The US Senate on Monday toughened laws against shark finning, hoping to save the ancient fish which experts fear is on the brink of extinction due to growing demand in Chinese restaurants.

Tens of millions of sharks are killed each year by fishermen who slice off their fins -- a delicacy in Chinese cuisine -- and leave them to die in the water. Sharks live long and have few offspring, compounding risks to their survival.

The United States banned finning in 2000 and has enforced restrictions in the Atlantic and Gulf of Mexico. The new rules close a loophole that permitted trade in the Pacific so long as sharks were not finned onboard the vessel, triggering a booming clandestine industry.

The legislation cruised through the House of Representatives in early 2009 but had languished in the Senate, which approved the measure without objection on one of the last days of its session.

"Shark finning has fueled massive population declines and irreversible disruption of our oceans," said Senator John Kerry, who championed the bill.

"Finally we've come through with a tough approach to tackle this serious threat to our marine life," the Democrat from Massachusetts said.

The bill does not ban the sale of shark fin, which is readily available in many upscale Chinese restaurants in the United States.

But conservationists welcomed the bill, saying it would curb a burgeoning but largely undocumented US trade in shark fins.

"This legislation will help address not only an unspeakably cruel practice of removing fins from live animals and then releasing them to suffer a slow death," said Nancy Perry, vice president for government affairs at The Humane Society of the United States.

"It will also help address on the macro level the rapid decline of shark populations," she said.

Environmental groups estimate that up to 73 million sharks are killed each year around the world for fins, leading to declines of up to 90 percent of some species of sharks -- which have swum the oceans since the age of the dinosaurs.

Despite campaigns from activists, demand for shark fins is seen as growing as China becomes increasingly prosperous.

Matt Rand, director of the shark conservation campaign at the Pew Environmental Group, said he recently heard of shark fins selling in California for an unprecedented 800 dollars a pound, or about 1,750 dollars a kilogram.

"The United States is a major shark exporter," Rand said. "I think this legislation sends a big signal that the United States is concerned about the decline of shark populations, not just in its own waters but in international waters as well."

Sharks are caught almost exclusively for fins. While the law does not ban the killing of sharks, all fins entering the United States must have an accompanying carcass.

In one notable incident in 2002, the US Coast Guard seized a Hong Kong-chartered, Hawaii-registered ship that was hauling nearly 65,000 pounds (30,000 kilograms) of just fins -- meaning tens of thousands of sharks died.

While closing loopholes, the Senate bill also opened one. To win support from North Carolina's senators, the law makes an exception for one shark -- the smooth dogfish.

Fishermen in the southeastern US state kill the shark for all of its meat instead of just the fins, but objected to the ban because they cut off fins in their ships.

"We had hoped they would adjust their practices so there wouldn't be any loophole," Perry of the Humane Society said. "But that was done to get the legislation over the finish-line."