Sharks, rays live longer than thought: study

Sharks and rays live up to twice as longer as previously estimated, a study has found.

Researchers at James Cook University in Australia looked at 53 different populations of sharks and rays that scientists had already intensely studied.

They found that in nearly a third of populations the studies had underestimated the animals' ages.

"Questions arose over methods of ageing sharks after it was found that grey nurse sharks can live up to 40-years-old, double the length of time first thought, and the age of New Zealand porbeagle sharks had been underestimated by an average of 22 years," said Alastair Harry, a researcher at James Cook University.

Scientists usually measure shark age by counting growth rings in their vertebrae.

These measurements are confirmed by tagging animals and injecting them with a fluorescent marker or by measuring carbon accumulated in the animals from atmospheric testing of nuclear weapons in the 1950s.

"Age underestimation appears to happen because the growth rings cease to form or become unreliable beyond a certain size or age," Harry said.

"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 rather than just a few isolated cases," he said.

Harry said that accurate age estimation was important because it was used to manage fishery stocks.

The underestimation of longevity in orange roughy, a deep-sea fish, led to overly optimistic estimates of stock productivity, contributing to serious, long-term ecological and socio-economic impacts, researchers said.

Sharks and rays are less commonly targeted by commercial fishers but are still often caught as bycatch. That means the impacts of age underestimation may well take longer to become apparent, they said.