revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 422
5. POTENTIAL FOR HUMAN EXPOSURE
Most recently, the Northeast states and Eastern Canadian provinces issued their own mercury study,
including a comprehensive analysis of current mercury concentrations in a variety of fresh water sportfish
species (NESCAUM 1998). This study involved a large number of fish sampling sites in each state, many
of which were remote lake sites that did not receive point source discharges. Top level piscivores (i.e.,
predatory fish) such as walleye, chain pickerel, and large and smallmouth bass were typically found to
exhibit some of the highest concentrations, with average tissue residues greater than 0.5 ppm and
maximum residues exceeding 2 ppm. One largemouth bass sample was found to contain 8.94 ppm of
mercury, while one smallmouth bass sampled contained 5.0 ppm. A summary of the mean and
minimum–maximum (range) of mercury concentrations in 8 species of fish sampled is shown in
Table 5-10. This study also identified a relationship between elevated mercury levels in fish and certain
water quality parameters, including low pH, high conductivity, and elevated levels of dissolved organic
carbon.
Lake trout taken from Lake Ontario between 1977 and 1988 did show a progressive decline in mercury
contamination from 0.24 µg/g (ppm) in 1977 to 0.12 µg/g (ppm) in 1988 (Borgmann and Whittle 1991).
Samples of zooplankton taken from an Illinois lake in 1986 contained approximately 10 ng/g (ppb)
mercury; however, fish that fed on the zooplankton had whole body mercury concentrations ranging from
11.6 µg/kg (ppb) for inedible shad to 69 µg/kg (ppb) for edible largemouth bass, indicating
bioaccumulation was occurring up the aquatic food chain. Older fish generally had higher mercury
concentrations (Kohler et al. 1990). Mercury concentrations in crayfish taken from 13 Ontario lakes with
no known mercury inputs ranged from 0.02 to 0.64 µg/g (ppm); the concentrations were positively
correlated with organism weight and fish mercury concentrations (Allard and Stokes 1989). Brown trout
taken from Lake Ontario contained between 0.18–0.21 µg/g (ppm) mercury in unskinned fillets and
between 0.24–0.26 µg/g (ppm) mercury in skinned fillets, indicating that methylmercury is associated with
the protein fraction of fish tissue (Gutenmann and Lisk 1991).
Methylmercury constitutes over 99% of the total mercury detected in fish muscle tissue, with no detection
of inorganic or dimethylmercury (Grieb et al. 1990; Bloom 1992). Mercury levels were examined in
aquatic organisms taken from the Calcasieu River/Lake Complex in Louisiana. The order of enrichment
was as follows: shrimp (0.2 µg/g [ppm])