Mr. Knights pointed out that the United Nations Convention on International Trade in Endangered Species, which restricts trade on items derived from endangered or threatened species, denied protection to five shark species last year after nations that profit from the trade voted against it.

Shark populations cannot tolerate intensive fishing because sharks have few offspring and often do not reproduce until they are over 10 years old. Even by conservative estimates, more than 10 million shark fins moved through Hong Kong in 2008, the main distribution center for the trade. Fins sell for over $300 a pound.

Many marine biologists support tougher regulation of shark fishing itself.

“These bans go part way, but you’re still allowed to fish sharks without a permit,” said John Bruno, a marine ecologist at the University of North Carolina, Chapel Hill. “In North Carolina, there are shark derbies for fun, where they are hung by their tails. We think it’s O.K. to do that with this ocean predator, but we wouldn’t dream of doing it to a terrestrial animal like a bear.”

As a halfway measure to limit the fin trade, a growing number of countries, including the United States as of last December, prohibit the removal of shark fins at sea. Requiring fishing boats to take whole sharks to the dock limits the size of their catch and allows the authorities to inspect for endangered species. In the traditional shark fin trade, fishermen slice off the valued fins from a living shark and dump the still writhing body back in the water.

But shark-finning prohibitions are hard to enforce because they involve dockside inspections of numerous small boats and a sack of lucrative fins is easily hidden. When Dr. Bruno was at his university’s new research station in the Galápagos Islands of Ecuador this summer, he was summoned by local authorities to help identify sharks on a boat they had seized with more than 350 carcasses, fins already partly detached. Ecuadorean law also bans finning at sea.