The iconic whale sharks that congregate at Ningaloo Reef off Western Australia are declining in number, according to a recent study published in the Journal of Animal Ecology.
Researchers from the Australian Institute of Marine Science and Charles Darwin University have used 12 years of whale shark photographs from Ningaloo Reef to monitor and predict trends in population size. The scientists' models show a steady decline in numbers of the giant fish.
The study follows increased efforts over the last decade to learn more about the Ningaloo whale shark aggregation due to fears that over-harvesting outside of Australian jurisdiction could pose a threat to the gentle giants.
"Because these animals
migrate up to 12,000 kilometers, Australia's whale shark population is shared with many other countries in Southeast Asia and around the Indian Ocean,"says AIMS fish biologist Dr Mark Meekan.
"Although many countries including India and Taiwan have recently halted or reduced their commercial take of whale sharks, continued harvesting throughout Southeast Asia is probably still occurring."
Because whale sharks grow slowly and reproduce infrequently they are particularly vulnerable to over-exploitation.
Prior to the mid-180s, there were only 50 confirmed reports of whale sharks worldwide. In 2001 the species was listed as nationally threatened under the Environment Protection and Biodiversity and Conservation Act 1 .
Dr Corey Bradshaw from Charles Darwin University believes that understanding more about the movement patterns and reproductive biology of whale sharks may hold the key to their protection.
"A big gap in our understanding of whale sharks is how often they breed, and how many offspring they produce. 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 says.
In Western Australia, whale sharks are a huge draw card for the tourism industry, bringing in over $6 million each year.
Although it's too early to tell what a population decline could mean for this industry, scientists say that improved conservation and further research will be critical for the survival of the species.
"Piece by piece, scientific research is beginning to unravel the biology and ecology of what were previously very elusive creatures. Rigorous scientific studies on whale sharks are helping to protect the species and the tourism industries that rely on them," says Dr Meekan.
While Drs Bradshaw and Meekan believe their findings are only preliminary given the long generation time of whale sharks, they predict that evidence for declines will increase as science uncovers more information about the inter-continental activities of whale sharks.
Source: Bradshaw, C. J. A., Mollet, H. F., and Meekan, M. G. (2007). Inferring population trends for the world's largest fish from mark-recapture estimates of survival. Journal of Animal Ecology 76, 480-48.
Corey J. A. Bradshaw , Senior Research Fellow, Charles Darwin University
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Dr Mark Meekan , Scientist in Charge, AIMS Darwin Office,
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Wendy Ellery , AIMS Media Liaison
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