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Low oxygen levels, which have been found along southeastern Australia, are known to increase stress on fish. ((Ho New/Reuters))

Scientists fear the planet is on the brink of another mass extinction as ocean dead zones continue to grow in size and number.

More than 400 ocean dead zones — areas so low in oxygen that sea life cannot survive — have been reported by oceanographers around the world between 2000 and 2008.

That is compared with 300 in the 1990s and 120 in the 1980s.

Ove Hoegh-Guldberg, a professor at the ARC Centre of Excellence for Coral Reef Studies (CoECRS) and the University of Queensland in Australia, says there is growing evidence that declining oxygen levels in the ocean have played a major role in at least four of the planet's five mass extinctions.

"Until recently the best hypothesis for them was a meteor strike," he said.

"So 65 million years ago they've got very good evidence ... all the dinosaurs died because of smoke and stuff in the atmosphere from a meteor strike.

"But with the four other mass extinction events, one of the best explanations now is that these periods were preceded by an increase of volcanic activity, and that volcanic activity caused a change in ocean circulation.

"Just as we are seeing at a smaller scale today, huge parts of the ocean became anoxic at depth.

"The consequence of that is that you had increased amounts of rotten egg gas, hydrogen sulfide, going up into the atmosphere, and that is thought to be what may have caused some of these other extinction events."

Hoegh-Guldberg says up to 90 per cent of life has perished in previous mass extinctions and that a similar loss of life could occur in the next 100 years.

"We're already having another mass extinction due to humans wiping out life and so on, but it looks like it could get as high as those previous events," he said.

"So it's the combination of this alteration to coastlines, climate change and everything, that has a lot of us worried we are going to drive the sixth extinction event and it will happen over the next 100 years because we are interfering with the things that keep species alive."

Hearts and lungs

Scientists say ocean dead zones, which vary in size from one square kilometre to 70,000 square kilometres, have been found all over the world.

Particular hotspots include the Gulf of Mexico, off Namibia in the South Atlantic, in the Bay of Bengal, in the Baltic, the Black Sea, the tropical South Pacific, off China and southeastern Australia.

"We're seeing an expansion of areas of the ocean which are very low in oxygen and also very low in nutrients," Hoegh-Guldberg said.

"Climate change is driving changes to water circulation — so winds, strange weather patterns, have a consequence for how the ocean turns over and aerates and so on, and it's the winds which are delivering a lot of organic compounds into the deep sea.

"At the same time we are putting a lot of fertilizer off coastlines, those sorts of things are incubating these deep water anoxic zones.

"So it's the combination of those two things that are having a big change on how the ocean works."

He said organic matter building up in the sea is a huge problem.

"You get enormous amounts of organic carbon building up at depth, bacteria then likes to break down that organic matter and bacteria uses up the oxygen," he said.

"So then what you get is a substantial drop in oxygen — that then has the consequences for fishers, for the productivity of coastlines and so on."

Destructive path

Mark McCormick, an associate professor at CoECRS and James Cook University, said low oxygen levels increase stress on fish.

"We know from our recent work that increases in stress result in deformities, leading to poorer survival of fish larvae," he said.

"It has also been found they can cause fish to have smaller ovaries, produce fewer eggs, so larvae are also smaller and less likely to survive."

Hoegh-Guldberg said while the dead zones may only exist in pockets of ocean today, it will affect a far greater area in the future unless steps are taken to reduce the impact of human activities on the world's oceans and their life.