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Where Have the Largest Whale Sharks Gone?

Marine biologists have raised concerns about the whereabouts of the world’s biggest whale sharks after finding that the largest sharks observed in recent years were smaller than those recorded more than a decade ago.

Dr Ana Sequeira of The University of Western Australia’s Oceans Institute, who led the study, said it was important to know the size of whale sharks because it provided information about their population status. However, it’s difficult to obtain accurate size estimates as this needs to be done while they are freely swimming.

“A common technique is to compare the sharks with an object of known size while swimming alongside them. However, these estimates are often inaccurate,” Sequeira said. “We found the margin for error increased as the actual size of the target increased, which meant that big sharks of around 10–11 metres were mistakenly thought to be up to about 3 metres smaller.”

The new study, published in Royal Society Open Science (http://tinyurl.com/jle7hpm), “compared visual estimates of whale shark sizes with those obtained using an underwater stereo-video system”. This found that the largest sharks observed at Ningaloo Reef in recent years were smaller than those recorded at the same location more than a decade ago. “The majority of whale sharks seen at Ningaloo were juveniles with mean lengths of around 6 metres which, given the fact that the fish reach maturity when they are about 9 metres long, prompts the question: where are the adults?”

Study co-author Dr Mark Meekan of the Australian Institute of Marine Science said that, apart from groups of large females reported at two locations in the eastern Pacific Ocean, there was a lack of adult whale shark sightings around the world. “Co-occurrence of adult males and females ensures the survival of a species, so not knowing the whereabouts of adult whale sharks and how many still exist presents a challenge for understanding their conservation status,” he said.

Meekan said that more research is needed to help locate large whale sharks and to clarify the number of mature animals still in existence. “Understanding the whereabouts of the biggest whale sharks will also help us understand how human activity such as industrial developments, fisheries and boat strike might impact the animals,” he said.