Plastic pollution in the ocean has either come from the land due to littering or poor waste disposal (See "How does plastic get into the ocean from land?") or directly from the ocean itself because of fishing and shipping littering. A large review of sources of plastic to the ocean by an organisation known as the Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESMAP) summarised the ocean sources of plastic littering as follows:
Large shipping vessels are constantly travelling across the world’s oceans. It is estimated that 90% of world trade is carried about by the international shipping industry. These large shipping vessels have large crews and can travel for months. As a result, they generate significant amounts of waste daily such as wire straps, packaging materials, plastic sheets & boxes and sewage. This plastic waste can accidentally enter the ocean when weather conditions are poor or inadequate waste storage can result in deliberate plastic disposal into the ocean. In addition, the regular cleaning of ships requires the use of plastic abrasives which are directly released into the ocean.
Plastic fishing gear such as nets, traps, lines, ropes, buoys, bait boxes and bags are frequently found in the ocean due to accidental or deliberate disposal. The ways in which fishing gear is handled can depend on the fishing region, the type of fishery and the type and size of the fishing vessel. A report by Greenpeace suggests that plastic fishing gear makes up 10% of all the plastic waste in our oceans. This lost and abandoned fishing gear is often called ghost gear because it can continue to trap and entangle animals in the marine environment.
GESAMP (2016). “Sources, fate and effects of microplastics in the marine environment: part two of a global assessment” (Kershaw, P.J., and Rochman, C.M., eds). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/ UNEP/UNDP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP No. 93, 220 p.
Greenpeace (2019). Ghost Gear: The Abandoned Fishing Nets Haunting Our Oceans. Available at: https://storage.googleapis.com/planet4-international-stateless/2019/11/8f290a4f-ghostgearfishingreport2019_greenpeace.pdf. (Accessed: 26 June 2020).