The whale shark population has also fallen by approximately 40 percent over the past decade in Western Australian waters, the new study has found, suggesting that this once prevalent shark, which can reach lengths up to 42 feet, is undergoing a severe decline in certain regions.
"We are all very alarmed at our findings, which really did defy our expectations," co-author Ben Fitzpatrick, a University of Western Australia biologist, told Discovery News.
The researchers analyzed the largest-ever database of sightings and size information on whale sharks. The database represents a long-term, continuous record of sightings -- 4,436 in total -- as well as photo ID information concerning age and size, all pertaining to whale sharks at Ningaloo Reef, Western Australia.
Because the sharks gather seasonally at the picturesque reef from March to June, a profitable industry has been built around "dive with sharks" activities. Usually by air sightings, tour operators regularly gather information on the sharks, compiled in the extensive database.
Fitzpatrick and his colleagues not only detected the population drop at the reef, but they also discovered the sharks have shrunk in body length by an average of over 6.5 feet. The overall reduction appears to be due to the disappearance of older, larger females, along with some males, within whale shark groups.
"I think it is mostly because the larger animals are being hunted for food and other products, such as for soup fins," explained Barry Brook, another co-author of the study and director of the Research Institute for Climate Change and Sustainability at The University of Adelaide.
"The larger the fin, the more valuable it is," Brook added.
The findings are published in the latest issue of Biological Conservation.
The scientists believe a selection effect may also be at work, whereby pressures are forcing smaller, younger whale sharks to breed earlier, but they believe this is just "a minor piece of the puzzle."