Sources and Variations of Mercury in Tuna

While the bulk of human exposure to mercury is through
the consumption of marine fish, most of what we know about
mercury methylation and bioaccumulation is from studies
of freshwaters. We know little of where and how mercury
is methylated in the open oceans, and there is currently a
debate whether methylmercury concentrations in marine
fish have increased along with global anthropogenic mercury
emissions. Measurements of mercury concentrations in
Yellowfin tuna caught off Hawaii in 1998 show no increase
compared to measurements of the same species caught
in the same area in 1971. On the basis of the known increase
in the global emissions of mercury over the past century
and of a simple model of mercury biogeochemistry in the
Equatorial and Subtropical Pacific ocean, we calculate
that the methylmercury concentration in these surface waters
should have increased between 9 and 26% over this 27
years span if methylation occurred in the mixed layer or in
the thermocline. Such an increase is statistically inconsistent
with the constant mercury concentrations measured in
tuna. We conclude tentatively that mercury methylation in
the oceans occurs in deep waters or in sediments.