Dead Zones in Fisheries

The popular term dead zones refers to bodies of water almost devoid of aquatic life-forms, except perhaps bacteria. Increasingly recognized as a serious threat to marine ecosystems, the number of dead zones—now more than 400 worldwide and covering some 245,000 square kilometers—has roughly doubled each decade since the 1960s. Dead zones are scientifically measured by the quantity of dissolved oxygen in a given body of water, which can range from normoxia (normal to high levels of oxygen) to anoxia (devoid of oxygen). Hypoxia—the intermediate state—occurs when there is less than 2 milliliters of oxygen per liter of water. Typically, dead zones are hypoxic waters that can be graded as mild, severe, or persistent, depending on the duration and intensity of diminished oxygen levels. This entry ...