of nuclear weapons. Many of these were explosionsdetonated several kilometres in the air.

One powerful result of the blasts was the temporary atmospheric doubling of anisotope called carbon-14.

Carbon-14 is a naturally occurring radioactive element that is often used byarchaeologists and historians to date ancient bones and artefacts. Its rate ofdecay is constant and easily measured, making it ideal for providing ageestimates for anything over 300 years old.

However, it is also a by-product of nuclear explosions. Fallout from the ColdWar tests saturated first the air, and then the oceans. The isotope graduallymoved through food webs into every living thing on the planet, producing anelevated carbon-14 label, or signature, which still persists.

This additional radioisotope also decays at a steady rate – meaning that theamount contained in bone formed at one point in time will be slightly greaterthan that contained in otherwise identical bone formed more recently.

Using bomb radiocarbon data prepared by Steven Campana, Ong, Meekan, andcolleagues set about testing the carbon-14 levels in the growth rings of twolong-dead whale sharks stored in Pakistan and Taiwan. Measuring theradioisotope levels in successive growth rings allowed a clear determinationof how often they were created – and thus the age of the animal.

“We found that one growth ring was definitely deposited every year,” Dr Meekansaid.

“This is very important, because if you over- or under-estimate growth ratesyou will inevitably end up with a management strategy that doesn’t work, andyou’ll see the population crash.”

One of the specimens was conclusively established as 50 years old at death –the first time such an age has been unambiguously verified.

“Earlier modelling studies have suggested that the largest whale sharks maylive as long as 100 years,” Dr Meekan said.

“However, although our understanding of the movements, behaviour, connectivityand distribution of whale sharks have improved dramatically over the last 10years, basic life history traits such as age, longevity and mortality remainlargely unknown.

“Our study shows that adult sharks can indeed attain great age and that longlifespans are probably a feature of the species. Now we have another piece ofthe jigsaw added.”

Whale sharks are today protected across their global range and are regarded asa high-value species for eco-tourism. AIMS is the world’s leading whale sharkresearch body, and the animal is the marine emblem of Dr Meekan’s home state,Western Australia.

Drs Ong, Meekan, and Campana were aided by Dr Hua Hsun Hsu from the King FahdUniversity of Petroleum and Minerals in Saudi Arabia, and Dr Paul Fanning fromthe Pakistan node of the UN Food and Agricultural Organisation.

Image: AIMS researcher Mark Meekan swimming with a whale shark. (Horizontal)Credit: Wayne Osborn

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