hether persistent toxic substances contributed to
lamprey is one prime factor and commercial over-exploita-
the extirpation of the fisheries resources in the
tion is another.  There have been good programs since the
Great Lakes basin has been a longstanding
1970s for controlling the sea lamprey.  These programs
controversy within the Great Lakes scientific community.
have been instrumental in re-establishing current day lake
In the late 1980s, after the discovery of high levels of
trout populations in Lake Superior and parts of Lake
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCCD) in Lake
Huron.  In conjunction with sea lamprey larvicide treat-
Ontario fish and herring gulls, U.S. EPA sampled sediment
ments of Lake Ontario tributaries, which began in 1971,
cores from Lake Ontario to reconstruct a history of con-
attempts to reestablish lake trout populations through
tamination.  Subsequently, these data were used in a
stocking of yearlings continued. Adult lake trout were
retrospective risk assessment for lake trout by Dr. Philip
present in the late 1970s, however, no sac fry from natural
Cook, who reported his results to the Great Lakes Water
reproduction were observed until 1986.
Quality Board on July 15, 1993 in Chicago at its 102
Meeting as part of the Water Quality Board’s reevaluation of
Dr. Cook and his colleagues graphically show that for at
dioxins.  Individual publications on the components of the
least a period of 40 years (1940 to1980) feral lake trout
retrospective risk assessment have been published in peer
reproduction was compromised due to the presence of
reviewed scientific literature.  The synthesis was presented
high levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)
by Dr. Cook at the February 1999 Special Meeting of the
and TCDD-like contaminants in Lake Ontario (Cook et al.
Great Lakes Science Advisory Board to Assess Scientific
2003).  Of all the possible PCDD, PCDF, and PCB conge-
Issues in Lakewide Management Plans for Critical Pollut-
ners, only 21 are known to be highly toxic to fish. These
ants, and was included in the 1997 - 1999 Priorities and
potent chemicals have chlorine substitution patterns that
Progress under the Great Lakes Water Quality Agreement
create molecular geometries with planar conformations
report.  Dr. Cook presented the completed retrospective
similar to that of TCDD.  Lake trout embryos and sac-fry are
risk assessment to the SAB’s Work Group on Ecosystem
very sensitive to toxicity associated with maternal expo-
Health on September 12, 2002 in Windsor, Ontario.
sures to TCDD and the 20 structurally related chemicals.
Over the last 15 years research has established the toxicity
Historically, the Great Lakes had large stable lake trout
of the various congeners relative to TCDD and a set of
populations until the last century and in each case, at some
toxicity equivalence factors (TEFs) based on trout early life-
point in time, there was an extreme decline of the lake
stage mortality has been developed.  TCDD induced early
trout populations.  In Lake Ontario, that decline started
life toxicity in lake trout resembles a non-infectious,
earlier than in the other Great Lakes, such that by 1960,
edematous condition known as blue-sac and exhibits a very
lake trout were completely extirpated.  There are a number
steep dose-response curve with a threshold beginning
of factors in the Great Lakes that certainly had major
around 30 pg TCDD/g egg and ending with 100 percent
contributions to the decline of lake trout populations and
mortality occurring near 100 pg TCDD/g egg.
the loss of reproducing populations.  Predation by sea
Dr. Cook’s retrospective risk assessment involved predic-
tion of exposure and bioaccumulation of the TCDD-like
congeners for years when egg samples were not available
for analysis or when lake trout were absent from the
In conjunction with sea lamprey larvicide treatments
ecosystem.  Historical TCDD toxicity equivalence concen-
of Lake Ontario tributaries, which began in 1971,
) were predicted from
trations in lake trout eggs (TEC
attempts  to  reestablish  lake  trout  populations
radionuclide dated sediment core sections.  This assess-
ment of exposures of lake trout embryos over time relative
through stocking of yearlings continued. Adult lake
to recent conditions required: 1. fine resolution of radionu-
trout were present in the late 1970s, however, no
clide dating profiles in two sediment cores; 2. reference
core specific biota-sediment accumulation factors (BSAFs)
sac fry from natural reproduction were observed
for each TCDD-like chemical in lake trout eggs; and 3. an
until 1986.
adjustment of the BSAFs for the effect of temporal changes
in the chemical distributions between water and sediments.