Fears that whale shark numbers are in decline have been confirmed even though a large number of countries have ceased harvesting the world's largest fish.
'Because these animals migrate up to 12,000 km, Australia's whale shark population is shared with many other countries in South-East Asia and around the Indian Ocean,' Dr Mark Meekan, of the Australian Institute of Marine Science, said.
The same enormous migrations make whale sharks a difficult species to study, and until recently little has been known about their population numbers or biology. However, the long life spans of the giant sharks, believed to be more than 70 years, indicate that they will take a long time to recover from over-harvesting.
The last commercial whale shark fishery has recently closed, but there is still some local fishing and possibly some illegal harvesting.
Photographs taken of whale sharks congregating off Ningaloo Reef in Western Australia over the past 12 years show that numbers of sharks are declining.
Charles Darwin University’s Dr Corey Bradshaw, said that depending on the model used to analyse survival rates, the decline may be quite gradual or as fast as 10 per cent a year.
'A big gap in our understanding of whale sharks is how often they breed and how many offspring they produce,' Dr Bradshaw said.
'We also know very little about how populations are connected, and this information will tell us a lot about the probability of future decline and potential extinction.'
Dr Bradshaw studied photographs to see which sharks had not been sighted in a particular year, and used this to calculate how likely it was that a particular shark had died or simply not been photographed.
Whether the age at maturity was taken at an optimistic 12-13 years or a more realistic 20-25, there was no question the sharks were disappearing faster than they should be.