Many Sharks Much Older Than Previously Thought

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Nuclear bomb testing during the 1950s and 60s is helping scientists to determine that many sharks today are much older than previously thought.

Earlier this year it was announced that some great white sharks swimming in the ocean now are 70 years old or more. A new study, published in the journal Marine & Freshwater Research, has found that many sand tiger sharks and likely other shark species too are much older than scientists had suspected.

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“Validated lifespan for C. taurus (sand tiger shark) individuals in the present study reached at least 40 years for females and 34 years for males,” according to lead author Michelle Passerotti and colleagues.

They added “that ages of large adult sharks were underestimated by 11–12 years,” prior to the study.

Passerotti, a researcher at the National Marine Fisheries Service of the Southeast Fisheries Science Center, and her team came to that conclusion after analyzing sand tiger shark vertebrae from different life stages. The sharks hailed from waters off of the southeastern United States and South Africa.

Here’s where the bomb fallout comes in: Detonation of nuclear bombs during tests in the 50s and 60s produced “artificial” radiocarbon in the atmosphere. This is known as the bomb effect. The global carbon exchange cycle has thankfully been reducing the nuclear bomb-released radiocarbon, such that levels of bomb carbon were 100 percent above normal between 1963 and 1965, but about 20 percent above normal (i.e., pre-nuclear bomb testing) in the 1990s.

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The radiocarbon pulse in the environment therefore created, and continues to create, time stamps, so scientists can measure radiocarbon levels in something (in this case, the shark vertebrae) and pair those measurements with a reference chronology.

The technique has been used to better date trees, but in more recent years, it’s been applied to sharks, which can be notoriously difficult to study. That’s because many sharks have large ocean ranges, swim in deep water, grow up discreetly in “nurseries,” and can be hard to tag-release-recapture.

The researchers suspect that sandbar sharks may be much older than presently thought as well.

The good news is that some sharks appear to enjoy impressive longevity. If you see a large great white in the ocean, for example, that shark could be a healthy senior citizen by human standards.

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The bad news is that sharks that mature late, have long life spans and produce small litters tend also to have the lowest population growth rates and the longest generation times. This makes them very vulnerable to extinction, since it’s hard for them to recover population losses.

Great whites, sand tiger sharks and other long-lived sharks therefore are more sensitive to fishing pressures, climate change and other human-caused environmental problems.

Photo: A sand tiger shark. Credit: ThinkStock