For many years, U.S. power companies and the electrical industry used compounds known as polychlorinated
biphenyls (PCBs) to insulate electrical transformers and other equipment. PCBs were also used as fluids in industrial equipment in many manufacturing sectors. By the 1970s, it was recognized that PCBs were toxic to wildlife and humans, causing damage to the reproductive,neurological, and immune systems at high exposures. As a result, PCBs were banned in the late 1970s. Nevethless, these compounds still persist in the environment because they break down very slowly. Ongoing cleanup efforts have cost millions of dollars. Contaminants can reach the ocean through atmospheric deposition. For example, mercury is released into the air when large quantities of coal and other fuels containing trace amounts of the element are burned, from the incineration of mercury-containing medical wastes,and from other human-induced sources. Ultimately, that mercury rains down into lakes, rivers, and the ocean. Once deposited in sediments, mercury may be converted by aquatic organisms into methylmercury, a more toxic form of the element.Many contaminants stay in the environment for a long time and become more concentrated through the food chain. Among the most troubling contaminants are heavy metals, such as mercury and cadmium, and “persistent organic pollutants,” such as PCBs, dioxin, and DDT, which remain in the marine sediment for a long time. Marine life takes up such contaminants from polluted sediments. Because these contaminants are much more soluble in fat than water, they are excreted slowly and build up in the tissues of fish and shellfish. They also accumulate in the tissues of people who eat contaminated
fish. Progress has been slow in reversing PCB contamination in the thirty years since they were banned. Physical removal of PCB-contaminated sediments by dredging or other methods has had limited success. Likewise, even though atmospheric levels of mercury have dropped from their peak levels in the 1980—thanks to regulatory actions including mandated mercury controls on coal power plants and more strict management of mercury wastes—mercury levels in fish remain high in many areas.