• SharkieSophie

Great Whites are the Bomb

Updated: Apr 26

Understanding how rapidly sharks grow and how long they live can be very important to determine how their populations may be able to bounce back after significant declines. Despite being considered 'vulnerable' by the International Union for the Conservation of Nature (IUCN, 2020) and being protected internationally by the Convention on International Trade in Endangered Species (CITES) and the Convention on Migratory Species, conservation of the great white shark (Carcharodon carcharias) has been hindered by gaps in our knowledge regarding their growth rates and longevity (Hamady et al, 2014).


The great white shark (Image source: www.theconversation.com/)

White sharks are between 1.2 - 1.5 m total length at birth and are thought to reach maturity between 8 and 12 years old. Male great whites must reach the critical size of 3.6 m before they become sexually mature and females must be 4.8 m. White sharks continue to grow throughout their lives, even as adults. This means that a large shark (generally speaking) is an older shark. There are several anecdotes that great white sharks may reach 7 or even 8 m in length, but the largest individual to have been accurately measured totalled 6m. The largest white shark alive today is a ~6 m female named Deep Blue, that is thought to be 50 years old.



Traditionally, the age of individual sharks has been estimated using a method known as 'vertebral band analysis'. As a shark grows, the cartilaginous skeleton grows by annual deposition on top of the previously existing cartilage. This means that, if we look at the certain structures, we can see rings of growth. Extracting a vertebra from the spinal column of a shark and counting how many bands have been laid down can give us its age, akin to ageing a tree using the rings (Hamady et al, 2014).


White shark vertebra in cross-section, with red dots identifying annual deposition rings

However, because this method can sometimes be inaccurate, recently scientists have also been using radiocarbon-dating to calculate age in sharks. This involves taking minute slices of the vertebra and analysing the assimilation of dietary carbon isotopes into the collagen (a structural protein found in cartilage) of each slice. Carbon naturally occurs in the environment as a stable element when it has 12 - 13 neutrons in the nucleus of the atom (known as carbon-12 and carbon-13). However, carbon can also be found as an isotope, meaning that there is a different number of neutrons in the nucleus. These isotopes are unstable and the nucleus loses energy by radiation. For instance, carbon-14 is commonly found naturally in the environment and decays at a consistent rate over thousands of years. This means that, if we measure the carbon-based radioactivity of something, we can use the known decay rate to calculate its age. This is commonly known as 'carbon-dating'.


What is seriously cool is that there have been periods in history when human beings have altered the ratio of carbon isotopes in the natural environment... with bombs. Testing of thermonuclear weapons during the 1950s and 1960s caused dramatic increases in atmospheric carbon-14, which mixed into the ocean and became assimilated into the tissues of marine animals through their food. In sharks, the vertebral bands with higher carbon-14 will have been laid down during the period of atomic testing. So basically, we can actually detect bomb detonation in a white sharks skeleton!



This has practical applications because the assimilation of heightened carbon-14 into the vertebral collagen can be used as a time stamp; giving us the exact year that a vertebral ring was deposited. This gives us a reliable reference point within the vertebra with which we can use to calculate a shark's age more accurately (Kerr et al, 2006).


Recently, a research team used a combination of vertebral band analysis and radiocarbon analysis to estimate the ages of large white sharks caught in the northwestern Atlantic Ocean which were stored as museum specimens. This allowed them to estimate how long great white sharks can live...

Large great white known as Deep Blue (Image source: www.divephotoguide.com)

They found that white sharks have remarkable longevity - they may live longer than 70 years! This means that white sharks have significantly longer lives than previously thought - making them amongst the longest lived of all sharks!


This may have serious implications for the conservation of great whites, because it may mean that they mature at an older age than previously thought or that their reproductive potential over their entire lifespan is lower than we anticipated. This may mean the population declines the white shark has experienced may be harder to come back from than we had hoped (Hamady et al, 2014).


“These findings change the way we model white shark populations and must be taken into consideration when formulating future conservation strategies”

- G.B. Skomal, Researcher



References

Hamady LL, Natanson LJ, Skomal GB & Thorrold SR (2014). Vertebral Bomb Radiocarbon Suggests Extreme Longevity in White Sharks. PLoS One, 9:1, e84006. Access online.


Kerr LA, Andrews AH, Cailliet GM, Brown TA & Coale KH (2006). Investigations of Δ14C, δ13C, and δ15N in vertebrae of white shark (Carcharodon carcharias) from the eastern North Pacific Ocean. Environmental Biology of Fishes, 77, 337–353. Access online.


By Sophie A. Maycock for SharkSpeak.

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