Recent progress on our understanding of the biological effects of mercury in fish and wildlife in the Canadian Arctic.

This review summarizes our current state of knowledge regarding the potential biological effects of mercury (Hg) exposure on fish and wildlife in the Canadian Arctic. Although Hg in most freshwater fish from northern Canada was not sufficiently elevated to be of concern, a few lakes in the Northwest Territories and Nunavut contained fish of certain species (e.g. northern pike, Arctic char) whose muscle Hg concentrations exceeded an estimated threshold range (0.5-1.0 µg g(-1) wet weight) within which adverse biological effects begin to occur. Marine fish species generally had substantially lower Hg concentrations than freshwater fish; but the Greenland shark, a long-lived predatory species, had mean muscle Hg concentrations exceeding the threshold range for possible effects on health or reproduction. An examination of recent egg Hg concentrations for marine birds from the Canadian Arctic indicated that mean Hg concentration in ivory gulls from Seymour Island fell within the threshold range associated with adverse effects on reproduction in birds. Mercury concentrations in brain tissue of beluga whales and polar bears were generally lower than levels associated with neurotoxicity in mammals, but were sometimes high enough to cause subtle neurochemical changes that can precede overt neurotoxicity. Harbour seals from western Hudson Bay had elevated mean liver Hg concentrations along with comparatively high muscle Hg concentrations indicating potential health effects from methylmercury (MeHg) exposure on this subpopulation. Because current information is generally insufficient to determine with confidence whether Hg exposure is impacting the health of specific fish or wildlife populations in the Canadian Arctic, biological effects studies should comprise a major focus of future Hg research in the Canadian Arctic. Additionally, studies on cellular interactions between Hg and selenium (Se) are required to better account for potential protective effects of Se on Hg toxicity, especially in large predatory Arctic fish, birds, and mammals.

Effects of methylmercury on epigenetic markers in three model species: mink, chicken and yellow perch.

We previously reported that methylmercury (MeHg) exposure is associated with DNA hypomethylation in the brain stem of male polar bears. Here, we conveniently use archived tissues obtained from controlled laboratory exposure studies to look for evidence that MeHg can disrupt DNA methylation across taxa. Brain (cerebrum) tissues from MeHg-exposed mink (Neovison vison), chicken (Gallus gallus) and yellow perch (Perca flavescens) were analyzed for total Hg levels and global DNA methylation. Tissues from chicken and mink, but not perch, were also analyzed for DNA methyltransferase (DNMT) activity. In mink we observed significant reductions in global DNA methylation in an environmentally-relevant dietary exposure group (1 ppm MeHg), but not in a higher group (2 ppm MeHg). DNMT activity was significantly reduced in all treatment groups. In chicken or yellow perch, no statistically significant effects of MeHg were observed. Dose-dependent trends were observed in the chicken data but the direction of the change was not consistent between the two endpoints. Our results suggest that MeHg can be epigenetically active in that it has the capacity to affect DNA methylation in mammals. The variability in results across species may suggest inter-taxa differences in epigenetic responses to MeHg, or may be related to differences among the exposure scenarios used as animals were exposed to MeHg through different routes (dietary, egg injection), for different periods of time (19-89 days) and at different life stages (embryonic, juvenile, adult).

Derivation of screening benchmarks for dietary methylmercury exposure for the common loon (Gavia immer): rationale for use in ecological risk assessment.

The current understanding of methylmercury (MeHg) toxicity to avian species has improved considerably in recent years and indicates that exposure to environmentally relevant concentrations of MeHg through the diet can adversely affect various aspects of avian health, reproduction, and survival. Because fish-eating birds are at particular risk for elevated MeHg exposure, the authors surveyed the available primary and secondary literature to summarize the effects of dietary MeHg on the common loon (Gavia immer) and to derive ecologically relevant toxic thresholds for dietary exposure to MeHg in fish prey. After considering the available data, the authors propose three screening benchmarks of 0.1, 0.18, and 0.4 µg g(-1) wet weight MeHg in prey fish. The lowest benchmark (0.1 µg g(-1) wet wt) is the threshold for adverse behavioral impacts in adult loons and is close to the empirically determined no observed adverse effects level for subclinical effects observed in captive loon chicks. The remaining benchmarks (0.18 and 0.4 µg g(-1) wet wt) correspond to MeHg levels in prey fish associated with significant reproductive impairment and reproductive failure in wild adult loons. Overall, these benchmarks incorporate recent findings and reviews of MeHg toxicity in aquatic fish-eating birds and provide the basis for a national ecological risk assessment for Hg and loons in Canada.

Toxicity of methylmercury injected into eggs of thick-billed murres and arctic terns.

Mercury (Hg) has been increasing in some marine birds in the Canadian Arctic over the past several decades. To evaluate the potential reproductive impact of Hg exposure, eggs of two species of arctic-breeding seabirds, the thick-billed murre and arctic tern, were dosed with graded concentrations of methylmercury (MeHg) and artificially incubated in the laboratory to determine species differences in sensitivity. Based on the dose-response curves, the median lethal concentrations (LC(50)) for thick-billed murre and arctic tern embryos were 0.48 and 0.95 µg g(-1) Hg on a wet-weight (ww) basis, respectively. Compared with published LC(50) values for other avian species, the murres and terns had a medium sensitivity to MeHg exposure. LC(50) values were also calculated for the actual Hg concentration measured in the embryos, that is, the maternally-deposited Hg plus the injected MeHg dose. This increased the LC(50) values to 0.56 µg g(-1) Hg ww in the thick-billed murre and to 1.10 µg g(-1) Hg ww in the arctic tern. Although muscarinic acetylcholine and N-methyl-D-aspartic acid glutamate receptor levels have been correlated with increasing Hg concentrations in brains of adult birds, no significant associations were found in brain tissue of the murre or tern embryos. The incidence of gross external anatomical deformities was 4.3 % in the murre embryos and 3.6 % in the tern embryos. However, given that the eggs were taken from wild populations, it is unlikely that the deformities observed in this study were due to MeHg exposure alone.

Selenium accumulation and reproduction in birds breeding downstream of a uranium mill in northern Saskatchewan, Canada.

Selenium (Se) concentrations in aquatic invertebrates and bird eggs collected along the treated effluent receiving environment of the Key Lake uranium mill in northern Saskatchewan were significantly greater than from nearby reference areas, and in some cases (e.g., eggs of common loons--Gavia immer) were higher than commonly used thresholds for adverse reproductive effects in birds (i.e., 5 µg/g dry weight in diet; 12-15 µg/g dry weight in eggs). Mean Se concentrations in tree swallow (Tachycineta bicolor) eggs reached a maximum of 13.3 µg/g dry weight at the point of treated effluent discharge and exhibited a gradient of decreasing Se concentrations with increasing distance from the effluent discharge, probably reflecting both effluent dilution and local site fidelity by nesting swallows. In some cases, high intra-clutch variability in Se concentrations in mallard (Anas platyrhynchos) and tree swallow eggs was observed in high-Se sites, suggesting that a single egg randomly sampled from a nest in an area of higher Se exposure may not be representative of Se concentrations in other eggs from the same nest. Overall, tree swallow reproductive success was similar in both exposed and reference areas.

Mercury, selenium and neurochemical biomarkers in different brain regions of migrating common loons from Lake Erie, Canada.

Common loons (Gavia immer) can be exposed to relatively high levels of dietary methylmercury (MeHg) through fish consumption, and several studies have documented MeHg-associated health effects in this species. To further study the neurological risks of MeHg accumulation, migrating loons dying of Type E botulism were collected opportunistically from the Lake Erie shore at Long Point (Ontario, Canada) and relationships between total mercury (THg), selenium (Se), and selected neurochemical receptors and brain enzymes were investigated. THg concentrations were 1-78 µg/g in liver; and 0.3-4 µg/g in the brain (all concentrations reported on a dry weight basis). A significant (p < 0.05) positive correlation was found between THg in liver and THg in 3 subregions of the brain (cerebral cortex: r = 0.433; cerebellum: r = 0.293; brain stem: r = 0.405). THg varied significantly among different brain regions, with the cortex having the highest concentrations. Se levels in the cortex and cerebellum were 1-29 and 1-10 µg/g, respectively, with no significant differences between regions. Se was not measured in brain stem due to insufficient tissue mass. There were molar excesses of Se over mercury (Hg) in both cortex and cerebellum at all Hg concentrations, and a significant positive relationship between THg and the Hg:Se molar ratio (cortex: r = 0.63; cerebellum: r = 0.47). No significant associations were observed between brain THg and the N-methyl-D-aspartic acid (NMDA) receptor concentration, nor between THg and muscarinic cholinergic (mACh) receptor concentration; however, brain THg levels were lower than in previous studies that reported significant Hg-associated changes in neuroreceptor densities. Together with previous studies, the current findings add to our understanding of Hg distribution in the brain of common loons, and the associations between Hg and sub-lethal neurochemical changes in fish-eating wildlife.

Spatial gradients of methylmercury for breeding common loons in the Laurentian Great Lakes region.

Much of the Laurentian Great Lakes region is a mercury-sensitive landscape, in which atmospheric deposition and waterborne sources of mercury (Hg) have led to high concentrations of bioavailable methylmercury (MeHg) in predatory fish and piscivorous wildlife. Efforts since the early 1990s have established the common loon (Gavia immer) as the primary avian indicator for evaluating the exposure and effects of MeHg in North America. A regional Hg dataset was compiled from multiple loon tissue types and yellow perch (Perca flavescens), a preferred prey fish species for loons. Hg exposure in loons and perch was modeled to develop male and female loon units (MLU and FLU, respectively), standardized metrics that represent the estimated blood Hg exposure of a male or female loon for a given loon territory or water body. Using this common endpoint approach to assess loon Hg exposure, the authors demonstrate spatial trends in biotic Hg concentrations, examine MeHg availability in aquatic ecosystems of the Great Lakes region in relation to landscape-level characteristics, and identify areas with potentially significant adverse reproductive impacts to loons and other avian piscivores. Based on 8,101 MLUs, seven biological Hg hotspots were identified in the Great Lakes region. Policy-relevant applications are presented.

Evidence for sex differences in mercury dynamics in double-crested cormorants.

Aquatic fish-eating birds can demethylate methylmercury in their livers. In this study, we determined whether a previously documented male bias in mercury concentration in double-crested cormorants ( Phalacrocorax auritus ) was due entirely to the depuration of mercury into eggs or might also in part be related to sex differences in methylmercury demethylation or biliary excretion capability in the liver. We found egg depuration accounted for less than a fifth of the mercury concentration difference between males and females, hence not entirely explaining the sex difference. Females had a significantly steeper slope for the negative relationship between percent methylmercury (i.e., percentage of total mercury that is methylmercury) and total mercury concentration than did males. This suggests that females have a greater capacity to demethylate methylmercury, which might be reducing the amount of methylmercury available for depuration to eggs. We also found a significant negative relationship between methylmercury concentration and liver mass for females only; thus females might also have a greater capability to excrete methylmercury compared to males. Therefore, we conclude that the male bias in mercury concentration might also result from females having a greater capability to excrete mercury compared to males.

Predicting mercury concentrations and fluxes in the water column and sediment of lakes with a limited dataset.

The purpose of the present study was to evaluate the mercury (Hg) Environmental Ratios Multimedia Ecosystem Sources (HERMES) model on two Ontario, Canada lakes (Harp and Dickie) and to include modifications to enable the model to estimate the major model input variables that tend to be missing for lakes with limited datasets. No significant differences were found for either sediment solid or bulk water total mercury (THg) when the HERMES model was applied to the two Ontario lakes, regardless of whether all available data were altered during application or only the 10 variables that tend to cause the most variation in model output (i.e., concentration of THg in atmosphere, water inflow THg concentration, water inflow rate, water volume, surface area, mean depth, suspended particulate matter concentration, settling rate of solids in water column, water temperature, and precipitation rate). Since measured sediment and water THg values do not exist for most lakes removed from industrial activities, empirical relationships were incorporated into the HERMES model framework to provide a method to double-check model output for lakes where this information is unavailable.

Investigation of spatial trends and neurochemical impacts of mercury in herring gulls across the Laurentian Great Lakes.

Herring gulls (Larus argentatus) bioaccumulate mercury (Hg) but it is unknown whether they are exposed at levels of neurological concern. Here we studied brain tissues from gulls at five Great Lakes colonies and one non-Great Lakes colony during spring of 2001 and 2003. Total brain Hg concentrations ranged from 0.14 to 2.0 microg/g (dry weight) with a mean of 0.54 microg/g. Gulls from Scotch Bonnet Island, on the easternmost edge of the Great Lakes, had significantly higher brain Hg than other colonies. No association was found between brain Hg concentration and [3H]-ligand binding to neurochemical receptors (N-methyl-D-aspartate, muscarinic cholinergic, nicotinic cholinergic) or nicotinic receptor alpha-7 relative mRNA expression as previously documented in other wildlife. In conclusion, spatial trends in Hg contamination exist in herring gulls across the Great Lakes basin, and herring gulls accumulate brain Hg but not at levels associated with sub-clinical neurochemical alterations.

In vitro and whole animal evidence that methylmercury disrupts GABAergic systems in discrete brain regions in captive mink.

The effects of mercury (Hg) on key components of the GABAergic system were evaluated in discrete brain regions of captive juvenile male American mink (Neovison vison) using in vitro and in vivo (whole animal) experimental approaches. In vitro studies on cortical brain tissues revealed that inorganic Hg (HgCl(2); IC50=0.5+/-0.2microM) and methyl Hg (MeHgCl; IC50=1.6+/-0.2microM) inhibited glutamic acid decarboxylase (GAD; EC 4.1.1.15) activity. There were no Hg-related effects on [(3)H]-muscimol binding to GABA(A) receptors (IC50s>100microM). HgCl(2) (IC50=0.8+/-0.3microM) but not MeHgCl (IC50>100microM) inhibited GABA-transaminase (GABA-T; EC 2.6.1.19) activity. In a whole animal study, neurochemical indicators of GABAergic function were measured in brain regions (occipital cortex, cerebellum, brain stem, and basal ganglia) of captive mink fed relevant levels of MeHgCl (0 to 2microg/g feed, ppm) daily for 89d. No effects on GAD activity were measured. Concentration-dependent decreases in [(3)H]-muscimol binding to GABA(A) receptors and GABA-T activity were found in several brain regions, with reductions as great as 94% (for GABA(A) receptor levels) and 71% (for GABA-T activity) measured in the brain stem and basal ganglia. These results show that chronic exposure to environmentally relevant levels of MeHg disrupts GABAergic signaling. Given that GABA is the main inhibitory neurotransmitter in the mammalian nervous system, prolonged disruptions of its function may underlie the sub-clinical impacts of MeHg at relevant levels to animal health.

Variation of cholinergic biomarkers in brain regions and blood components of captive mink.

Studies are increasingly using cholinergic parameters as biomarkers of early neurotoxicity, but few have characterized this system in ecologically relevant model organisms. In the present study, key neurochemicals in the cholinergic pathway were measured and analyzed from discrete parts of brain and blood from captive mink (Mustela vison). Similar to other mammals, the regional distribution of cholinergic parameters in the brain could be ranked from highest to lowest as: basal ganglia > occipital cortex > brain stem > cerebellum (F (3,192) = 172.1, p < 0.001). Higher variation in cholinergic parameters was found in the cerebellum (coefficient of variation = 34.9%), and the least variation was measured in the brain stem (19.7%). Variation was also assessed by calculating the difference between the lowest and highest measures among individual animals: choline acetyltransferase (1.6x fold difference), cholinesterase (2.0x), muscarinic receptor levels (2.4x), acetylcholine (3.7x), nicotinic receptor levels (3.9x), and choline transporter (5.0x). In blood samples, activity and inter-individual variation of cholinesterase was highest in whole blood and lowest in plasma and serum. By using captive mink of a common genetic source, age, gender, and rearing conditions, these data help establish normal levels, ranges, and variations of cholinergic biomarkers among brain regions, blood components, and individual animals. Such information may better enable the utility of cholinergic biomarkers in environmental assessments.

Neurochemical and electrophysiological diagnosis of reversible neurotoxicity in earthworms exposed to sublethal concentrations of CL-20.

BACKGROUND, AIM, AND SCOPE: Hexanitrohexaazaisowurtzitane (CL-20) is a relatively new energetic compound sharing some degree of structural similarity with hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a known neurotoxic compound. Previously, we demonstrated using a noninvasive electrophysiological technique that CL-20 was a more potent neurotoxicant than RDX to the earthworm Eisenia fetida. In the present study, we investigated the effect of CL-20 exposure and subsequent recovery on muscarinic acetylcholine receptors (mAChRs) to further define the mechanism of reversible neurotoxicity of CL-20 in E. fetida. MATERIALS AND METHODS: We used a noninvasive electrophysiological technique to evaluate neurotoxicity in CL-20-treated worms, and then measured how such exposures altered levels of whole-body mAChR in the same animals. RESULTS AND DISCUSSION: A good correlation exists between these two types of endpoints. Effect on mAChR levels was most prominent at day 6 of exposure. After 7 days of recovery, both conduction velocity and mAChR were significantly restored. Our results show that sublethal concentrations of CL-20 significantly reduced mAChR levels in a concentration- and duration-dependent manner, which was accompanied with significant decreases in the conduction velocity of the medial and lateral giant nerve fibers. After 7-day post exposure recovery, worms restored both neurochemical (mAChR) and neurophysiological (conduction velocity) endpoints that were reduced during 6-day exposures to CL-20 concentrations from 0.02 to 0.22 microg/cm(2). CONCLUSIONS AND PERSPECTIVES: Our findings support the idea that CL-20 induced neurotoxic effects are reversible, and suggest that CL-20 neurotoxicity may be mediated through the cholinergic system. Future studies will investigate other neurotransmission systems such as GABA, glutamate, and monoamine. Ion channels in the nerve membrane should be examined to further define the precise mechanisms underlying CL-20 neurotoxicity.

Bone lead levels and lead isotope ratios in red grouse from Scottish and Yorkshire moors.

Leg and foot bones of adult and juvenile red grouse (Lagopus lagopus scoticus) were collected from hunter-shot birds on two Scottish estates (Glendye and Invermark) and one Yorkshire estate in September, 2003. The lead content of bones was measured by atomic absorption spectrophotometry, and corresponding stable lead isotopes (Pb(204, 206, 207, 208)) by inductively coupled plasma mass spectrometry. At the Glendye (N=111) and Invermark (N=85) estates, relatively few birds (5.4% and 3.5%, respectively) had highly elevated bone lead concentrations (>20 microg/g dry weight). In bones of these highly exposed birds, a combination of Pb(206):Pb(207) and Pb(208):Pb(207)ratios was consistent with ingestion of lead gunshot available in Europe. By contrast, Yorkshire grouse experienced a high incidence (65.8%) of bone lead >20 microg/g. The Pb(206):Pb(207) and Pb(208):Pb(207)ratios in bones of these highly exposed birds were consistent with a combined exposure to ingested lead gunshot and lead from galena mining in the region. Lead isotope ratios also indicated that lead from UK gasoline combustion and fallout from atmospheric particles was not a likely source of elevated lead in bones of either Scottish or Yorkshire grouse. Suggested management options for the three moors include adopting nontoxic shot for all game shooting on the estates, allowing heather (Calluna vulgaris) vegetation to grow tall in lead shot fall-out zones to reduce physical access to high densities of lead shot already present, and provision of calcareous grit across moors to reduce lead assimilation from all ingested sources of lead.

Is dietary mercury of neurotoxicological concern to wild polar bears (Ursus maritimus)?

Polar bears (Ursus maritimus) are exposed to high concentrations of mercury because they are apex predators in the Arctic ecosystem. Although mercury is a potent neurotoxic heavy metal, it is not known whether current exposures are of neurotoxicological concern to polar bears. We tested the hypotheses that polar bears accumulate levels of mercury in their brains that exceed the estimated lowest observable adverse effect level (20 microg/g dry wt) for mammalian wildlife and that such exposures are associated with subtle neurological damage, as determined by measuring neurochemical biomarkers previously shown to be disrupted by mercury in other high-trophic wildlife. Brain stem (medulla oblongata) tissues from 82 polar bears subsistence hunted in East Greenland were studied. Despite surprisingly low levels of mercury in the brain stem region (total mercury = 0.36 +/- 0.12 microg/g dry wt), a significant negative correlation was measured between N-methyl-D-aspartate (NMDA) receptor levels and both total mercury (r = -0.34, p < 0.01) and methylmercury (r = -0.89, p < 0.05). No relationships were observed among mercury, selenium, and several other neurochemical biomarkers (dopamine-2, gamma-aminobutyric acid type A, muscarinic cholinergic, and nicotinic cholinergic receptors; cholinesterase and monoamine oxidase enzymes). These data show that East Greenland polar bears do not accumulate high levels of mercury in their brain stems. However, decreased levels of NMDA receptors could be one of the most sensitive indicators of mercury's subclinical and early effects.

The effects of mercury on muscarinic cholinergic receptor subtypes (M1 and M2) in captive mink.

A combination of in vitro (competitive binding assays) and in vivo (tissues from animals exposed to dietary methyl mercury, MeHg) experimental procedures was employed to assess the effects of mercury (MeHg, HgCl(2)) on the two-key muscarinic cholinergic (mACh) receptor subtypes (M1, M2) in two brain regions (occipital cortex, brain stem) of captive mink (Mustela vison). In vitro, HgCl(2) and MeHg were equipotent in inhibiting [(3)H]-pirenzipine binding to the M1 receptor in the occipital cortex, but in the brain stem, MeHg was about 65x more potent than HgCl(2). For the M2 receptor, both HgCl(2) and MeHg were more potent at inhibiting [(3)H]-AFDX-384 binding in the occipital cortex than in the brain stem. Within each brain region, HgCl(2) was more potent at inhibiting [(3)H]-AFDX-384 binding than MeHg. In vivo exposure of captive mink to MeHg (0.5, 1, and 2ppm MeHg in the diet for 89 days) resulted in greater binding of radioligands to the M1 and M2 receptor in the occipital cortex, but not in the brain stem, when compared to control animals. Based on the in vitro results, we could not conclude which mACh receptor subtype or brain region was most sensitive to Hg, but the in vivo findings suggest that Hg preferentially affects mACh receptor subtype (M1 and M2) levels in the occipital cortex. By studying distinct mACh receptors, these results extend upon previous studies in laboratory rodents and wildlife that showed Hg to affect the global population of mACh receptors.

Trace element and metallothionein concentrations in seabirds from the Canadian Arctic.

Livers and kidneys were collected for five seabird species from the Canadian Arctic during the 1983 and 1991 to 1993 breeding seasons. Livers were analyzed for Cd, Hg, Pb, and Se, and kidneys were analyzed for Cd, Cu, Zn, and metallothionein (MT). Concentrations of the essential elements, Cu and Zn, were in agreement with those previously published in the literature. Thick-billed murres (Uria lomvia) from Ivujivik on the Hudson Strait/Hudson Bay coast in northwestern Quebec (Canada) had the highest mean renal concentrations of Cu, Zn, and Cd. Among the four species collected from Prince Leopold Island, northern fulmars (Fulmarus glacialis) had the highest hepatic concentrations of both Cd and Hg. The highest Se concentrations were found in northern fulmars and black-legged kittiwakes (Rissa tridactyla) from Prince Leopold Island. Hepatic Pb concentrations were low (<0.3 microg/g dry wt) in all species at all locations. Metallothionein concentrations were positively correlated with Cd and Zn for all species combined but were not correlated with Cu in any species. No significant relationships were found between MT and Cu or Zn in black guillemots (Cepphus grylle) or glaucous gulls (Larus hyperboreus). To our knowledge, this is the first report of MT concentrations and their relationships with trace metals in Arctic seabirds.

Effects of environmental methylmercury on the health of wild birds, mammals, and fish.

Wild piscivorous fish, mammals, and birds may be at risk for elevated dietary methylmercury intake and toxicity. In controlled feeding studies, the consumption of diets that contained Hg (as methylmercury) at environmentally realistic concentrations resulted in a range of toxic effects in fish, birds, and mammals, including behavioral, neurochemical, hormonal, and reproductive changes. Limited field-based studies, especially with certain wild piscivorous bird species, e.g., the common loon, corroborated laboratory-based results, demonstrating significant relations between methylmercury exposure and various indicators of methylmercury toxicity, including reproductive impairment. Potential population effects in fish and wildlife resulting from dietary methylmercury exposure are expected to vary as a function of species life history, as well as regional differences in fish-Hg concentrations, which, in turn, are influenced by differences in Hg deposition and environmental methylation rates. However, population modeling suggests that reductions in Hg emissions could have substantial benefits for some common loon populations that are currently experiencing elevated methylmercury exposure. Predicted benefits would be mediated primarily through improved hatching success and development of hatchlings to maturity as Hg concentrations in prey fish decline. Other piscivorous species may also benefit from decreased Hg exposure but have not been as extensively studied as the common loon.

Polychlorinated biphenyls, organochlorinated pesticides, and polybrominated diphenyl ethers in the cerebral cortex of wild river otters (Lontra canadensis).

We measured the levels of ortho-substituted polychlorinated biphenyls (PCB), organochlorinated pesticides (OCP), and polybrominated diphenyl ethers (PBDE) in the cerebral cortex of river otters (Lontra canadensis) trapped from Ontario and Nova Scotia between 2002 and 2004. The mean concentration of total PCBs was 70.9+/-12.1 ng/g l.w., and congeners 153, 180 and 138 accounted for nearly 60% of the sum. The mean concentration of total OCPs was 21.2+/-3.7 ng/g l.w., and hexachlorobenzene (32.6% of total) and DDE (28.1%) accounted for the majority. The mean concentration of total PBDEs was 3.2+/-0.6 ng/g l.w., and congeners 99 (44.9%), 153 (30.5%), and 100 (24.7%) were measured at the indicated percentages. There was no relationship between these residue data and concentrations of brain mercury or neurochemical receptors and enzymes as determined in earlier studies on these same animals.