Changes in mercury levels in Great Lakes fish between 1970s and 2007

A number of initiatives have curtailed anthropogenic mercury emissions in North America over the last two decades; however, various factors, including long-range transport of global emissions, may complicate the response of fish mercury levels to remedial actions. Since the Great Lakes of North America are together the largest surface freshwater body in the world and are under the influence of many complicating factors, trends of mercury in fish from the Great Lakes can reflect the overall impact of mercury management actions at local, regional, and perhaps global scales. Here we present a comprehensive view of mercury trends in Canadian Great Lakes fish using two large (total 5807 samples), different (fillet and whole fish), and long-term (1970s-2007) monitoring data sets. The spatial differences in lake trout and walleye mercury levels during this period have generally been within a factor of 2-3 with Lakes Erie and Superior having the lowest and highest concentrations, respectively. These spatial differences have diminished in the recent years (2000-2007). The concentrations have generally declined over the three decades (mid-1970s to 2007); however, in recent years, the concentration trends are flat in Lake Ontario walleye and appear to be increasing in Lake Erie walleye. There was a mismatch in the Lake Ontario lake trout and walleye temporal trends, which shows the importance of considering more than one fish species for proper spatial/temporal trend assessments.