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# How do they know how old a shark is?

By Blaise Jones

Determining the age of a shark is a difficult and imprecise without a complete dissection, which requires the animal to be dead.

It’s a Simple Matter of Weight Ratios
When it comes to aging all fishes (not just sharks), one method is to establish a size:age ratio. This is a mathematical formula that allows scientists to accurately guess at an animal’s age based upon the size measurements of the species at known ages. Creating a size:age ratio chart is a long and difficult process. It involves catching many different specimens of a single species, measuring their length and weight, and then determining their age using their otolith.

After a long time and a lot of hard work, a pattern connecting the size of an animal to its age should begin to appear. Once you have found this pattern you can use a fish’s size to guess its age. However, this is not an entirely accurate method of aging an animal. Several factors can alter the rate of growth in fishes. Everything from amount of predation, availability of food, and even slight variations in temperature can lead to changes in the rate of fish growth.

Otolith
The only reliable way of determining the age of a fish is to look at its otolith. The otolith is a bony structure in a fish’s head that helps it determine its location in three-dimensional space. As a fish grows older the otolith grows in layers, which form rings, similar to those found in trees and on turtle scutes. By counting the rings on a otolith, scute or inside a tree, scientists can determine the age of the fish, turtle or tree.

Shark otoliths are about the size of a grain of sand, however, which makes reading the rings very difficult.

Vertebrae rings
The most common structure used to measure a shark’s age is its vertebrae or, if it has one, dorsal spine. Both the dorsal spine and vertebrae grow in ringed layers like the otolith, allowing scientists to count them and determine a sharks age.

Unfortunately, recent research has shown that this method might not be entirely accurate. It has been discovered that the rate at which new rings form slows down once a shark reaches maturity, meaning that we need to find a new method of aging sharks.

Due to nuclear bomb testing throughout the Pacific Ocean during the 1950s and 60s, radioactive isotopes from carbon are found throughout the ocean and everything living in it. Radiocarbons have unique signatures to the bombs that formed them, meaning that if an animal has a trace of a specific radiocarbon that isotope proves the animal was alive when the bomb was tested. Using this method, scientists have been able to find adult sharks of many different species that are at least 70 years old, including the great white shark, which was previously thought to only live to be 40 years.

Perhaps the most astonishing discovery using this method involves the sluggish Greenland shark. This species of shark was always thought to live long, mostly due to its frigid habitat and lethargic lifestyle. In 2016, radiocarbon testing revealed one Greenland shark to be at least 272 years old and could be more than 500 years old.

That means the average, anonymous, mature Greenland shark may be older than the 185-year-old giant tortoise named Jonathan, the previous record-holder for world’s oldest living animal.

Sources

1. “Sharks: The Animal Answer Guide” by George H Burgess and Gene Helfman
2. “The Encyclopedia of Sharks” by Steve Parker
3. http://science.sciencemag.org/content/353/6300/702
4. http://thescientificfisherman.com/old-fish-age-chart/
5. http://slideplayer.com/slide/3502429/
6. http://www.ibtimes.com/science-behind-ageing-sharks-study-reveals-great-whites-can-live-more-70-years-1533020
7. http://nefsc.noaa.gov/nefsc/Narragansett/sharks/age.html