The oceans are running out of oxygen, says a report of the International Union for the Conservation of Nature (IUCN).
The report reveals that the oxygen levels in the oceans have decreased by roughly two percent since 1960. In the 1960s, only 45 dead zones were found where oxygen was effectively absent. However, the number has increased to 700 in the 2010s.
This decline is called deoxygenation which is caused largely by climate change. Humans are also responsible for it.
The IUCN report, which is a combined effort of 67 scientists from 17 countries, was presented on Saturday at the United Nations' climate conference in Madrid, reports the New York Times.
Low levels of oxygen are correlated with global warming, because warmer water retains less oxygen and heating induces stratification, so less of the essential combination of oxygen-rich and oxygen-poor layers is available.
By the end of this century, oceans are expected to lose about 3-4 percent of their oxygen, but the impact will be much greater in the nearest surface levels where many species are concentrated, and in the mid to high latitudes, reports the Guardian.
Intensive agriculture also has a major role to play. If surplus synthetic fertiliser from plants or waste from the meat industry flows off the land and into rivers and oceans, it feeds blooming algae and then decomposes oxygen.
Large fish, such as sharks, tuna, marlin, are particularly at risk as bigger fish consume more energy. Lack of oxygen in deep water forces these species to come close to shore where they can be subjected to over fishing.
"We have known about deoxygenation but we have not known the linkages to climate change and this is really worrying," Minna Epps from the IUCN told The BBC.
Dan Laffoley, also from the IUCN and the report's co-editor said, "Ocean oxygen depletion is menacing marine ecosystems already under stress from ocean warming and acidification. To stop the worrying expansion of oxygen-poor areas, we need to decisively curb greenhouse gas emissions as well as nutrient pollution from agriculture and other sources."