<P> Mercury is known to bioaccumulate in humans, so bioaccumulation in seafood carries over into human populations, where it can result in mercury poisoning . Mercury is dangerous to both natural ecosystems and humans because it is a metal known to be highly toxic, especially due to its ability to damage the central nervous system . In human - controlled ecosystems of fish, usually done for market production of wanted seafood species, mercury clearly rises through the food chain via fish consuming small plankton, as well as through non-food sources such as underwater sediment . </P> <P> The presence of mercury in fish can be a particular health concern for women who are or may become pregnant, nursing mothers, and young children . </P> <P> The consumption of fish is by far the most significant source of ingestion - related mercury exposure in humans and animals . Mercury and methyl mercury are present in only very small concentrations in seawater . However, they are absorbed, usually as methyl mercury, by algae at the start of the food chain . This algae is then eaten by fish and other organisms higher in the food chain . Fish efficiently absorb methyl mercury, but excrete it very slowly . Methyl mercury is not soluble and therefore not excreted . Instead, it accumulates, primarily in the viscera, although also in the muscle tissue . This results in the bioaccumulation of mercury, in a buildup in the adipose tissue of successive trophic levels: zooplankton, small nekton, larger fish, and so on . The older that such fish become, the more mercury they may have absorbed . Anything that eats these fish within the food chain also consumes the higher level of mercury that the fish have accumulated . This process explains why predatory fish such as swordfish and sharks or birds like osprey and eagles have higher concentrations of mercury in their tissue than could be accounted for by direct exposure alone . Species on the food chain can amass body concentrations of mercury up to ten times higher than the species they consume . This process is called biomagnification . For example, herring contains mercury levels at about 0.1 parts per million, while shark contains mercury levels greater than 1 part per million . </P> <P> Since the Minamata disaster, Japan has improved on its mercury regulation . During the 1970s Japan made strides to reduce mercury demand and production . Chief among these efforts was the reduction of inorganic mercury produced by mines . It was halted by 1974, and demand fell from 2,500 tons per year in 1964, its peak, to 10 tons per year in recent years . Since these initial strides, Japan has introduced a list of regulations governing the mercury content of a variety of materials . </P>

Where does the mercury in tuna come from