Methylmercury is held tightly to fish protein when absorbed through the gills or when contaminated food sources are eaten. In some cases, methylmercury levels in carnivorous fish, such as freshwater bass, walleye and pike, and marine shark and swordfish, bioaccumulates up to a million times greater than in the surrounding water. Although fish appear to be tolerant to large body burdens of methylmercury, there have been extensive human deaths in cases of severe poisoining. For example, in the 1950s, the Chisso Corporation in Minamata, Japan, released untreated effluent containing methyl mercury chloride into Minamata Bay. Once in the bay's sediments, the mercury was readily absorbed by marine species, contaminating the entire ecosystem. Fish consumed by local residents resulted in the deaths of more than 1000 individuals and severely impacted the developing fetuses of pregnant women.
In general, levels of mercury increase with fish size and age, although not always. Levels also vary by species and location. Bioaccumulation in fish is influenced by the amount of methylmercury present, which is in turn affected by local biogeochemical processes and by mercury inputs from atmospheric pollution.
Piscivorous (fish eating) predators such as loons, merganser ducks, osprey, eagles, herons, and kingfishers, generally have very high concentrations of mercury. Mercury has been detected in Common Loons from Alaska to Atlantic Canada, and blood concentrations have been correlated with levels in prey fish species. A recent survey of mercury in loons from five regions across the US and Canada has shown that blood mercury concentrations increased from west to east, with the highest levels in southeast Canada. High levels of mercury are suspected to impair the loon's reproductive success as well as cause growth related problems.These problems inevitably lead to an increased death rate and a decreased birth rate, resulting in a reduction in the abundance of natural populations.
In addition, mercury has been found in predatory mammals such as otters. It is thought that elevated mercury levels in otters may cause early mortality due to toxicity and behavioral changes. While the reproduction and behavior of bird species is generally affected by exposure to methylmercury, mammals most often suffer neurological effects. The severity of the toxic effects will depend on the degree of exposure, and may range from a slight impairment to reproductive failure or death.
In the past, mercury risk reduction strategies focused on restricting human consumption of heavily contaminated fish in order to protect human health. Such a strategy is clearly not adequate for the protection of wildlife. Species such as otter and mink cannot heed warning notices or fish consumption advisories. Since mercury is so widely distributed in the environment, their risk is real and immediate, especially when effects such as impaired growth and reproduction, neurological damage, kidney damage, and weight loss, which occur at relatively low concentrations, are considered.