The number of oxygen-deprived “dead zones” in the world’s oceans has been increasing since the 1970s and is now nearly 500, threatening fisheries as well as humans who depend on fish, the U.N. Environment Program announced in unveiling its first-ever Global Environment Outlook Year Book.

These “dead zones” are caused by an excess of nitrogen from farm fertilizers, sewage and emissions from vehicles and factories. In what experts call a “nitrogen cascade,” the chemical flows untreated into oceans and triggers the proliferation of plankton, which in turn depletes oxygen in the water.

While some of the fish might flee this suffocation, millions die every year, slow moving, bottom-dwelling creatures like clams, lobsters and oysters are even less able to escape.

The 500 dead zones — most in Europe and the U.S. East Coast — range from under a square mile to up to 45,000 square miles.

The single largest contributing factor is the contaminated rivers that flow into these areas, clean these up and the problem itself goes away.

The installation of AquaCannons with their high volume of oxygenated water injected into these rivers will dramatically speed up the recovery time.