Bomb tests reveal whale shark ages
The ages reached by whale sharks, the world’s biggest fish, have always been a mystery – until an international team of scientists came up with a way of estimating them based on the nuclear bomb-testing of the 1950s and ’60s.
The study, led by Joyce Ong of Rutgers University in the USA, has just been published, and explains how radiocarbon dating provides an accurate method of assessing the longevity of Rhincodon typus.
Whale sharks can weigh up to 36 tonnes and have been known to grow as long as 20m, although 12m is more typical. Like all sharks and rays, they lack the otoliths or bony structures that scientists use to assess the ages of other fish.
Whale shark vertebrae do feature distinct bands that are known to increase with age, but scientists have long differed about their significance, some arguing that new rings form annually, others that two form every year.
Ong’s research team examined the radioactive legacy of the Cold War nuclear arms race during which the USA, USSR, UK, France and China tested weapons in the atmosphere. This temporarily doubled levels of carbon-14, the naturally occurring radioactive element often used by archaeologists to date ancient bones and artefacts.
The fall-out from those explosions saturated the air and oceans, with the carbon-14 gradually moving through food webs and still lingering today. When the team tested carbon-14 levels in the growth rings of dead whale sharks stored in Taiwan and Pakistan, they found that they made it possible to assess their ages accurately.
One was estimated to be 50 years old when it died, making it the oldest known, and the other was reported to be 35.
“Accurate estimates of longevity, growth and mortality will better inform management and conservation efforts for whale sharks,” said Ong.
“The extended longevity, slow growth rates, late maturity and global connectivity of this species indicate high susceptibility to death caused by human impacts, such as ship strikes. Hence, this knowledge can help conservation managers adjust their strategies to be more effective.”
7 April 2020
Next steps include seeking the vertebrae of stranded sharks and studying further large, old whale sharks, she said. This would enable growth models to be refined and increase the accuracy of growth and natural mortality estimates.
Scientists at the University of Iceland, Australian Institute of Marine Science, King Fahd University of Petroleum & Minerals in Saudi Arabia and the Pakistan node of the UN Food & Agricultural Organisation also participated in the study, which is published in Frontiers in Marine Science.