Françoise Gaill, a French marine biologist and vice president of the Ocean & Climate Platform, who is also a scientific adviser at France’s National Centre for Scientific Research (CNRS), spoke to FRANCE 24.

FRANCE 24: What is a dead zone?

Françoise Gaill: Dead zones are hypoxic areas in the ocean, where the concentration of oxygen is below the norm. This can mean a decrease of up to 20 percent, which is already quite significant, but can reach up to a 50 percent drop in oxygen levels.

The lack of oxygen occurs in the ocean’s surface areas, between 50 and 400 metres deep. The shallowest waters are generally less affected since they have more contact with the air and therefore benefit from oxygenation, which is less available in deep water.

Dead zones are mostly found off the coast of the Americas, from California to Chile. West Africa is also affected, as is the western part of Indonesia in the Indian Ocean.

Although they mostly hug coastlines, we are starting to see some dead zones stretch from the Americas into the middle of the Pacific, far from the shore.