'Creeping dead zones' (CDZ) are oxygen-starved areas of the world's oceans.
Dawn of the dead
Creeping dead zones are areas where the water at the sea-floor is anoxic - meaning that it has very low (or completely zero) concentrations of dissolved oxygen. This situation is the result of coastal waters algae blooms due to nutrient pollution from sewage discharges and agriculture.
As the algae die and rot, they consume oxygen, and the oxygen dissolved in the water falls to levels unable to sustain marine life. CDZs are especially dangerous to fisheries because they afflict coastal waters where many fish spawn and spend most of their lives before moving to deeper waters.
These dead zones are occurring in many areas along the coasts of major continents, and they are spreading over larger areas of the sea floor. Well known CDZs have long afflicted the Baltic Sea and are an annual feature in the Gulf of Mexico where the Mississippi River brings down huge quantities of fertilisers from the agricultural heartlands of the United States. Many of the areas where increasing deoxygenisation has recently been observed are near the mouths of major river systems, and they're now spreading to other bodies of water, such as the Black Sea, Adriatic Sea, Gulf of Thailand and Yellow Sea. They're also appearing off South America, Japan, Australia and New Zealand.
The phenomenon is getting noticeably worse. According to the UN Environmental Programme (UNEP), the number of known oxygen-starved areas has doubled since 1990 to nearly 150, with some stretching 70,000 square kilometres, about the size of Ireland.