Extending this pattern into the future, the scientists calculated that by 2048 all species would be in collapse, which the researchers defined as having catches decline 90 percent from the maximum catch.

This applies to all species, from mussels and clams to tuna and swordfish, said Boris Worm, lead author of the study, which was published in the current edition of the journal Science.

Ocean mammals, including seals, killer whales and dolphins, are also affected.

"Whether we looked at tide pools or studies over the entire world's ocean, we saw the same picture emerging," Worm said in a statement. "In losing species we lose the productivity and stability of entire ecosystems. I was shocked and disturbed by how consistent these trends are -- beyond anything we suspected."

When ocean species collapse, it makes the ocean itself weaker and less able to recover from shocks like global climate change, Worm said.

The decline in marine biodiversity is largely due to over-fishing and destruction of habitat, Worm said in a telephone interview from Dalhousie University in Halifax, Nova Scotia.

OVER-EXPLOITATION

The loss of biodiversity makes ocean ecosystems less able to recover from the effects of global climate change, pollution and over-exploitation, Worm said.

He likened a diverse ocean environment to a diversified investment portfolio.

With lots of different species in the oceans, just as with lots of different kinds of investments, "You spread the risk around," Worm said. "In the ocean ecosystem, we're losing a lot of the species in our stock portfolio, and by that we're losing productivity and stability. by losing stability, we're losing the ability of the system to self-repair."

"This research shows we'll have few viable fisheries by 2050," Andrew Sugden, international managing editor of Science, told reporters at a telephone news briefing. "This work also shows that it's not too late to act."

To help depleted areas rebuild, marine-life reserves and no-fishing zones need to be set up, Worm and other authors of the study said. This has proven effective in places including the Georges Bank off the US Atlantic coast, he said.

With marine reserves in place, fishing near the reserves can improve as much as four-fold, Worm said.

Beyond the economic benefits to coastal communities where fishing is a critical industry, there are environmental benefits to rebuilding marine biodiversity, the scientists said.

Depleted coastal ecosystems are vulnerable to invasive species, disease outbreaks, coastal flooding and noxious algae blooms, they reported.

Certain kinds of aquaculture -- like the traditional Chinese cultivation of carp using vegetable waste -- can also be beneficial, according to the scientists. However, farms that aim to raise carnivorous fish are less effective.