Anthropogenic use of mercury (Hg) has had local and global consequences, with emissions to the atmosphere from fossil fuel combustion or waste incineration and subsequent long-range transport and deposition raising background levels by two to four times in the global environment. There are three main forms of Hg in the aquatic environment, inorganic Hgo, Hg2+ [Hg(II)], and organic CH3Hg+ [methylmercury or MeHg(I)], with MeHg(I) being of particular concern because of its potent neurotoxicity. Waterborne or dietborne exposures of fish to Hg(II) and MeHg(I) affect their growth, development, and reproduction, even in remote ecosystems. This chapter reviews the current understanding of the sources and cycling of Hg in the aquatic environment, and then focuses on the toxicity and fate of Hg(II) and MeHg(I) in fish.
