© Reuters: Ho New
Low oxygen levels, which have been found along south-eastern Australia, are known to increase stress on fish.
Australian 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.
Professor Ove Hoegh-Guldberg, of the ARC Centre of Excellence for Coral Reef Studies (CoECRS) and from the University of Queensland, 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.
