Some great whites may be ‘great greys’
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Cape Town - Some great white sharks may need to become known as grey white sharks, judging by the results of a new study which has found that these top ocean predators can live significantly longer than previously thought – at least into their seventies.
This means that some giants that are still around today may have been patrolling the False Bay shoreline as long ago as the 1940s, local shark expert Dr Alison Kock points out.
The study, led by scientists at the renowned Woods Hole Oceanographic Institution in the US, also found they grow much slower than previously believed. This raises concerns that white shark populations are considerably more sensitive to human-induced mortality than previously thought.
Their paper, Vertebral bomb radiocarbon, suggesting extreme longevity in white sharks, appeared last week in PLOS One, one of the journals and blogs from the US-based Public Library of Science.
Estimating age in white sharks can be challenging because vertebral growth bands don’t necessarily signify annual growth.
“Traditionally, ageing sharks has relied on the assumption that band pairs are annual. In many cases this has been proven correct for part or all of a species’ life; however, in more and more cases this is being disproven,” says fisheries biologist and co-author Lisa Natanson.
In previous studies, which assumed annual deposition of growth bands, the oldest white sharks identified were from the south-western Pacific Ocean at 22 years old and the western Indian Ocean at 23 years old.
In this study, the researchers looked at the well-documented presence of radiocarbon (C14) that was produced by thermo-nuclear device testing done during the 1950s and ‘60s.
This “fallout” was incorporated into the tissues of marine organisms living then. This can be used as a “time stamp” to help determine the age of an animal.
The scientists conducted radiocarbon analysis on collagen in vertebrae samples of eight white sharks that had been caught in the north-western Atlantic Ocean between 1967 and 2010. They determined age estimates of up to 73 years for the largest male shark and 40 years for a female.
“Our results dramatically extend the maximum age and longevity of white sharks compared to earlier studies,” says PhD student and lead author Li Ling Hamady.
“Understanding longevity of the species, growth rate, age at sexual maturity, and differences in growth between males and females are especially important for sustainable management and conservation efforts.”
Kock, research manager of the local Shark Spotters organisation, says the exciting new research confirms scientists’ suspicions that previous longevity estimates were too low.
“This means that white sharks, out of more than 500 (shark) species, are among the longest-lived sharks in the world,” she says.
“It also puts a thought-provoking perspective on the presence of these sharks in Cape waters, many of which could be older than most of us living here today and could have been visiting our shores since the 1940s.
“The results of this study also infer slower growth rates for this species, which means white sharks are even more vulnerable to human impacts, like fishing, than previously thought.”