A report from the US Environmental Protection Agency estimates that fish in about half of US lakes and reservoirs are contaminated with mercury levels considered dangerous to health.

http://www.tennessean.com/article/20091110/NEWS01/91110055/2066/NEWS03
http://www.oregonlive.com/environment/index.ssf/2009/11/epa_study_finds_high_levels_of.html
http://www.wfn.tv/gogreen/blog.php?blog=401333
http://www.ens-newswire.com/ens/nov2009/2009-11-11-091.asp