Whale sharks (Rhincodon typus) are the world’s largest known fish, measuring in at around 13 meters (43 feet) long. Despite efforts over the last decade to increase our knowledge of this giant creature, including a look at its mating behavior, the age of Whale Sharks have remained a mystery. To answer this question, scientists have looked in a rather peculiar place – the radioactive legacy of nuclear bomb tests conducted during the Cold War.
Found in decreasing numbers across all tropical and temperate seas, whale sharks are listed as an endangered species on IUCN’s Red List. In order to implement effective conservation and management strategies for the fish, accurate estimates of its age and growth rate are necessary. However, unlike some species of fish, sharks and rays lack a small bony structure called otoliths (commonly known as “earstones”) from which scientists can estimate their age.
Instead, in whale sharks, the clue lies in their vertebrae. Each vertebra has distinct bands, like the rings of a tree trunk, which are created as the shark grows older. But there have been discrepancies over the rate at which they are produced with some studies suggesting a new ring is formed every year, whilst others propose the rings are generated biannually.
To settle the debate, an international team of researchers have used another method to verify the ring rate – the amount of the isotope carbon-14. Used by archaeologists to date fossils, this naturally occurring radioactive element decays constantly over time. If there were to be an increase in carbon-14 in the environment at one point in the shark’s lifetime, you would expect that spike to also be found in the bone it developed at that time.
For this study, published in Frontiers in Marine Science, the team looked at the time of the Cold War (specifically the 1950s and 60s) when the USA, Soviet Union, Great Britain, France, and China conducted nuclear weapon tests. A by-product of these tests was the doubling of carbon-14 in the atmosphere. This worked its way into the oceans, through the food-web, and then into marine organisms, producing an elevated carbon-14 signature that still persists.
The team compared carbon isotope data from growth rings in the vertebrae of two long-dead whale sharks stored in Pakistan and Taiwan to two reference chronologies of carbon-14 in surface water and coral. This allowed the researchers to determine how often the rings were formed in the whale sharks, and therefore how old they were.
“We found that one growth ring was definitely deposited every year,” Dr Mark Meekan, a senior principal research scientist at the Australian Institute of Marine Science, said in a statement. “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 shark from Taiwan was determined to be 35 years old at death, whereas the shark from Pakistan was conclusively established as 50 years old at death. This achievement marks the first time the age of a whale shark has been unambiguously verified.
“Our study shows that adult sharks can indeed attain great age and that long lifespans are probably a feature of the species,” Dr Meekan continued. “Now we have another piece of the jigsaw added.”