Is the World's Largest Shark Shrinking?

Jennifer Viegas, Discovery News 25 Jul 08;

July 25, 2008 -- Humans have over-exploited the whale shark -- the world's largest living fish -- to such a degree that the ocean giants are actually shrinking in size, according to new research.

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."

Brook said that while the whale sharks enjoy protection in Ningaloo Reef waters, the sharks migrate over large distances, often traveling thousands of miles.

"Artisanal fisheries via harpoons for meat, for example, off the coasts of India and Indonesia, but mostly by Taiwanese and Chinese commercial fisheries" are likely responsible for the declines, he said, adding that whale shark meat is referred to as "tofu fish" due to its texture, which is also prized in shark fin soup and Chinese medicine.

Ship strikes also tend to kill larger adults, he said, though evidence for the strikes is hard to compile since resulting deaths would usually remain unknown.

These latest findings counter a study late last year by Brad Norman and Jason Holmberg of ECOCEAN, a research education and conservation organization. That report, based on multiple underwater images of the sharks, concluded that Ningaloo Reef whale sharks are thriving. Norman did, however, admit to Discovery News that the species is "rare" and "vulnerable to extinction."

Brook and his colleagues have authored a written response to Norman's paper, which is still under consideration by the journal, Ecological Applications. They say that while whale sharks receive some trade protection from the Convention on International Trade in Endangered Species, "it is difficult to police non-international trade or local hunting by indigenous people."

The migratory habits of whale sharks, Brook said, "mean it is impossible to protect the Ningaloo population once the sharks leave Australian waters for Indonesia and the Philippines."

The researchers urge officials to establish well-enforced international protection for the sharks. They also hope that collaborative tagging studies in the future will help to better identify and monitor whale shark migration routes.