Where have all the large, mature whale sharks gone?
A recent study of whale sharks sizes reveals disturbing data. Large, mature whales sharks are increasingly rare. In 1995 whale sharks over 10m in size were reported around the world from India to Belize.
Today, other than two adult populations in the Easter Pacific, it seems only whales sharks less than 7m are being reported.
Ana Sequeira, a research associate at the University of Western Australia and the study’s lead author, started looking at whale sharks around the coral reefs of Western Australia’s Ningaloo Coast and subsequently expanded the study worldwide.
'The majority of whale sharks seen at Ningaloo were juveniles with mean lengths of around six metres,' Sequeira said in a statement. 'Given the fact that the fish reach maturity when they are about nine metres long, it prompts the question: Where are the adults?'
The study of recorded sightings of the ocean's largest fish from 1995 to 2013 reveals that those of the largest individuals were limited and occurred mostly prior to 2006.
The report notes that marine vertebrates which have long life spans and are slow to reach maturity - such as whales sharks - are particularly susceptible to human impacts such as habitat loss, pollution and overexploitation by fisheries.
Measuring the size of large animals at sea is notoriously inaccurate. In Ningaloo the scientists have developed a method of stereo-video measuring which dramatically improves accuracy. They used this to calculate the likely over reporting of size of whale sharks over the study period and re-examined the data taking this into consideration - the big beasts still seem to be missing!
Study coauthor Mark Meekan of the Australian Institute of Marine Science suggests tagging and satellite tracking some of the larger whale sharks left around Ningaloo to learn more about their population and where they roam.
Whale sharks are listed as threatened by the International Union for Conservation of Nature and protected by governments in Australia, Mexico, and China. But they are still illegally hunted for their fins and oil in other parts of the world.
'Understanding the whereabouts of the biggest whale sharks will also help us understand how human activity, such as industrial developments, fisheries, and boat strikes, might impact the animals,' Meekan said.
The report was published in the Royal Society of Open Science.