Great white sharks roam the coastal waters of Australia. How many there are nobody knew. A recent study shed light on this well kept mystery.
It is a well-known fact that great white sharks are common inhabitants of Australian waters. But how many of them are actually there? The answer to that question has been a mystery for a long time. A recent study of CSIRO researchers published in Scientific Reports attempted an answer. The method the researchers used is called close-kin mark-recapture. They used tissue samples of juvenile sharks to look for half-sibling pairs. Great whites are viviparous. The females give birth to only one young at a time. The likelihood of finding a half-sibling couple is directly dependent on two factors: The size of the adult population - the less sharks there are the more likely you will find half-siblings - and the survival rate of adult sharks – the higher the survival rate, the more offspring an individual shark can produce, and therefore the more likely you’ll find half-siblings among juvenile sharks. Another clever aspect of the study was that the researchers didn’t use the traditional mark-recapture approach. That would be catching the same individual at least twice (capture and re-capture again). As this is almost impossible with highly migratory sharks, finding a half-sibling was considered a ‘re-capture’, as the genetic profile of an already recorded juvenile shark was ‘re-captured’ in its half-sibling.
Using complex statistical methods the CSIRO scientists extrapolated an estimate of the current adult and juvenile population from the likelihood of finding half-siblings among the juvenile sharks. They defined two different shark populations in Australia: the eastern and the southern-western population. For the former they estimated 750 adult and 5460 juvenile great whites. For the latter they found 1460 adults, and therefore almost twice as many. The number of juveniles in the south-west is still to be determined.
The researchers are expecting a positive trend of the shark population for the future. Sharks are protected in Australia since 1999. Since it takes the great whites 12-15 years to mature, the population only recovers very slowly. This is the worldwide first genetically based study to reliably estimate a population of great white sharks.
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Source:
https://www.nature.com/articles/s41598-018-20593-w
Photo credit: wikipedia.com, Terry Goss
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