A field experiment was conducted to determine the degree to which fish
accumulated methylmercury (MeHg) via their food or via passive uptake
from water through the gills. Finescale dace (Phoxinus neogaeus) were
held in 2000 L enclosed pens floating in an undisturbed, oligotrophic
lake in northwestern Ontario. Fish were exposed to water containing e
ither low (0.10-0.40 ng L-1), intermediate (0.45-1.30 ng L-1), or high
(0.80-2.1 ng L-1) concentrations of MeHg. Zooplankton with either low
(0.16-0.18 mu g g(-1) d.w.) or high (0.28-0.76 mu g g(-1) d.w.) conce
ntrations of MeHg were added daily to each pen. Fish fed zooplankton w
ith high concentrations of MeHg had significantly higher concentration
s of mercury in muscle after 32 days than fish fed zooplankton with lo
w concentrations of MeHg (ANCOVA, P<0.0001). Fish feeding on zooplankt
on with low concentrations of MeHg had the same amount of Hg in their
tissues as fish at the start of the experiment. Uptake from water was
at most 15%. This is the first experiment to confirm that food is the
dominant pathway of MeHg bioaccumulation in fish at natural levels of
MeHg.