Study Reveals Sharks and Rays Live Way More Than Previously Thought

Sharks and rays are some of the best studied marine animals. However, a new study by Dr Alastair Harry shows that we have long been wrong about a critical aspect of their biology, their lifespan. Apparently they live a lot longer, sometimes twice of what we previously thought.

Dr. Harry, a fisheries researcher at the James Cook University,  looked at 53 different populations of sharks and rays that scientists had studied in the past. According to his research, one out of the three studies he reviewed have seriously underestimated the age of sharks. For example, grey nurse sharks (Carcharias taurus) can live to be at least 34-years-old, and not 22 as previously thought whereas the common thresher (Alopias vulpinus) can live to be 38 years old instead of 20. 

"Questions arose over methods of ageing sharks after it was found that grey nurse sharks can live up to 40-years-old, double the length of time first thought, and the age of New Zealand porbeagle sharks had been underestimated by an average of 22 years." said Dr Harry. [1]

Great whiteshark (credit)

The problem when ageing a shark

A common method used by scientists to measure a shark's age is by counting growth rings in their vertebrae, the same method used to measure a tree's growth. However, sharks aren't  trees and sometimes sharks simply stop growing, which means counting the rings on a shark's vertebrae might give wrong results. This is why researchers often confirm their findings using other methods too, like  chemical marking and bomb-carbon dating.  

However, this is not the case with all the published papers:

"Age underestimation appears to happen because the growth rings cease to form or become unreliable beyond a certain size or age. Across the cases I studied age was underestimated by an average of 18 years, and up to 34 years in one instance. From the amount of evidence we now have it looks like the problem is systemic rather than just a few isolated cases." said Dr Harry. [1]

Implications

According to Dr Alastair Harry, accurate ageing of sharks is very important for many reasons. For example, many of the models used by fisheries to make decisions about the fish that can be caught can be very wrong, putting overwhelming pressure to certain species.

"It could lead to inefficient prioritisation of research, monitoring and management measures. If it's as widespread and common as it seems from this study, the impacts could also be substantial from a wider scientific perspective, affecting the many disciplines that also use baseline life history data." said Dr Harry. [1]

Fun Fact: Longest-living shark needs 150 years to reach sexual maturity

Here's a quick fun fact for you. The Greenland shark (Somniosus microcephalus) holds the world's record for longest lifespan between all vertebrate species. In 2016, a study on 28 specimens determined by radiocarbon dating that the oldest of the sharks sampled was 392 ± 120 years old. The same study also concluded that it takes about 150 years for them to reach sexual maturity! [3]

Greenland shark (credit)

References

• https://www.sciencedaily.com/releases/2017/09/170929093259.htm
• Onlinelibrary.wiley.com/doi/10.1111/
• http://science.sciencemag.org/content/353/6300/702

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