An Australian university study published on Friday found a number of errors in the way the age of sharks and rays have been measured, and which actually live 18 years longer than previously estimated.James Cook University researcher Alastair Harry conducted a study on 53 sharks and rays after which it was discovered that a bull shark can live up to the age of 40, twice as long as had been estimated earlier, reports Efe news.A majority of the scientists usually measure sharks' age by counting the rings in their vertebrae injecting them with a fluorescent marker or by measuring carbon accumulated in the animals from atmospheric testing of nuclear weapons in the 1950s.The expert explained that errors in the age calculation appeared to have occurred owing to the growth rings ceasing to form or becoming unreliable beyond a certain size or age."Across the cases I studied age was underestimated by an average of 18 years and up to 34 years in one instance. From the amount of evidence we now have it looks like the problem is systemic," the researcher said.Age calculation for these animals, which often end up trapped in fishermen's nets, is important for the management of marine resources.