BBC: [2] Data from atomic bomb tests conducted during the Cold War have helped scientists accurately age the world’s biggest fish.
[3] Whale sharks are large, slow moving and docile creatures that mainly inhabit tropical waters.
[4] They are long-lived but scientists have struggled to work out the exact ages of these endangered creatures. [5] But using the world’s radioactive legacy they now have a workable method that can help the species survival. [6] Whale sharks are both the biggest fish and the biggest sharks in existence. [7] Growing up to 18m in length, and weighing on average of about 20 tonnes, their distinctive white spotted colouration makes them easily recognisable. [8] These filter feeders live on plankton and travel long distances to find food. [9] They are very popular with tourists in many locations, often allowing divers to swim alongside them.
[10] However, the species is now classified as endangered because of over-fishing in places like Thailand and the Philippines.
[11] Much about the species remains a mystery, especially how to age them correctly.

Researchers say this is fundamental to understanding their growth rates – information that’s considered crucial to saving the species in the long term.