Oxygen levels in the oceans are falling because of climate change and pollution, which could harm countless species of fish.
The finding comes from a major study by the conservation organisation IUCN.
Scientists have known for decades that nutrient levels in the oceans were changing. Now researchers say climate change is making the loss of oxygen far worse, threatening fish such as tuna, marlin and sharks, because larger fish need more energy.
More than 700 marine areas around the world now suffer from low oxygen. In the 1960s the number was only 45.
Farms, factories and warming seas
Scientists say nutrient pollution happens when nitrogen and phosphorus from farms and factories wash into the sea.
They had also known that this affects the oxygen in seawater, but until now it was thought to affect only coastal waters.
In recent years, however, climate change has increased the scale of the threat. As greenhouse gases push up carbon dioxide, the oceans have to absorb more heat, and warmer water holds less oxygen.
Scientists estimate that the oceans lost 2 per cent of their oxygen between 1960 and 2010.
Source: BBC Bangla. First published in Bengali on The Agro News.
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