Page 846 - The Toxicology of Fishes

826 The Toxicology of Fishes

PCDFs were greatest in fish collected in Lake Ontario. The potencies of PCDD, PCDF, and planar PCBs
are summed by the use of 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents (TEQs) (van den Berg et al.,
1998). TEQs calculated based on fish-specific TCDD toxicity equivalency factors (TEFs) were 70, 30
(±13), 28, 7, and 6 pg/g in fish collected from Lakes Ontario, Michigan, Huron, Erie, and Superior,
respectively (DeVault et al., 1989). Slightly greater concentrations of PCDDs and PCDFs were reported
in Great Lakes fish collected earlier (Stalling et al., 1983). These data do not allow direct comparisons
of TEQs, but the reported concentrations are consistent with the findings of DeVault and coworkers
(1989). Concentrations of PCDDs and PCDFs in lake trout collected from Lake Michigan were less in
1988 (12.7 pg/g) (Wright and Tillitt, 1997), as compared with the earlier collections and consistent with
a decline over time. The most complete evaluation of temporal trends of PCDDs and PCDFs in lake
trout has come from predictions based on analysis of dated sediment cores (Cook et al., 2003). Estimated
concentrations of TEQs in lake trout from Lake Ontario were maximum (250 to 275 pg/g) in the mid-
1960s and declined exponentially from the 1970s to the 1990s. A detailed analysis of this data and the
subsequent risk analysis are presented later in this chapter.
The current trends in concentrations of chlorinated hydrocarbons are not expected to decrease as much
as previous years; instead, they are expected to slowly decline as the number of point sources continues
to decrease. In the case of PCBs, although the concentrations entering the lakes from point sources is
now less than at the peak of releases in the late 1960s, large stores of PCBs are still present in the
terrestrial environment and in contaminated sediments; thus, PCBs are still entering the lakes. Never-
theless, concentrations of PCBs in fish tissues have decreased by a factor of approximately 25 since
maximum concentrations were reached in the lower Great Lakes in the mid-1970s. Although there are
differences among species and locations, in general the trends for persistent halogenated hydrocarbons
in Great Lakes fish during the last decade are either slightly decreasing or fluctuating around an apparent
steady state (Hickey et al., 2006; Tillitt et al., 1998). Most POPs have been banned in the industrialized
countries of North America and Europe from manufacture and continued new use; however, long-range,
atmospheric transport is also responsible for new releases of POPs into the Great Lakes (Baker, 1997).
The northern latitudes of the lakes, along with the large surface area, make atmospheric deposition an
important source of contaminant loading. It is difficult with the current datasets to ascertain whether
concentrations of chlorinated hydrocarbons have attained a new equilibrium or whether the rates of
decline are slowing. Regardless, changes in concentration are difficult to distinguish on an annual basis,
and it appears that current concentrations of HAHs and organochlorine pesticides will not decline greatly
over the next decade.
Concentrations of mercury in lake trout were measured as part of the National Contaminants Biomon-
itoring Program (NCBP) of the U.S. Department of Interior (Schmitt et al., 1985, 1990). Other monitoring
programs, such as those of the U.S. Environmental Protection Agency (USEPA), did not include mercury
in their list of targeted analytes until this century. The concentrations of mercury in lake trout from the
Great Lakes remained fairly constant during the period from the mid-1970s until the mid-1980s. Concen-
trations of mercury were 0.15 to 0.33 µg/g in Lake Superior, 0.12 to 0.17 µg/g in Lake Michigan, 0.15
to 0.29 µg/g in Lake Huron, 0.12-0.17 µg/g in Lake Erie, and 0.30 to 0.40 µg/g in Lake Ontario fish
(Schmitt et al., 1990). The Canadian Department of Fisheries and Oceans (DFO) has maintained extensive
contaminant monitoring efforts in the Great Lakes for the past three decades and has monitored mercury
in the aquatic ecosystem through measurements in rainbow smelt (Osmerus mordax). From 1981 to 2002,
concentrations of mercury in rainbow smelt from the Great Lakes ranged from 0.02 to 0.1 µg/g in Lake
Superior, 0.05 to 0.07 µg/g in Lake Huron, 0.02 to 0.04 µg/g in Lake Erie, and 0.04 to 0.08 µg/g in Lake
Ontario (Great Lakes Binational Toxics Strategy, http://www.epa.gov/glnpo/bns/dioxin/index.html).

Aryl-Hydrocarbon-Receptor-Mediated Toxicity in Fish

Biological responses elicited by dioxin-like PCBs, PCDDs, and PCDFs are triggered by binding to a
cellular protein: the aryl hydrocarbon receptor (AhR). This protein is highly conserved in vertebrates
and is a member of the PAS family (per–arnt–sim gene family) of basic helix–loop–helix regulatory
proteins. Its role as a ligand-activated transcription enhancer is well understood in the context of induction