Many species of marine life are being threatened by global warming and nutrient pollution, according to a report released by a conservation group at the UN Climate Change conference in Madrid.
The study, the biggest such survey ever undertaken, found that 700 ocean sites are suffering from low oxygen levels compared to just 45 in the 1960s, the International Union for Conservation of Nature (IUCN) said.
"We are now seeing increasingly low levels of dissolved oxygen across large areas of the open ocean," said Dan Laffoley, a co-editor of the report and senior advisor in marine science at IUCN.
"This is perhaps the ultimate wake-up call from the uncontrolled experiment humanity is unleashing on the world’s ocean as carbon emissions continue to increase."
The authors said species such as tuna, marlin and sharks are sensitive to low oxygen levels due to their relatively large size and energy demands.
The IUCN report also found that these species are being forced into oxygen-rich shallow waters where they are more vulnerable to overfishing.
Nutrient pollution is a major cause of oxygen loss in coastal waters.
It comes mainly from increasing run-off from agricultural fertilisers, sewage, animal and aquaculture waste. They cause excessive growth of algae that depletes oxygen as they decompose.
The report found the situation is also being made worse by global warming.
Increasing levels of carbon dioxide are enhancing the greenhouse effect, warming up the planet. Much of that heat is absorbed by the oceans but warmer waters hold less oxygen.
According to the findings, if the situation does not change, then the oceans are expected to lose 3-4% of their oxygen levels by the year 2100. But the report says the losses are predicted to be more concentrated in the upper 1000m of the water column which is the highest in marine diversity.