Worthless waters: by midcentury, seas' value may be drained.

The biological riches of the oceans will be spent within decades if current trends continue. A global analysis of marine ecology predicts that wild seafood will effectively disappear by midcentury.

"People have fished for as long as we've dwelled on the planet," says study leader Boris Worm. "Within our lifetime, it's going to be over."

Worm, a marine biologist at Dalhousie University in Halifax, Nova Scotia, and an international team of scientists examined data from dozens of localized studies of changes in marine biodiversity. In aggregate, the studies offer a panoramic view of how ecosystems respond to species depletion. The researchers also considered whether species have recovered in places where people discontinued fishing.

In one data set after another, the researchers found that each loss of biodiversity--for example, the disappearances of gray whales, dolphins, and salmon from the North Sea, or the collapse of cod populations in Massachusetts Bay--increased the likelihood of subsequent losses and cut the odds of ecological recovery.

Possible countermeasures to fish declines include setting off new areas as marine reserves and altering management of unsustainable fisheries and destructive coastal activities. Marine reserves and fisheries closures increase species diversity by an average of 23 percent, Worm and his team find.

The researchers report their findings in the Nov. 3 Science.

Projected into the future, the trends suggest that by 2048, catches of all marine organisms will fall to less than 10 percent of their historic highs.

Worm says that he came up with that estimate by crunching data on his laptop computer as he proctored a student examination at his university. He was stunned. Disbelieving his computer, he redid the math by hand and confirmed the result.

"Those students I was overseeing ... they'll see the end of seafood," says Worm.

The year 2048, he says, offers "avery tangible deadline of when we're going to hit the bottom of the barrel."

"My guess is it will happen sooner than that," says Elliott A. Norse, president of the Marine Conservation Biology Institute in Bellevue, Wash. By relying on simple extrapolation, the new study has underestimated the immediacy of the threat, he says. It doesn't account for China's exploding demand for seafood, for example, or for the impact of climate change on oceans.

The new study is still impressive, Norse says, because it "carefully quantifies and confirms what a lot of smart people ... have been saying for a long time."

The oceans provide valuable "ecosystem services" other than food, says Jane Lubchenco, a marine ecologist at Oregon State University in Corvallis. These include recycling sewage into usable nutrients and fostering marine ecotourism. Moreover, coral reefs, mangroves, and other features of healthy oceans and coasts protect people from tsunamis and hurricanes.

"Loss of species diversity is detrimental to [those] human interests" Lubchenco says.

Despite the bleak new finding, says Norse, "there is an uplifting message here: If we exercise restraint ... and start treating our Earth as if our lives depend on it, we're going to be OK. It is not 2048 or 2040. We still have some time."