And models of global warming indicate the trend will continue because oxygen in the air mixes less readily with warmer water. Large fish such as tuna or swordfish avoid, or are unable to survive, in regions starved of oxygen.

"Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economies," according to the scientists writing in the journal Science.

The north of the Indian Ocean, along with the Arabian Sea and the Bay of Bengal, is also oxygen-low but the available data showed no substantial change in the size of the oxygen-minimum zone in recent decades.

Lothar Stramma, lead author at IFM-GEOMAR in Kiel, Germany, said there were signs the oxygen-low bands between 300 and 700 metres depths were getting wider and moving into shallower coastal waters.

"The expansion of the oxygen-minimum zones is reaching more to the continental shelf areas," he told Reuters. "It's not just the open ocean." That could disrupt ever more fisheries.

Problems of lower oxygen supply add to woes for the oceans led by over-fishing as the world struggles to feed an expanding population. A UN conference in 2002 set a goal of trying to reverse declines in fish stocks by 2015.

The scientists said levels of dissolved oxygen in the oceans had varied widely in the past and more study was needed. "We are far from knowing exactly what will happen," Stramma said.

In the most extreme case, at the end of the Permian period about 250 million years ago, there were mass extinctions on land and at sea linked to high levels of carbon dioxide and extremely low oxygen levels in the waters.

The UN Climate Panel said last year that global warming, stoked by human use of fossil fuels, would push up temperatures and bring more droughts, floods, heatwaves and rising sea levels. More and more species would be at risk of extinction.

Thursday's study showed that a swathe of the eastern Pacific from Chile to the United States and a smaller part of the eastern Atlantic, centred off Angola, were low in oxygen.

Stramma said the oxygen-poor regions were away from major ocean currents that help absorb oxygen from the air. And warmer water is less dense and so floats more easily -- that makes it less prone to mix with the deeper levels of the oceans.