Some sharks and rays can twice as long as previously estimated, a James Cook University researcher has found out
Scientists usually measure shark age by counting growth rings in their vertebrae. These rings are often assumed to show age in the same way as the rings of a tree. But a growing number of studies have now demonstrated that counting growth zones on calcified structures, can underestimate true age.
Dr Alastair Harry from James Cook University looked at 53 different populations of sharks and rays that scientists had already intensely studied. He found in nearly a third of populations the studies had underestimated the animals' ages.
Harry looked at two methods of checking whether the age estimated from counting rings is correct: chemical marking and bomb-carbon dating. In the former, researchers capture an animal and inject it with fluorescent dye that is taken up by its spine, making a permanent mark. When the animal is recaptured later, it is possible to count how many bands have formed since this known date. In the second method, scientists can look for carbon traces of 1950s nuclear-bomb tests in animals that were alive then, and use this to estimate age.
“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,” he said.
Age was likely to have been underestimated in nine of 29 genera and 30% of the 53 populations studied, including 50% of those validated using bomb carbon dating. Length and age were strongly significant predictors of occurrence, with age typically underestimated in larger and older individuals.
The study has wide-ranging implications. If age information is wrong, models that guide fisheries’ decisions about how many animals can safely be caught will also be wrong.