With a spate of recent incidents involving conflict between humans and marine creatures — specifically sharks — it would perhaps bring no comfort to people affected by or generally scared of the toothy animals to know that scientists have been estimating the longevity of sharks mostly wrong till now. Sharks, and even rays, live much longer than previously thought, some even twice as long, new research has shown.
Most current methods for determining the age of sharks rely on counting the growth rings on their vertebrae — similar to what is done with trees and their trunks. In recent research published in the journal Fish and Fisheries, Alastair Harry from the James Cook University in Australia found this method led to often underestimating the age of a given individual, and he found the discrepancy to be quite prevalent across numerous species.
“Age underestimation appears to happen because the growth rings cease to form or become 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 rather than just a few isolated cases,” Harry said in a statement Friday.
For his research, Harry decided to look at 53 populations of sharks and rays that have already been thoroughly studied by scientists, because doing so would give him data to compare notes with.
“Age was likely to have been underestimated in nine of 29 genera and 30 percent of the 53 populations studied,” he wrote in the paper, which was titled “Evidence for systematic age underestimation in shark and ray ageing studies.”
A grey nurse shark seen at Green Island, South West Rocks in New South Wales, Australia, Oct. 3, 2005. Photo: Richard Ling/Wikimedia Commons
The grey nurse shark (Carcharias taurus) can live up to the age of 40 years, which is twice as long as it was hitherto thought to live till. Similarly, the New Zealand porbeagle shark can live longer by an average of 22 years than was thought till now.
“Current perceptions of shark and ray life histories are informed to a large extent by growth studies that assume calcified ageing structures are valid throughout life. The widespread age underestimation documented here shows this assumption is frequently violated, with potentially important consequences for conservation and management,” Harry wrote in the paper.
Even though sharks and rays are not common targets for fishing, various genera and species of both families of marine creatures often get entangled in nets meant for fish, and end up as bycatch. And since age estimation of these animals is often the only parameter used to estimate their populations, underestimating their ages could lead to biases in data.
“It could lead to inefficient prioritization of research, monitoring and management measures. If it’s as widespread and common as it seems from this study, the impacts could also be substantial from a wider scientific perspective, affecting the many disciplines that also use baseline life history data,” Harry said in the statement.
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