How Can You Tell How Old a Shark Is?

In this installment of Ask a Marine Biologist, Dr. David Shiffman answers a question from an 11-year-old shark fan (relayed by his mother).

Question: How can scientists tell how old sharks are? Does it hurt? Do they release all the sharks they study? With Greenland sharks, how can they carbon-date the eye? —James H., Washington (11 years old) as relayed by his mother, Serenity H.

Answer: There are a few ways scientists can tell how old a shark is. The most common way is the same way we can tell how old a tree is: count the rings! Ring patterns come from seasonal changes in resource availability, which leads to seasonal changes in growth patterns and a series of annual rings. In the case of sharks, these rings are found on the vertebrae. This link to a NOAA shark research lab shows how scientists use this method to age sharks. Check it out.

Another way is catch-and-release tagging programs. For example, if a tagged shark is caught again 20 years later, we know that species can live at least 20 years. This method is notably less precise than the others, but with a huge dataset of many thousands of recaptured sharks, you can still learn a lot.

Because Greenland sharks have unusual skeletal structures, scientists couldn’t use regular vertebral growth rings in a recent study. Instead, they used a different method called bomb radiocarbon. The testing of nuclear weapons in the 1950s and ’60s fundamentally changed the planet in a way we can measure. Animals born after that point have evidence of this “bomb pulse” in their tissues. And those animals born before it don’t have that signature, or have a measurably different version of it (this is of course a huge oversimplification of complex mathematical and biogeochemical principles, but it works for our purposes here). The Greenland shark researchers in the paper you asked about used eye lenses, because this tissue doesn’t change much as you age and grow. They found an amazing result: Greenland sharks are probably the longest-lived vertebrate animals on Earth! (Take this result with a grain of salt because this aging method has a margin of error, but Greenland sharks still probably live a really long time.) Also, sometimes, as in the case of great white sharks a few years ago, scientists use a combination of bomb radiocarbon and traditional vertebral ring work.

Unfortunately, there’s no way for scientists to access vertebrae or eye lenses of a live shark. We often try to take advantage of already dead sharks rather than sacrificing sharks just for a single tissue sample, but that’s not always possible. Marine biologists take this responsibility very seriously, and we use it to gather important data that is used to protect entire species. We need to know how long sharks live, in addition to how often they reproduce, in order to come up with fisheries management plans and population recovery plans. So that’s how you can tell how old a shark is, and why that data matters.

Ask a Marine Biologist is a biweekly column where Dr. David Shiffman answers your questions about the underwater world. Topics are chosen from reader-submitted queries as well as data from common internet searches. If you have a question you’d like answered in a future Ask a Marine Biologist column, or if you have a question about the answer given in this column, email Shiffman at WhySharksMatter@gmail.com with subject line “Ask a marine biologist.”

Dr. David Shiffman is a marine conservation biologist specializing in the ecology and conservation of sharks. An award-winning public science educator, David has spoken to thousands of people around the world about marine biology and conservation and has bylines with the Washington Post, Scientific American, New Scientist, Gizmodo and more. Follow him on Twitter, Facebook and Instagram, where he’s always happy to answer any questions about sharks.

The views expressed in this article are those of David Shiffman, and not necessarily the views of Sport Diver or Scuba Diving magazines.