Shark in foreground with diver swimming nearby.

Whale sharks can reach 18 metres in length and, according to a new technique, at least 50 years of age. Credit: Wayne Osborn

Zoology

Nuclear bomb tests help to expose a giant fish’s true age

Radioactive fallout from the cold war shows that one whale shark studied was 50 years old when it died.

Using the radioactive carbon spewed into the atmosphere by cold war-era nuclear-bomb tests, scientists have developed a precise method for pinning down the age of whale sharks, giant fish that some researchers have suggested can survive for a century or longer.

Like rings in a tree trunk, the layers in a shark’s vertebrae can reveal its age. But scientists disagree about how often these layers are deposited, leading to wide-ranging estimates of how long a whale shark (Rhincodon typus) can live.

To settle the debate, Joyce Ong at Rutgers University in New Brunswick, New Jersey, and her colleagues measured the amount of carbon-14 in vertebrae from two whale sharks. Because carbon-14 decays at a known rate, measuring the ratios of different carbon isotopes in adjacent layers shows how much time has passed between layers’ formation.

The scientists’ technique showed that a 10-metre-long female was 50 years old when it died after becoming entangled in fishing gear. A second shark studied by the team was 35 when it died.

Accurate information about the longevity of marine species is crucial for managing conservation efforts, the researchers say.