A white shark, age unknown. Image via Flickr/Ken Bondy
While we humans generally find sharks absolutely fascinating (how many other animals make such compelling viewing that they have their own week of TV programming?), we still don’t know all that much about them. And it turns out some things we thought we did know about the beasts of the deep aren’t even accurate.
We recently learned that, despite being a fish, sharks share more in common with our mammalian species than we thought. And now a new study published in PLOS One has found that great whites, without a doubt the most badass shark out there, might live a lot longer than previously believed.
A team led by researchers at the Woods Hole Oceonographic Institution in Massachusetts used an interesting method to age white sharks caught in the northwest Atlantic Ocean from 1967 to 2010, and it’s pretty awesome. Sharks are usually aged by counting the bands in their vertebrae, much like counting rings in a tree trunk. But that's not entirely accurate, because sometimes the bands are so thin they can't even be seen under a microscope. So the study authors turned to something else: nuclear bombs.
It works like this: When thermonuclear weapons were tested in the 50s and 60s, radioactive carbon entered the ocean and permeated the tissues of marine creatures. The researchers therefore measured the levels of radiocarbon in the sharks' vertebrae to better understand the band count. "The rapid rise in radiocarbon in the ocean can be used as a time stamp to determine the age of an organism," they wrote.
A photomicrograph of a section of vertebrae, where the red dots indicate 'growth bands.' Image via PLOS One
What they found was that the sharks were much older than you’d think by reading their vertebrae bands. The oldest female specimen out of the eight sharks tested was 40 years old, and the oldest male had reached the ripe old age of 73. In the past, the oldest sharks identified were just 22 and 23.
The authors suggested the discrepancy could be because previous studies looked at sharks in the Pacific and Indian Oceans (though why sharks in the Atlantic would live over twice as long is unknown), or more likely because the method of band-counting is flawed, either because sharks stop growing new bands on their vertebrae after a certain age, or because of the readability issues.
While it’s nice to know these kick-ass sharks can live to a grand old age, their longevity doesn’t protect them against being killed, as many of them are each year. They might not look it, but Great Whites are vulnerable to extinction, and the fact they grow older than we thought could bring added urgency to conservation efforts.
“Age estimation is particularly important to the development of conservation and sustainable management strategies as most demographic variables required for adequate population assessments, such as longevity, growth rate, and age at sexual maturity, include an age component,” the authors wrote. One particular worry is that, as sharks live longer, they may not reproduce until they’re older, which means it would take longer for the population to bounce back after overfishing.
The researchers suggested that, as a result, the sharks could be “considerably more sensitive to human-induced mortality than previously thought.” The only silver lining is that we at least now know more about shark demographics, and can hopefully develop conservation efforts to suit them. As Simon Thorrold, one of the study's authors, told the Australian, "Those bombs were good for something at least!"