It's not easy to like hammerhead sharks. Ugly to the point of freakishness, stupid beyond mere brutishness, the species is one of nature's least agreeable creations. Nonetheless it was bad news last week when the journal Science published a paper revealing that the unloved hammerhead, as well as many other species of shark, is vanishing faster than we imagined. If the beleaguered predators - consumed as delicacies, hunted as trophies and inadvertently caught by fishing fleets seeking other game - should ever disappear entirely, the world's oceans could be in serious trouble.

Biologist Ransom Myers of Dalhousie University in Nova Scotia led a team of researchers in examining the logbooks of U.S. fishing boats operating in the northwestern Atlantic from 1986 to 2000. The fleets were hunting swordfish and tuna using what are known as longlines, cables stretching as long as 20 miles that are equipped with more than 500 baited hooks. Toss a line overboard, and up should come your desired prey - plus a lot of other hungry fish that you didn't mean to snag. "Longlines are designed to collect large marine predators," says biologist Julia Baum, lead author of the Science paper. "But sharks often swim in the same area as tuna and swordfish, and that means they can get caught too."

If the boat has room in its hold, the sharks may be kept with the rest of the catch and sold for their meat. If not, their fins, prized on the market for shark-fin soup, may be cut off and the rest of the animal - alive and bleeding - tossed back. In 2000, former President Bill Clinton signed an order banning shark finning, but that covers only U.S. fleets, and whether all of them comply is hard to know.

Using the number of sharks caught on longlines as an index of the number surviving in the wild, the Dalhousie team came up with some alarming figures. From 1986 to 2000, nearly all shark species may have declined at least 50%, with the populations of some approaching collapse. Tiger-shark populations are down 65%, the legendary white shark has fallen 79%, and the hammerhead is in the worst shape of all, down a staggering 89%.

Unless you're a shark gourmand, the disappearance of such lethal beasts might not seem like a bad thing. For marine life, however, it could be a disaster. Despite their ferocity, sharks ensure a kind of order in the oceans. Sitting at the top of the food chain, they keep other large predators in check, regulating who gets to eat whom and who gets to survive and thrive. Want to preview an ocean after the sharks have gone? Picture Yugoslavia after the Soviets: a bloodbath. "We know from studying lakes that top predators have disproportionate effects on their ecosystems," says Baum.

The Dalhousie team believes that there is still time to save the sharks before the most imperiled species are entirely wiped out, but conservationists must act fast. The best protection method is to establish marine reserves - areas of ocean where fishing is off limits for a while so the vanishing species can catch its breath and rebuild its ranks. Such a strategy works for imperiled commercial fish and could work for sharks too, although it would take more time because sharks breed relatively slowly.

Marine reserves, however, are generally effective only if the fleets agree to catching fewer fish for a while, as opposed to merely moving their operations outside the protected zone. The latter not only maintains the pressure on the failing species that swim outside the boundaries but also increases pressure on other species in the new fishing grounds. If fishermen do heed the warnings of the Dalhousie study, a healthy marine balance may still be maintained. If they don't, the world's oceans could grow leaner, and meaner, than ever.