The world’s oceans have lost around 2 per cent of their oxygen on average over half a century, alarming scientists who have warned of the trend’s impact on fisheries and endangered species.

The number of low oxygen sites along coasts globally has spiralled from 45 in the 1960s to around 700 now, according to a peer-reviewed report by 67 researchers for the International Union for Conservation of Nature (IUCN).

Presented at the ongoing UN climate talks in Madrid , the research projects …