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.