Temporal changes in mercury concentrations of large-bodied fishes in the boreal shield ecoregion of northern Ontario, Canada

Much of the mercury (Hg) in freshwater fish of the boreal shield ecoregion is believed to originate from atmospheric deposition. As such, declines in fish Hg concentrations would be expected in response to recent declines in atmospheric Hg deposition in this ecoregion. We compared recent (2005–2010) and historic (1974–1981) muscle total mercury concentrations ([THg], standardized to a fish body mass of 1 kg) in seven fish species (five piscivores, two benthivores) from 73 lakes in northern Ontario (Canada) using a paired-comparisons approach. The rate of bioaccumulation (i.e., slopes of loge[THg] vs loge total length relationship) increased for walleye (Sander vitreus) but did not change significantly for any other species. There was no significant decline in mean [THg] between recent and historic time periods for any species. In fact, recent mean [THg] were slightly higher (< 0.08 ppm) than historic mean [THg] for all species, and this difference was significant for northern pike (Esox lucius). The magnitude of the temporal change in northern pike declined significantly from south to north over the study area but there were no discernible geographic patterns in the temporal change in [THg] for any other species. This study shows that [THg] of most large-bodied fish species in boreal shield lakes are not declining in response to the decline in atmospheric Hg deposition.

► In northern lakes the mercury in fish originates mainly from atmospheric deposition.
► Atmospheric deposition of mercury has recently been declining in North America.
► We predicted a similar decline in fish mercury levels in the boreal shield ecoregion.
► No such decline was evident in this region from the late 1970s to the present.
► Fish mercury levels in this region are not currently limited by atmospheric inputs.