A key part of wildlife conservation is knowing an animal's age. For scientists studying the world's largest fish and shark, the whale shark, that's not been very easy to come by.
However, a new study has shown how atomic bomb tests carried out in the 1950s and 1960s could help to solve this long-standing puzzle by using nuclear isotopes to help fill in the gaps for the first time ever.
The study was published in Frontiers in Marine Science.
Radioactive element to count age
One powerful result of the nuclear bomb tests from the Cold War era was the temporary atmospheric doubling of carbon-14, an isotope. Carbon-14 is a radioactive element that naturally appears in the atmosphere and is then absorbed by every living creature on Earth. This is what makes it ideal when estimating the age of some animals.
Measuring amounts of the radioactive isotope Carbon-14 (a byproduct of nuclear bomb detonations) in a shark's skeleton can help scientists determine their age. In a new study from @aims_gov_au@MarkMeekan, Whale Sharks' vertebrae were dated at 50 years old https://t.co/eD3GnbkKA7— The Shark Trust (@SharkTrustUK) April 6, 2020
Moreover, as the decaying of the isotopes is regular and predictable, this shows a good indication of how old something is.
When it comes to whale sharks, which lack certain structures called otoliths, scientists have been left to approximate their age. Similar to rings on a tree trunk, whale sharks' vertebrae have distinct bands, and it was known that these increase with time. However, the issue was that scientists were unsure whether these bands formed every six months or every year.
So a team of scientists from Rutgers University in New Jersey, U.S., the University of Iceland, and from the Australian Institute of Marine Science (AIMS) in Perth, Australia, has used atomic bomb tests' isotope residue to come to a conclusion.
Measuring the radioisotope levels in the growth rings of long-dead whale sharks provided the researchers with a clear answer. "We found that one growth ring was definitely deposited every year," said Dr. Mark Meekan from AIMS.
Dr. Meekan continued "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."
The team concluded that one of the specimens was 50 years old at its time of death — the first time a true age has been given to a whale shark.
"Although our understanding of the movements, behavior, 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," Dr. Meekan said. "Now we have another piece of the jigsaw added."