The two-thirds of the planet covered by seas provide one fifth of the world's protein -- but 75 percent of fish stocks are now fully exploited or depleted, a Hanoi conference that ended Friday was told.

Warming seas are bleaching corals, feeding algal blooms and changing ocean currents that impact the weather, and rising sea levels could in future threaten coastal areas from Bangladesh to New York, experts said.

"People think the ocean is a place apart," said Peter Neill, head of the World Ocean Observatory. "In fact it's the thing that connects us -- through trade, transportation, natural systems, weather patterns and everything we depend on for survival."

Marine ecosystems and food security were key concerns at the Global Conference on Oceans, Coasts, and Islands, an international meeting of hundreds of experts from governments, environmental groups and universities.

"There is a race to fish, but in wild capture fisheries right now we can catch no more," said Steven Murawski, fisheries chief science advisor at the US National Oceanic and Atmospheric Administration (NOAA).

"We catch 100 million metric tonnes per year, and that's been very flat globally. Our only hope is if we conserve and rebuild stocks," he said, adding that sustainable aquaculture could help make up the shortfall.

The current plunder is risking long-term sustainability with "too many fishing boats taking too many fish and not allowing the stocks to regenerate," said Frazer McGilvray of Conservation International.

"Once the oceans are gone, we're gone. The oceans sustain the planet."

The world has already seen the effects of over-fishing, experts said.

North Atlantic cod fisheries collapsed in the 1990s, anchovies previously disappeared off Chile, herring off Iceland and sardine off California.

Sixty-four percent of ocean areas fall outside national jurisdictions, making it difficult to reach international consensus or to stop illegal fishing -- a growing concern as high-tech ships scour the high seas.

"It's the Wild West. It's a very small number of boats but the technology allows them to take enormous amounts of fish," said Neill.

"They take only the high commercial product and they throw the bycatch overboard. The waste is extraordinary."

Marine life is also being harmed by climate change, said Murawski.

"We've seen that fish populations go up and down with variations in the climate," he said. "Increasingly we are starting to see long term change affect the productivity, the distributions, the migrations."

The trend is speeding up, Murawski said.

"Our forecasts are wrong," he said. "The melt-off is much faster than has been forecast in the models."

Meanwhile land-based pollution puts heavy strain on oceans, said Ellik Adler of the UN Environment Programme.

"Rivers of untreated sewage, factories, refineries, oil industry discharge their effluent into the marine environment, and this causes huge damage," he said. "Marine pollution has no political borders."

There are few easy fixes, experts said, but one initiative now being considered is setting up a global network of marine protected areas.

"You've got to get agreements between countries," said consultant Sue Wells, whose has worked in coastal East Africa. "Some developed countries have already closed some areas, and most coastal countries are now considering it."

Satellites could monitor no-catch areas, she said, while inspiration could come from South Pacific fishing communities.

"They have taboo areas, coral reef sanctuaries, where fish would be saved for bad weather periods or major festivals and feast," she said. "They know if they leave an area and don't fish there, they'll have much better stocks."

It is a view that has been lost in modern times, she said, where the common view now was "if I don't go and fish it, someone else will."

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