How Nuclear Bombs Helped Scientists To Calculate The Age Of Whale Sharks?

How Nuclear Bombs Helped Scientists To Calculate The Age Of Whale Sharks?

• “How old do sharks get up to?” is a frequent question a shark scientist gets asked from the public. There are over 500 different species of sharks, and so the answer varies if we know it. However, many times the answer is “we don’t know” because it’s quite hard to measure! For some fish, scientists can examine the ear bones (otoliths) to determine the animal’s age. 
• When sectioned the otoliths show a pattern of rings, like those rings one can see in a tree that can then be counted. Larger fish are often trickier since they can be so long-lived that counting rings becomes nearly impossible. Not to mention sharks, skates, and rays lack otoliths and have their age calculated from growth bands formed in the vertebrae (similar to those the otolith rings). So how are these predators aged? Enter radiocarbon dating.

• “Above-ground testing of thermonuclear weapons in the 1950s and 60s increased the ratio of carbon 14 isotopes in the atmosphere that were then mixed into the ocean, passed up food webs and incorporated into marine organisms. As a result, the timing of the deposition of bands can be validated by comparing carbon isotope values within vertebrae, with an isotope baseline chronology of known age,” explained a new paper in the Frontiers journal. Radiocarbon dating works by comparing the three different isotopes of carbon: the lighter, stable isotopes of ¹²C and ¹³C, and the heaviest isotope ¹⁴C (radiocarbon), which is radioactive. When living things die, tissue is no longer being replaced and the radioactive decay of ¹⁴C becomes apparent. 
• This decay is used as a clock that researchers can used to calculate how much time has passed, since every living thing has the same amount of ¹⁴C compared to ¹²C as our atmosphere (this is called the ¹⁴C:¹²C ratio). Radiocarbon dating doesn’t produce exact dates, but if know the ¹⁴C:¹²C ratio at the time of death and the ratio today, you can estimate a time period. Radiocarbon dating was used when determining the age of the longest-living vertebrates known on Earth, the Greenland sharks (Somniosus microcephalus). 
• It isn’t a new age-testing method, with the first radiocarbon dates being conducted in 1949 by Professor Willard Libby, who won a Nobel Prize for this discovery. But even today scientists are trying to validate the timing of the production of growth bands in animals, with many tagging individuals with chemical markers (such as oxytetracycline or OTC), allowing for the tag to act as a time stamp that allows the rate of deposition of subsequent growth bands to be determined.
• Accurate and reliable estimates of the age and growth are crucial to effective sustainable management strategies for any species, especially vulnerable elasmobranches. If they calculations for their age aren’t accurate it can lead an ineffective management regime or, worse case scenario, the stock collapse of exploited species (similar to what happened to orange roughy, Hoplostethus atlanticus). This is the opposite of what scientists want for their study species, especially one as charismatic as whale sharks (Rhincodon typus). 
• Easily identifiable by their blue skin that is peppered with constellations across their whole body, they are the largest species of shark today reaching lengths of 30 feet or more and weighing more than 20 tons. They are seen in all tropical waters as they prefer warm water, are famous for their migrations to various “hot spots” around the world, and are currently listed on the IUCN Red List of Threatened Species as Endangered due to threats to these gentle giants face like marine pollution, boat strikes, and accidental entanglement in fishing nets. 

• As is true for many sharks, our understanding of their movements, behavior, connectivity and distribution has improved, but their basic life history traits remain largely unknown and are frequently inferred. However, a new study has shed light on their longevity using bomb radiocarbon assays to provide the first validated age estimates for whale sharks. They can get up to at least 50 years old, and this research shows that growth bands in sectioned vertebrae could provide accurate estimates of sharks aged up to at least 50 years old including large species like great white sharks, short fin makes (Isurus oxyrinchus), sandbar (Carcharhinus plumbeus), and porbeagles (Lamna nasus). 
• The new study also looked at growth models for whale sharks, and tentatively confirms supposed sightings of whale sharks that are over 65 feet (20 meters) in total length (TL). They predict that a L_∞ 72 ft (21.9 m) TL, which is close to the largest maximum length ever recorded in the wild from Taiwan in March 1987 (65 ft/20 m) and close to maximum sizes recorded in other locations. This suggests that the previous growth models could underestimate maximum sizes of whale sharks, but until more research is done with more individuals this cannot be completely proven. 
• These results of greater observed longevity and size have important implications for conservation of whale sharks, as it means it can be further susceptible to sources of anthropogenic mortalities. “We are hopeful that the demographic data we have provided in this study will help to improve the accuracy of population models (e.g., persistence, survival) and hence, better inform management and conservation efforts for this iconic species,” commented the authors.