The practice is particularly crude and cruel, critics say. The "finners" pull the sharks onto the boat, hack off some or all of their four fins, then throw the shark, usually still alive, back into the water. Unable to swim, the sharks sink to the bottom of the sea and die.
"Not only is it horrible to look at," says Peter Knights, the executive director of Wildaid, a conservation group, "but it's sheer waste. Ninety-five percent of the shark is thrown overboard."
Shark meat is considered inferior to other kinds of commercially fished species, such as tuna or swordfish, experts say. While it would be difficult for fishermen to store entire shark carcasses, the fins take up little space. They can be dried and kept for months.
The fins are sold primarily to China for shark fin soup, considered a delicacy and a symbol of affluence. According to a report by the group Wildaid, shark fins are now among the most expensive seafood products in the world, selling for $700 per kilogram on the Hong Kong market.
Demand for shark fins has increased with China's economic growth, the report says. Shark fin soup was originally served as part of formal banquets by the Ming dynasty. But with the creation of a new middle class in China with disposable income, what was once rare is now common at weddings and corporate banquets.
Knights says it's "new money with old ideas -- a bad combination." Wildaid and the Chinese Wildlife Conservation Association conducted a survey in China early this year and found that 35 percent of those interviewed said they had consumed shark fin soup in the last year. That adds up to a consumer base of more than 100 million people. But Wildaid says few are aware of the shark-finning process or the effect on the shark population.
Conservationists estimate some shark populations have declined as much as 80 percent or more in the last 50 years. While there may be a variety of reasons for that decline, they say the shark fin market is only making the situation worse.
Some countries now have a ban on shark finning, including the United States, Australia and Ecuador. Wildaid is campaigning for a global ban. But until that happens, the conservation group is targeting consumers to cut demand.
Knights compares shark finning with the war on drugs. "Burning the cocoa fields doesn't do it. You're always fighting a losing battle until you can dissuade people not to use this anymore."
To raise awareness in China, Wildaid enlisted the help of celebrities, including NBA basketball superstar Yao Ming and action movie star Jackie Chan.
At a Beijing press conference, Yao pledged never to eat shark fin soup, "under any circumstances," and asked "for the sake of our future, please join us." Yao is one of several athletes who have already made slick public service announcements that lobby against the illegal wildlife trade.
In one dramatic ad, the 7-foot-6 basketball star runs down the court and leaps to block a bullet aimed at a giant elephant. It became a favorite on YouTube. Wildaid hopes Yao will shoot another PSA soon -- this one against shark finning.
Knights is confident an education campaign can work. He points to elephant ivory, which was subject to a vigorous public awareness campaign; most people now consider it politically incorrect to keep ivory pieces in their home or to wear as jewelry.
"Prior to '89," Knights says, "before ivory was banned, 270 tons were stored in Hong Kong and all over." Today, under the ban, the illegal trade of ivory still goes on, but elephant populations, which were once under serious threat, "are relatively safe," Knights says, as a result of both the ivory ban and a drop in demand through education.
Knights hopes Wildaid's campaign on behalf of sharks will soon make shark fin soup no longer welcome on the banquet table.