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April 7 (UPI) -- Thanks to the atomic remnants of Cold War nuclear bomb tests, scientists now have a better idea of how long whale sharks live. Researchers typically count bony structures called otoliths to determine the age of fish, but whale sharks don't have otoliths, making it more difficult to estimate the lifespan of the massive fish.
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The vertebrae of whale sharks boast distinct bands, similar to tree rings. The bands accumulate as the whale ages, but scientists have struggled to determine exactly how long each band takes to form.
Some previous studies determined a new ring forms every year, while others have claimed a new ring forms every six months.
Because the vertebrae rings trap environmental signatures in them, scientists decided to use shifting levels of the isotope carbon-14 to calibrate the growth rate of the vertebrae rings.
Carbon-14 is naturally occurring and often used to date ancient bones, but it's also a byproduct of nuclear weapons. During the Cold War, nuclear weapons tests by the United States, Soviet Union, Great Britain, France and China produced a large spike in Carbon-14 in the atmosphere. Decades later, that can be found trapped in the bones of aging fish.
"The concentration of Carbon-14 rose in the atmosphere as the number of tests increased. It then tailed off as atmospheric tests were reduced and eventually banned," Mark Meekan, senior researcher with the Australian Institute of Marine Science at the University of Western Australia, told UPI in an email. "We know when this spike peaked, so we can calibrate this against the Carbon-14 taken up in the vertebrae."
Scientists have previously measured Carbon-14 levels in the bones of long-living fish, but never before in whale shark remains.
"Carbon-14 isotopes have wide applications in ecology -- everything from tracing pathways in food webs, looking at turnover in soils by earthworms and for tracking the illegal trade in animal skeletons and body parts," Meekan said. "For whale sharks we have to rely on the occasional stranding of animals or access to vertebrae that have been collected in fisheries. So getting our hands on samples has been a problem and has hampered the technique being used in the past."
Fortunately, Meekan and his colleagues were able to test the vertebrae rings of two well-preserved whale shark specimens stored at museums in Pakistan and Taiwan.
The data, published this week in the Frontiers in Marine Science, showed whale shark vertebrae rings form every year. The updated calibration for ring growth suggests whale sharks can live upwards of 50 years.
Whale sharks are still hunted in Southeast Asia and are also vulnerable to ship strikes. The research could be used to inform conservation efforts.
"If you know the age and size of the animal you can calculate growth rate. This is a critical variable as it determines the resilience of populations to threats such as fishing," Meekan said. "Fast growing species have fast rates of replacement and can withstand relatively high losses, whereas slow growing species have low rates of replacement and are much less resilient. We need to adjust conservation strategies accordingly."
In followup studies, scientists plan to test additional whale shark remains to more accurately model the species' growth rates and longevity.
