In 1994 United Nations Food and Agriculture Organization (UNFAO) estimated global by-catch rates to be as high as 27 million tons of fish discarded by fisheries each year. Many individuals of non-target species perish as by-catch in the cast of each drift net. As a result, many such species are now endangered. Species caught as by-catch include sharks, dolphins, whales, turtles, sea birds, and other marine mammals. Since nets are placed and may not be retrieved for days, air-breathing mammals that become tangled in the nets drown if they are unable to free themselves. In certain areas, exemption from punitive measure due to the unintentional by-catch of marine mammals, as outlined by the Marine Mammal Protection Act, is extended to commercial drift fishermen. In the 1990s, drift net fisheries were responsible for 30,000 tons of sharks and skates in global by-catch annually. While filming National Geographic's Incidental Kill in the California Channel Islands where swordfish and various sharks swim north, the divers discovered that many drift net boats had placed nets that night. The nets were one mile long each and nearly high placed to target swordfish and thresher sharks. They swam half the length of one net and in that length discovered 32 dead blue sharks in the net as well as 2 hammer head sharks, a sea lion, and a manta ray all of which were thrown back into the ocean when the net was hauled in. Although long line fisheries are the main contributor to sea bird by-catch, sea birds are also caught in drift nets in significant numbers. Studies conducted on 30 small-scale drift net fisheries in the Baltic Sea estimate that 90,000 sea birds die annually in drift nets. Bycatch is thrown back to the ocean either dead or with injuries that may result in death. If not eaten, dead animals decompose, as bacteria use oxygen to break down the organic matter. Large amounts of dead matter decomposing in the ocean causes the surrounding levels of dissolved oxygen to decrease. Environmental damage Drift nets lost or abandoned at sea due to storms causing strong currents, accidental loss, or purposeful discard become ghost nets. Synthetic nets are resistant to rot or breakdown, therefore ghost nets fish indefinitely in the oceans. Marine animals are easily tangled in ghost nets as are the predators the dead animals attract. The float line on the net allows it to be