So-called ‘dead zones,’ oxygen-starved areas of the world’s oceans that are devoid of fish, top the list of emerging environmental challenges, the United Nations Environment Programme warned in its global overview. The spreading zones have doubled over the last decade and pose as big a threat to fish stocks as overfishing. The new findings tally nearly 150 dead zones around the globe, double the number in 1990, with some stretching 27,000 square miles. Dead zones have long afflicted the Gulf of Mexico and Chesapeake Bay, but are now spreading to other bodies of water, such as the Baltic Sea, Black Sea, Adriatic Sea, Gulf of Thailand and Yellow Sea, as other regions develop, UNEP said. They are also appearing off South America, Japan, Australia and New Zealand. The main cause is excess nitrogen run-off from farm fertilizers, sewage and industrial pollutants. The nitrogen triggers blooms of microscopic algae known as phytoplankton. As the algae die and rot, they consume oxygen, thereby suffocating everything from clams and lobsters to oysters and fish. The growing frequency of dust and sand storms is another concern, especially storms caused by land degradation and desertification in Mongolia and northern China. Scientists have recently linked similar storms, originating in the Sahara, with damage to coral reefs in the Caribbean, UNEP said.