<P> Marine debris is mainly discarded human rubbish which floats on, or is suspended in the ocean . Eighty percent of marine debris is plastic--a component that has been rapidly accumulating since the end of World War II . The mass of plastic in the oceans may be as high as 100,000,000 tonnes (98,000,000 long tons; 110,000,000 short tons). </P> <P> Discarded plastic bags, six pack rings, and other forms of plastic waste which finish up in the ocean present dangers to wildlife and fisheries . Aquatic life can be threatened through entanglement, suffocation, and ingestion . Fishing nets, usually made of plastic, can be left or lost in the ocean by fishermen . Known as ghost nets, these entangle fish, dolphins, sea turtles, sharks, dugongs, crocodiles, seabirds, crabs, and other creatures, restricting movement, causing starvation, laceration, infection, and, in those that need to return to the surface to breathe, suffocation . </P> <P> Many animals that live on or in the sea consume flotsam by mistake, as it often looks similar to their natural prey . Plastic debris, when bulky or tangled, is difficult to pass, and may become permanently lodged in the digestive tracts of these animals . Especially when evolutionary adaptions make it impossible for the likes of turtles to reject plastic bags, which resemble jellyfish when immersed in water, as they have a system in their throat to stop slippery foods from otherwise escaping . Thereby blocking the passage of food and causing death through starvation or infection . </P> <P> Plastics accumulate because they don't biodegrade in the way many other substances do . They will photodegrade on exposure to the sun, but they do so properly only under dry conditions, and water inhibits this process . In marine environments, photodegraded plastic disintegrates into ever - smaller pieces while remaining polymers, even down to the molecular level . When floating plastic particles photodegrade down to zooplankton sizes, jellyfish attempt to consume them, and in this way the plastic enters the ocean food chain . Many of these long - lasting pieces end up in the stomachs of marine birds and animals, including sea turtles, and black - footed albatross . </P>

Marine pollution by chemical thermal and radioactive wastes