As we cannot be around to observe their whole lives, it can be very difficult to understand how quickly sharks grow, how their lives change as they get bigger and what the maximum size they can reach is. This is not just information we seek out of sheer curiosity. Understanding how sharks age and grow are vital to determining how they might respond to population declines and to design management plans for their conservation. So how do scientists figure out how old sharks are?
Learning to Love the Bomb
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 (Caillet et al, 1985; Hamady et al, 2014).
However, because this method can sometimes be inaccurate, recently scientists have also been using "radiocarbon-dating" to calculate age in sharks (Caillet et al, 1985; Kett et al, 2006; Hamady et al, 2014).
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). 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, so the carbon isotopes decays at a consistent rate. This means that, if we measure the carbon-based radioactivity of something, we can use the known decay rate to calculate its age (Caillet et al, 1985; Kett et al, 2006; Hamady et al, 2014).
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 shark's skeleton! (Kett et al, 2006; Hamady et al, 2014)
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 (Kett et al, 2006).
Growing Old and White
To give you a real world example, a research team used a combination of vertebral band analysis and radiocarbon dating to estimate the ages of large white sharks (Carcharodon carcharias), to try to estimate how long great white sharks might live (Hamady et al, 2014).
They found that great whites 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! (Hamady et al, 2014).
Life Plans
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; Rigby et al, 2022) 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 has been hindered by gaps in our knowledge regarding their growth rates and longevity (Hamady et al, 2014).
White sharks are thought to reach sexual maturity between 8 and 12 years old (males must reach the critical size of 3.6 metres before they become sexually mature and females must be 4.8 metres) and whilst here are several anecdotes that great white sharks may reach 7 or even 8 metres in length, the largest individual to have been accurately measured was 6 metres (Hamady et al, 2014).
These findings may have serious implications for the conservation of great whites, because it may mean that they mature at an older age than previously thought and/or that their reproductive potential over their entire lifespan might be lower than we anticipated (Hamady et al, 2014).
This may mean the population declines the white shark has experienced may be harder to come back from than we had hoped. Conservationists will need to consider these new findings when designing management strategies for great whites in the future (Hamady et al, 2014).
References
Cailliet GM, Natanson LJ, Welden BA & Ebert DA (1985). Preliminary studies on the age and growth of the white shark, Carcharodon carcharias, using vertebral bands. Memoirs of the Southern California Academy of Sciences, 9:4, 49-60. Access online.
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.
Rigby CL, Barreto R, Carlson J, Fernando D, Fordham S, Francis MP, Herman K, Jabado RW, Jones GCA, Liu KM, Lowe CG, Marshall A, Pacoureau N, Romanov E, Sherley RB & Winker H (2022). Carcharodon carcharias (amended version of 2019 assessment). The IUCN Red List of Threatened Species 2022: e.T3855A212629880. Access online.
By Sophie A. Maycock for SharkSpeak
