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chapter nine
Mercury contamination
of lake trout ecosystems
R.A. (Drew) Bodaly
Department of Fisheries and Oceans, Freshwater Institute


Karen A. Kidd
Department of Fisheries and Oceans, Freshwater Institute
Contents
Introduction
Mercury concentrations in lake trout populations in small Boreal lakes
Factors affecting mercury concentrations in lake trout
Management of mercury exposure from consumption of lake trout
Effects of mercury on fish
Possible future trends
Summary
References
Introduction
Mercury is a widespread contaminant in freshwater fish and is currently causing great
concern because of its potential impact on the health of humans and wildlife. Most mercury
(Hg) in fish flesh is present as methyl mercury (MeHg) (Bloom, 1992). This organic form
of mercury is a powerful neurotoxin and in large doses causes motor, sensory, and devel-
opmental problems in humans and other vertebrate animals (Clarkson, 1992). Because of
the concerns over human health impacts, Canadian provincial and federal agencies mon-
itor Hg concentrations in fish from lakes with important fisheries and in commercial
shipments. If total Hg (both methyl mercury and inorganic Hg) concentrations exceed the
Canadian limit for commercial sale (0.5 µg g
-1
), consumption advisories are issued and

commercial sales are restricted. Though high concentrations of persistent pesticides have
occasionally been the cause, mercury is by far the most common reason for fish consump-
tion advisories in North American freshwaters (e.g., Quebec Ministère de l’Environnement
et de la Faune, 1995; United States Environmental Protection Agency, 1998; Ontario Min-
istry of the Environment, 2003). For example, in Ontario 95% of fish consumption
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advisories in lakes were related to mercury, and consumption advisories for this contam-
inant applied to 1206 of the 1595 lakes tested (Ontario Ministry of the Environment, 2003).
In the United States in 1997 mercury accounted for 78% of the fish consumption advisories
in freshwaters (United States Environmental Protection Agency, 1998). Present-day expo-
sure of humans to MeHg results almost wholly from the consumption of fish (Clarkson,
1992).
Lake trout (Salvelinus namaycush) frequently have high concentrations of mercury
because of their position at the top of food chains and because the Boreal lakes that they
inhabit often have conditions favorable for mercury bioaccumulation. As a result, most
lake trout populations in Boreal lakes have fish consumption advisories. In this chapter
we review current knowledge concerning Hg in lake trout populations in the southern
Shield lakes of Ontario, Quebec, Minnesota, and New York.
Factors affecting Hg in freshwater fish are outlined, with discussion of the reasons
why lake trout are frequently highly contaminated with Hg. Emphasis is on lakes that do
not receive direct anthropogenic discharges of Hg but rather receive their Hg from atmo-
spheric sources and local weathering of the earth’s crust. Examples of Hg in lake trout
populations are given, especially to demonstrate how Hg varies with size and trophic
position of the fish and with the food-web structure of the lake. Approaches to managing
mercury contamination in freshwater systems are outlined, including sampling needed to
determine existing levels and advisory systems to advise the public of recommended
consumption limits. Finally, some speculation is made about future trends of Hg in lake
trout populations.
Mercury concentrations in lake trout populations in small Boreal lakes
Mercury concentrations in lake trout in Boreal lakes are frequently high, and most popu-

lations of lake trout have at least some fish with Hg concentrations greater than the
Canadian marketing limit of 0.5 µg g
-1
. For example, 74% of the lake trout lakes in Ontario
(excluding the Great Lakes) contain lake trout with Hg concentrations greater than the
Canadian limit, and therefore consumption advisories exist for this species (Ontario Min-
istry of the Environment, 2003). In Quebec, 80 of 105 lake trout populations (76%) that
have been sampled had consumption advisories (Quebec Ministère de l’Environnement
et de la Faune, 1995). Braune et al. (1999) noted that mercury in lake trout from northern
Canadian lakes usually exceeds consumption guidelines. Fish consumption advisories
exist for lake trout in all regions that have been sampled, demonstrating that mercury is
a widespread problem for this species and its consumers.
Large differences in Hg concentrations are observed in fish from lakes in close prox-
imity to one another. Within a region, mean Hg concentrations in predatory fish vary
fivefold or more, even after standardization of these data for fish size and/or age. For
example, in northern Quebec lakes standardized concentrations of Hg varied about five-
fold in lake trout (Schetagne and Verdon, 1999), and predatory fish in six lakes in north-
western Ontario varied three- to fourfold (Bodaly et al., 1993). Similarly, mean Hg (stan-
dardized for fish size) in lake trout in almost 100 lakes from all regions of Ontario varied
more than 20-fold, from 0.05 to more than 1 µg g
-1
(McMurtry et al., 1989). Stafford and
Haines (1997) also found mean Hg concentrations in lake trout from 120 randomly chosen
lakes in Maine to vary more than eightfold, from 0.11 to 0.91 µg g
-1
. These studies dem-
onstrate that Hg concentrations in lake trout populations can vary considerably within a
limited geographic area.
Despite the high variability in lake trout Hg concentrations within regions, some
geographic trends remain evident. In southern Ontario, 74% of lakes have Hg in lake trout

above 0.5 µg g
-1
and 26% have Hg in lake trout exceeding 1.5 µg g
-1
; in northern Ontario,
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only 58% of lakes have Hg in lake trout greater than 0.5 µg g
-1
and only 8% have Hg in
lake trout higher than 1.5 µg g
-1
(Ontario Ministry of the Environment, 1997). In contrast
to Ontario, mercury concentrations in lake trout from Quebec are lower in southern regions
(Outaouais and Fleuve Saint-Laurent: 26 of 46 populations tested with recommended
consumption limits less than four meals per month) than in the more remote northern
areas (Lac Saint-Jean and Gaspésie–Côte-Nord: 22 of 26 lake trout populations with
restricted consumption; La Grande Rivière and Grande Rivière de la Baleine: 32 of 33
populations; excluding reservoirs) (Quebec Ministère de l’Environnement et de la Faune,
1995). The underlying causes of these trends are not yet understood but may be related
to differences in geologic or atmospheric sources of Hg.
Factors affecting mercury concentrations in lake trout
With the exception of lakes that have received direct discharges of Hg (e.g., the English-
Wabigoon river system in northwestern Ontario; Parks and Hamilton, 1987), most Hg
entering freshwater systems today is probably atmospheric in origin. This mercury orig-
inates from natural sources (such as geologic weathering, volcanic eruptions, and ocean
degassing) and from anthropogenic sources (such as burning of coal, oil, and municipal
wastes and industrial processes). Atmospheric concentrations of mercury in the northern
hemisphere have increased since industrial times, and between one-half and three-fourths
of the Hg in the atmosphere is anthropogenic in origin (Swain et al., 1992). This atmo-
spherically derived Hg is mainly in inorganic forms and enters lakes directly and indirectly

via watershed runoff. Atmospheric deposition of Hg to temperate and Arctic lakes is now
about two to three times preindustrial rates (Lockhart et al., 1995). Whether increases in
atmospheric deposition rates have caused increases in concentrations of Hg in freshwater
fish, including lake trout, is unclear but some evidence suggests this (Kelly et al., 1975;
Johnson, 1987; Swain and Helwig, 1989; Rolfhus and Fitzgerald, 1995).
Climate and rates of atmospheric deposition of Hg to lakes and their watersheds are
similar within a given region and therefore cannot explain lake-to-lake differences in Hg
concentrations in fish. The high variability in mercury levels in lake trout populations
within regions must therefore be related to the physical, chemical, and biological charac-
teristics of lakes and their watersheds. The lake-specific characteristics that are believed
to affect Hg concentrations in freshwater fish include the rate of supply of inorganic Hg
and methyl mercury to lakes and their watersheds, the trophic position and growth rates
of different fish species, and the physical and chemical characteristics of lakes and their
watersheds. The relatively high concentrations of Hg in lake trout in Boreal lakes are
probably mainly the result of the trophic position of lake trout in freshwater systems, the
relatively large size and age of many individual lake trout, and the chemical conditions
of Boreal lakes that tend to promote high Hg concentrations in fish.
Almost all of the Hg in fish muscle is MeHg (Bloom, 1992; Lasorsa and Allen-Gil,
1995; Hammerschmidt et al., 1999), and this is the form of mercury that is accumulated
in aquatic food webs. Lakes and their biota receive MeHg from three sources: precipitation,
runoff from the surrounding watershed, and in-lake methylation of inorganic mercury
(Rudd, 1995). Inputs of MeHg from precipitation are not sufficient to account for Hg in
fish in the Boreal lakes of North America, and in-lake production of MeHg by methylation
of inorganic mercury is thought to be a significant source of mercury to food chains and
fish (Rudd, 1995).
MeHg production and its bioavailability are affected by chemical factors, and many
of the conditions known to favor Hg methylation are observed in the Boreal lakes that
support lake trout populations. For example, low pH and high dissolved organic carbon
(DOC) concentrations are common in Boreal lakes, and these factors tend to be associated
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with high Hg in fish in lakes (McMurtry et al., 1989; Wiener et al., 1990; Driscoll et al.,
1994). The significant relationship between DOC and Hg in fish may be, at least in part,
a result of the inputs of DOC-associated MeHg from wetlands in a lake’s catchment (St.
Louis et al., 1994).
The physical characteristics of lakes also affect Hg bioaccumulation in lake trout and
other fish species. A study of six lakes in the Canadian Shield in northwestern Ontario
that varied in their surface area from 89 to 35,000 ha but were similar in other chemical
and physical characteristics revealed that lake size exerted a strong influence on Hg
concentrations in fish (Bodaly et al., 1993). Concentrations of mercury in yearling yellow
perch ranged from 0.04 µg/g
-1
in the largest lake studied to 0.14 µg/g
-1
(w/w) in the
smallest lake that was studied and decreased in a regular pattern with lake size (Figure 9.1).
These differences in mercury concentrations between the smaller and larger study lakes
were also seen in predatory and planktivorous species. The methylation of mercury by
microorganisms is a temperature-dependent process. The cooler epilimnetic temperatures
in large lakes decrease mercury methylation rates and subsequent inputs of mercury to
the food web (Figure 9.1) (Bodaly et al., 1993). In contrast, McMurtry et al. (1989) observed
that Hg in lake trout was positively related to lake area in a study where the ratio of
catchment to lake area was not kept constant as in Bodaly et al. (1993). Lakes with pro-
portionately larger catchment areas will have greater inputs of mercury from drainage
basin runoff and, most likely, higher mercury concentrations in the fish from these systems.
Other studies have demonstrated that the watershed size in relation to the lake size is
important in determining Hg concentrations in fish in Boreal lakes (Suns and Hitchin,
1990; Evans, 1986).
Fish obtain most of their mercury from their food and only a small proportion directly
from the water via uptake across the gills (Hall et al., 1997; Rodgers, 1994; Harris and
Snodgrass, 1993). Therefore, Hg in fish is influenced strongly by Hg concentrations in

Figure 9.1 Mean mercury concentrations in axial muscle of yearling yellow perch from six lakes in
northwestern Ontario. Vertical bars are mercury concentrations (with 95% confidence intervals and
number of fish), and points are mean epilimnetic water temperatures (June–August, 1986–1989).
Lakes are arranged in order of smallest (Green, 89 ha) to largest (Trout, 35,000 ha). From Bodaly
et al., 1993, Canadian Journal of Fisheries and Aquatic Sciences 50: 980–987.
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their diet (Borgmann and Whittle, 1992; Harris and Snodgrass, 1993; Harris and Bodaly,
1998). Top predators such as lake trout contain the highest concentrations of mercury in
part because they tend to feed on prey with high mercury concentrations. The MeHg they
are accumulating is successively concentrated from the base of the food web because it is
much more efficiently absorbed and accumulated (Mason et al., 1996) and excreted more
slowly (Trudel and Rasmussen, 1997) by organisms than the inorganic forms of Hg.
Concentrations of MeHg increase from prey to predator, and high-trophic-level organisms
tend to have the greatest concentrations of mercury in their tissues (Kidd et al., 1995;
Cabana et al., 1994). As an example, in Lake Michigan mean dry weight concentrations
of MeHg increase through the pelagic food web from 0.01 in zooplankton to 0.21 in the
insectivorous bloater to 0.59 µg g
-1
in lake trout (Mason and Sullivan, 1997), and similar
increases are seen in Boreal lake food chains.
In recent studies of mercury accumulation through food webs, the trophic position of
fish and invertebrates has been characterized using tissue ratios of stable nitrogen isotopes
(
15
N/
14
N). The heavier isotope of nitrogen is enriched from primary producers to primary
consumers, from primary consumers to secondary consumers, and so on up through the food
web by an average of 3 to 5 parts per thousand (per mil; Peterson and Fry, 1989). This
enrichment in the heavy isotope provides a continuous relative measure of an organism’s

trophic positioning within the food web and also reflects dietary habits over a period of
months to years (Hesslein et al., 1993). Kidd et al. (1995) used stable nitrogen isotope ratios
in fish muscle to quantify the trophic transfer of mercury through several food webs in
northwestern Ontario. They found a highly significant relationship between muscle con-
centrations of Hg and the trophic position of fish (as quantified by stable nitrogen isotope
analyses) in the six lakes examined (Figure 9.2). Similar relationships have been observed
for mercury and other persistent pollutants in other freshwater and marine food webs
(reviewed by Kidd, 1998). From the initial work done with this technique, it is evident
from the differences in the slope of this relationship that the accumulation of Hg through
food webs varies considerably from lake to lake; such variation may be related to varying
efficiencies of carbon transfer in these systems.
The length of the underlying food chain also significantly affects the concentration of
Hg in top predators such as lake trout. Such effects were unequivocally demonstrated by
Cabana et al. (1994) and Cabana and Rasmussen (1994). They categorized temperate lakes
into three classes based on the length of the pelagic food chain leading up to the top predator
lake trout using the presence or absence of important prey species: Class 1 lakes had the
shortest food chains with no mysids (a zooplanktivorous crustacean) or pelagic prey fishes
(rainbow smelt [Osmerus mordax], lake cisco [Coregonus artedi], lake whitefish [Coregonus
clupeaformis], alewife [Alosa pseudoharengus], and others); Class 2 lakes had an intermediate
food chain length because of the presence of pelagic prey fishes; Class 3 lakes had the
longest food chains because of the presence of both pelagic prey fishes and mysids. They
found that the concentrations of mercury in lake trout increased 3.6-fold from the Class 1
to Class 3 lakes and were significantly related to the stable nitrogen isotope ratios in this
species. It is likely that manipulations (both intended and accidental) of freshwater food
webs will influence the concentrations of Hg in top predators. Vander Zanden and Ras-
mussen (1996) observed that Hg concentrations in lake trout were considerably higher in
lakes that had introduced populations of smelt. They hypothesized that this was the result
of increased food chain lengths in the systems that had introductions of this exotic species.
Concentrations of Hg in fish are also affected by the amount of its diet that it uses for
growth relative to metabolism. The MeHg present in a fish’s diet is efficiently absorbed

and retained in its tissues; the excretion rates of MeHg are slow compared to that of Hg
and are slower in older than in younger fish (Trudel and Rasmussen, 1997). Young,
immature fish use a large proportion of their dietary carbon intake for growth and
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generally have low Hg concentrations in their tissues. This is often termed growth dilution,
and although the MeHg is not “diluted,” it results in concentrations in fast-growing,
immature predators that can be similar to that of their prey. Larger, mature fish tend to
be slow growing, and these fish use most of their ingested carbon for metabolism and
reproduction (not growth) while retaining most of the ingested mercury. These fish there-
fore tend to have higher concentrations of this contaminant (Harris and Snodgrass, 1993;
Rodgers, 1994; Harris and Bodaly, 1998), although Stafford and Haines (2001) did not find
a relationship between growth rate and mercury in a lake trout population. As predatory
fish grow, they tend to eat larger prey items with higher concentrations of contaminants,
resulting in increasing concentrations of Hg with size (Figure 9.3). This relationship
between Hg concentrations and fish size is commonly seen in temperate and Boreal lakes
(e.g., Wiener et al., 1990).
Management of mercury exposure from consumption of lake trout
The primary objective of provincial and federal agencies in Canada for managing mercury
in lake trout populations is to reduce health risks to humans. Fish are collected from lakes,
Figure 9.2 Relations between total mercury (µg.g
-1
wet weight) and standardized δ
15
N (per mil) of
fish muscle from seven species from the Northwestern Ontario Lake Size Series (Kidd et al., 1995,
Water, Air, and Soil Pollution 80: 1011–1015.). [The δ
15
N of the obligate benthivore, white sucker,
generally increased with decreasing lake size, likely as a result of differences in in-lake cycling or
sources of nitrogen. For this reason, δ

15
N of all fish were standardized to the mean δ
15
N of white
sucker using the following formula: δ
15
N
fish
− δ
15
N
white sucker
+ 6.8 (to account for the fact that suckers
are secondary consumers and that δ
15
N increases an average of 3.4 per mil with each trophic level)].
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and the concentrations of mercury in axial muscle from individual fish are determined.
Muscle tissue is typically analyzed for mercury because it is the main tissue consumed
by people. The relation between mercury concentrations and fish size is then determined,
and mercury concentrations are presented in relation to fish size in terms of the recom-
mended human consumption limit for each species sampled (Ontario Ministry of the
Environment, 1997; Quebec Ministère de l’Environnement et de la Faune, 1995). Advice
on the sizes of fish fit for consumption is delivered to the public through booklets for
anglers and signs posted on lake shores, and through the dissemination of information to
communities. In aboriginal communities, fish are often an important and significant part
of the diet. For this reason advice on the species, sizes, and quantities of fish that are safe
to eat is based on consumption information specific to these communities (Health and
Welfare Canada, 1984).
Safe consumption limits are based on recommendations from Health and Welfare

Canada for maximum allowable intake of MeHg per day (0.47 µg/kg
-1
body weight/day;
Health and Welfare Canada, 1984). Health Canada is currently recommending that Hg
intake by children and by women of childbearing age not exceed 0.2 µg/kg
-1
body
weight/day.
Sampling lakes to determine Hg concentrations in fish and its risk to human consum-
ers is relatively straightforward. From each lake, at least 20 fish of each species likely to
be caught and consumed by people should be obtained. Ideally, this sample should include
a wide range of fish lengths and weights to ensure that analyses are conducted on all sizes
of fish likely to be consumed. This eliminates the need for extrapolation of mercury
concentrations to fish sizes not sampled and ensures a statistically reliable relationship
between Hg and fish size for each species. To compare the concentrations of Hg in fishes
across lakes, statistical methods are generally used to standardize Hg for differences in
size and age. Several techniques have been used including regressions and transforma-
tions, alternative techniques such as multivariate analysis of univariate and bivariate
statistics (Somers and Jackson, 1993), or polynomial regressions with indicator variables
(Tremblay et al., 1998). As noted above, this information is often used by governmental
agencies to advise sport fishers of recommended fish consumption limits for various water
bodies (e.g., Ontario Ministry of the Environment, 1997).
Figure 9.3 Relationship between Hg concentrations and fish length in lake trout from three lakes in
northwestern Ontario. Data from Fudge et al., 1994 Canadian Data Report of Fisheries and Aquatic
Sciences 921.
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Effects of mercury on fish
Although the main emphasis of research and management of mercury in freshwater fish
populations has been placed on the human health implications, high MeHg concentrations
may be affecting the fish themselves. There has been little research to date on the effects

of Hg on fish at environmentally realistic concentrations or on possible effects on fish-
eating wildlife (Wiener and Spry, 1996). However, there is some evidence that MeHg may
impair reproduction in freshwater predators. For example, Friedmann et al. (1996) found
that environmentally relevant MeHg concentrations of 0.1 and 1.0 µg g
-1
fed to juvenile
walleyes affected their growth and gonadal development. Latif et al. (2001) examined the
effect of MeHg in water, also at realistic concentrations, on walleye egg development and
found significant reductions in egg survival at higher MeHg concentrations. Also, fathead
minnows consuming food with elevated MeHg concentrations were found to show
reduced spawning success (Hammerschmidt et al., 2002). The toxicologic significance of
MeHg to fish is an important area for future research.
Possible future trends
Future trends in Hg levels in freshwater fish generally and lake trout specifically are
difficult to forecast because many factors influence Hg concentrations in these organisms.
There is some recent evidence that rates of atmospheric deposition of Hg in the northern
hemisphere have recently begun to decrease (Engstrom and Swain, 1996). Some studies
suggest that mercury concentrations in fish are related to rates of atmospheric deposition
(Kelly et al., 1975; Swain and Helwig, 1989; Rolfhus and Fitzgerald, 1995). For this reason,
reductions in atmospheric transport and deposition of Hg may lead to general decreases
of this contaminant in lake trout populations.
Climate warming may have significant effects on Hg concentrations in fish in Boreal
lakes by affecting rates of mercury methylation and the supply of MeHg to food chains.
As noted above, Bodaly et al. (1993) found significant relationships between epilimnetic
temperatures and Hg in fish in Boreal lakes. Climate warming may produce warmer
and/or deeper epilimnia in the Boreal zone. Because mercury methylation probably takes
place mainly in epilimnetic sediments and is known to be temperature dependent, climatic
warming could increase rates of methylation. On the other hand, lower inputs of DOC to
Boreal lakes with decreased precipitation and runoff (Schindler and Gunn, this volume)
could in turn reduce the supply of MeHg to lakes from their watersheds.

The acidification of lakes by atmospheric deposition of pollutants may be increasing
Hg concentrations in lake trout in small Boreal lakes. Experimental lake acidification was
observed to increase Hg in fish (Wiener et al., 1990) and Hg in fish in lakes has often been
observed to be negatively related to pH (e.g., McMurtry et al., 1989). Also, Hg methylation
is stimulated by sulfate addition (Gilmour et al., 1992), and atmospheric sulfate deposition
has increased concurrently with acidic deposition. Fortunately SO
2
emissions have
declined substantially in recent years, with an approximate 40% decline in total North
America emissions since 1980 (Jeffries et al., 2003).
The recent introduction and the spread of rainbow smelt into freshwater systems may
also increase Hg concentrations in lake trout (Franzin et al., 1994; Cabana and Rasmussen,
1994; Futter, 1994). Because the presence of smelt is believed to increase the length of the
food chain, lake trout from lakes with smelt tend to have higher Hg when compared to
the same species from lakes without smelt (Vander Zanden and Rasmussen, 1996;
Akielaszak and Haines, 1981).
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Intensive fishing tends to decrease Hg in freshwater fish, at least temporarily (Verta,
1990). Exploitation will tend to decrease fish densities, increase growth rates, and decrease
the mean age of the population, all of which will tend to decrease Hg in lake trout
populations. Therefore, the presence of sport and commercial and subsistence fisheries on
lake trout lakes may reduce Hg concentrations, at least on a size-adjusted basis.
Summary
Mercury is present in all freshwater fish. Lake trout, because of their piscivorous nature,
are particularly susceptible to accumulating high concentrations of this contaminant. Most
lake trout populations surveyed have mean concentrations of mercury greater than the
0.5 µg g
-1
Canadian standard for human consumption, although there is a large amount
of variation among lakes even within a given region. Mercury enters freshwater systems

primarily from atmospheric deposition and is converted to MeHg, the form that is effi-
ciently bioaccumulated through food webs. Factors affecting mercury concentrations in
lake trout include the length of the food chain, the size and age of individual fish, and
physical and chemical characteristics of lakes and their watersheds. It is difficult to predict
future contaminant trends in predatory fish, including lake trout, in Boreal lakes. Declines
in atmospheric deposition, increased fishing pressure, and a reduced supply of DOC to
lakes from their watersheds may reduce mercury concentrations in lake trout. On the other
hand, the spread of rainbow smelt populations into lakes and climate warming might
increase the concentrations of mercury in lake trout and the risk to humans and fish-eating
wildlife. At present, it is not possible to predict how these opposing factors will affect the
concentrations of a widespread contaminant in freshwater fish.
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