By Matt Soniak
It turns out that age is a lot more than just a number.
Consider great white sharks, for example. These fearsome fish are flagged as "vulnerable" on the International Union for the Conservation of Nature's Red List of Threatened Species, and are protected by international trade agreements like the Convention on International Trade in Endangered Species. Knowing how old these sharks can get, how fast they grow, and when they can start making little sharks helps scientists figure out how healthy and stable their populations are, which is vital for designing conservation and management programs.
Normally, biologists estimate the ages of sharks by counting the layers of tissue that form on their vertebrae, the way botanists count growth rings in tree trunks. This isn't a perfect method, though. Some of the tissue layers can be too thin to differentiate, even with a microscope, and in some sharks the tissue accumulates at different rates, screwing up the count.
Recently, researchers from the Woods Hole Oceanographic Institute in Massachusetts dated some vertebra samples from great whites using a different method, and estimated that one shark had lived into its 70s — more than twice as long as what scientists had thought was the maximum age for the species. Not only does this suggest that World War II-era sharks could still be alive today, but it also makes a strong case for rethinking the way we protect these amazing predators.
Instead of using microscopes and tissue layers to get these new estimates, the scientists turned to carbon-14 — a variant, or isotope, of the element carbon. Carbon-14 is generated by cosmic rays and normally shows up in small traces in the atmosphere, but human activity — from burning coal and oil to testing nuclear weapons — can lead to reductions or spikes in carbon-14 concentrations. Since carbon-14 oxidizes into carbon dioxide, is absorbed by plants and the oceans, and makes its way around the food chain, almost every living thing has its share of carbon-14 hanging out in its body.
By comparing the concentrations of carbon-14 in the layers of tissue on a shark's vertebrae to known levels in the atmosphere at different dates, specific layers can be time-stamped and a more accurate age estimate can be made.
The Woods Hole Researchers looked at vertebrae from eight different sharks caught in the Atlantic Ocean between 1967 and 2010, and dated them by analyzing the carbon-14. They aged the largest male at 73, half a century older than the previous oldest-known shark, a 23-year-old from the Indian Ocean.
Of course, the vertebrae they dated were in a museum, and not still inside a shark. But if they had been from a living shark, that animal would have been born in 1941. It would have been swimming around when the Japanese attacked Pearl Harbor, when JFK was shot, when man landed on the moon, when Jaws came out — and still swimming around today.
If we've been underestimating the maximum age for sharks, the researchers say, population projections and conservation plans based on old data will need to be revisited. Animals that live long, grow slowly, and reproduce later in life are much more sensitive to pressures like hunting and fishing and environmental changes. They are also more vulnerable to extinction because their low population growth rates make it harder to bounce back and replace lost individuals. If sharks regularly live into what we consider the "golden years," new conservation plans will have to take that longevity — and the problems that go with it — into consideration.