Fatty acid profiles of lake trout reveal the importance of lipid content for interpreting trophic relationships within and across lakes

Fatty acid profiles increasingly are being used to quantify foraging patterns of consumers, but the associated interpretation may vary with the tissue type and its lipid content. For salmonids, lipid deposits can be found in both dorsal and ventral (“belly flap”) areas of muscle tissues. However, it is uncertain whether belly flap and dorsal muscle fatty acid profiles are similar in natural populations of fish. We examined how fatty acid profiles of belly flap compared to those of dorsal muscle and the consequent impacts on dietary inferences. Fatty acid profiles were derived from lake trout (Salvelinus namaycush) caught in five North American lakes: Champlain, Flathead, Michigan, Ontario, and Swan. Fatty acid profiles were most similar between tissues when lipid content of muscle was > ~10%, the threshold below which similarities decreased and thus increasingly affected dietary inference. Some fatty acids commonly used as trophic indicators preferentially accrued in one tissue over the other depending on lipid content of the tissues. Regardless of tissue type, fatty acid profiles were specific to each lake indicating that food web structures were distinctive over a broad geographic range. Fatty acid profiles of tissues from lakes Michigan and Ontario were highly similar, so were those from Flathead and Swan lakes, whereas those from Lake Champlain were distinct, having comparatively high proportions of 18:1n-9. We conclude that lipid storage areas like belly flaps likely provide a more accurate signal than muscle when using fatty acids to investigate dietary patterns, particularly when muscle lipid levels are low.     

Diet predictions of Lake Ontario salmonines based on fatty acids and correlations between their fat content and thiamine concentrations

Thiamine Deficiency Complex (TDC) limits early life stage survival of salmonines. Consuming fatty prey has been hypothesized as a cause of thiamine deficiency; however, this relationship has not been evaluated in the Laurentian Great Lakes where TDC occurs. We found that alewife (Alosa pseudoharengus) have higher lipid content than other common Lake Ontario prey fish. In addition, alewife were predicted as the most consumed prey for brown trout (Salmo trutta), Chinook salmon (Oncorhynchus tshawytscha), coho salmon (O. kisutch), lake trout (Salvelinus namaycush), and steelhead trout (O. mykiss); however, the relative importance of alewife in diet composition varied within and among species. Overall, species with greater predicted consumption of alewife had lower egg and muscle thiamine concentrations. Negative correlations between thiamine concentrations and both lipid content and fatty acid concentrations (mg/mg of wet tissue) were limited to brown trout. Similarly, negative correlations between fatty acid proportions (i.e., cumulative proportions of polyunsaturated fatty acids [PUFA] and monounsaturated fatty acids [MUFA]) and thiamine concentrations were only observed for brown and lake trout. Combining data from all species produced curvilinear correlations between thiamine concentrations (egg and muscle) and fatty acid composition (eggs and belly flap). Proportions of PUFAs had negative correlations with thiamine concentrations while proportions of MUFAs had positive correlations. These results provide evidence that, in some cases, salmonine fatty acid composition negatively correlates with thiamine concentrations in Lake Ontario; however, additional research is needed to confirm that this mechanism causes TDC in salmonines, and to understand additional factors potentially associated with TDC.     

Reducing Levels of Mirex,Aroclor 1254, and DDE by Trimming and Cooking Lake Ontario Brown Trout (Salmo Trutta Linnaeus) and Smallmouth Bass (Micropterus Dolomieui Lacepede)

Trimming of Lake Ontario smallmouth bass (Micropterus dolomieui Lacepede) and brown trout (Salmo trutta Linnaeus) was found to reduce levels of mirex, Aroclor 1254, and DDE by 43% to 64%. Trimming included the removal of the skin, dorsal and ventral fat, and the entire lateral line. Of the cooking methods tested (smoking, broiling, baking, and deep frying), only deep frying produced consistently high reductions of all three contaminants.     

Salmonine Consumption as a Source of Mirex in Human Breast Milk near Rochester,New York

Fifty-seven pre-natal classes were attended to recruit 30 donors of human milk samples in the vicinity of Rochester, NY. Donors were categorized into two groups of 15 Lake Ontario fish eaters (LOFE) and non-Lake Ontario fish eaters (NLOFE). A comparison of mirex concentrations in breast milk of Lake Ontario fish eaters with non-Lake Ontario fish eaters indicated no significant difference in median mirex concentrations (0.09 vs 0.06 ng/g milk). When the Lake Ontario fish eaters were divided into panfish and salmonine consumers, significant differences in mirex concentration in breast milk were observed. Women eating Lake Ontario salmonines have significantly higher concentrations of mirex (median = 0.21 ng/g milk) in their breast milk than Lake Ontario panfish consumers (median = 0.03 ng/g milk) and the NLOFE group (median = 0.06 ng/g milk). Photomirex concentrations were consistently higher in both donor groups than mirex and were significantly higher in the breast milk of salmonine consumers than in both the panfish consumers and the NLOFE group. The relatively high concentrations of photomirex in the breast milk of all donors (LOFE and NLOFE) suggest volatilization and an atmospheric transport of this contaminant. The child of a woman salmonine consumer, breast feeding daily at the median concentration level observed for a month, has a loading of mirex (816 ng mirex/kg) that is equivalent to consuming three Lake Ontario salmon meals at the FDA action level and that represents 2.9 times more mirex than an adult male should consume per month according to the NYS Department of Health advisory.     

Organochlorine Contaminant Residues in Spottail Shiners (Notropis Hudsonius) from the Niagara River

Collections of young-of-the-year spottail shiners (Notropis hudsonius) were used to determine spatial distribution (1981) of organochlorine residues and their temporal trends in the Niagara River. PCB and mirex residues in spottail shiners ranged from 164 to 573 ng/g and ND-18 ng/g, respectively. PCB and mirex residues exceeded the aquatic life objectives (Great Lakes Water Quality Agreement of 1978) at all sites except Frenchman's Creek, where mirex was non-detectable. Residues for heptachlor, heptachlor epoxide, aldrin, endrin, chlordane, thiodan, dieldrin, BHC, and DDT were low or non-detectable in Niagara River spottail shiners. Chlorinated benzene and octachlorostyrene residues were also present in most Niagara River spottail shiner samples analyzed. Dioxin (2,3,7,8- TCDD) was present in all Niagara River spottail shiner collections and ranged from 4 to 60 pg/g. The highest dioxin residues were found at Cayuga Creek, N. Y. PCB residues have declined significantly since 1975 in Niagara-on-the-Lake spottail shiners; however, residues increased in 1980 and 1981, although they were below the 1975 concentrations. Mirex residues in spottail shiners from Niagara-on-the-Lake have also declined significantly since 1978, suggesting reduced mirex inputs to Lake Ontario.     

Organochlorine Contaminant Levels in Waterbird Species from Hamilton Harbour,Lake Ontario: an IJC Area of Concern

The levels of organochlorine (OC) contaminants in eggs and tissues of waterbird species nesting in Hamilton Harbour, Ontario, an International Joint Commission Area of Concern, were monitored between 1981 and 1992. PCBs, DDE, and mirex were present at the highest concentrations of the 29 organochlorines and one trace metal measured. Most contaminants in the various species tested showed a declining temporal pattern during that period. Double-crested cormorant eggs generally had higher concentrations of contaminants than those in herring gull eggs which in turn were higher than those in eggs of black-crowned night-herons, Caspian terns and common terns. The levels of contaminants in the eggs reflect the dietary preferences of the species with the fish-eating birds containing consistently higher accumulations of contaminants when compared to the herbivorous Canada goose for which the levels were consistently much lower for all compounds. The concentrations of contaminants detected are among some of the highest in the Great Lakes but when compared to other sites on Lake Ontario, the levels in Hamilton Harbour are generally equal or lower. Elevated levels of contaminants such as PCBs, mirex, and DDE were also detected in liver and muscle tissues of migrant waterfowl species from the harbor. Despite habitat degradation and continued presence of contaminants, the harbor supports a large number and wide variety of waterbird species. Except for black-crowned night-herons, the nesting populations of colonial waterbirds have increased between 21 and 1061 % since the last survey in 1987.     

Lake Ontario Sediment - Mirex Relationships

Sediment samples collected from within the eastern basin of Lake Ontario, the Oswego River and harbor indicate that mirex, a persistent, chlorinated hydrocarbon continues to accumulate in harbor and offshore lake sediments. A high statistical relationship exists between mirex and organic carbon concentrations in deep water lake sediments. Mirex-contaminated sediments are accumulating in the deeper waters (>100m) of the lake at rates ranging from about 2.2–7.0 mm/yr. Nearshore bottom sediments are subject to redistribution, providing a continuing source of mirex to lake organisms.     

A Screening-Level Mass Balance Analysis of Mirex Transport and Fate in the Oswego River

A mass balance approach was used to evaluate the fate of mirex in the Oswego River. The objectives of this research were 1) to assess the magnitude and extent of mirex contamination in the Oswego River, 2) to quantify the transport, fate, and distribution of mirex in the river, and 3) to estimate mirex export to Lake Ontario via the Oswego River. Field data collected as part of a 1990 Oswego River Mirex Study, in addition to other existing data, were used to develop a water quality model describing the transport and fate of chlorides, total suspended solids, and mirex in the Oswego River from Fulton to Lake Ontario. Long-term and short-term loading scenarios were evaluated to assess the possible magnitude of the initial mirex discharge to the Oswego River in 1965 as well as the subsequent export resulting from a given loading. Field data and model results suggest that a short-term mirex discharge occurring in the mid-1960s cannot account for the water column concentrations observed in 1990 or the mirex mass in Lake Ontario sediments attributable to the Oswego River. Similarly, field data and model results suggest that resuspension of the 1990 in-place mirex mass cannot account for the water column mirex concentrations observed in 1990. This suggests that there may be a continuing mirex source to the Oswego River. Based on the 1990 field data, the estimated Oswego River mirex inventory was 10 kg and export to Lake Ontario averaged 42 g/day.     

Additional Link in the Food Web Does Not Biomagnify a Persistent Contaminant in Lake Ontario: The Case of Cercopagis pengoi

Cercopagis pengoi is a new and abundant non-indigenous predator species in the Lake Ontario food web. We explored the impact of this predator on the levels of a chlorinated hydrocarbon in the pelagic food web through assessments of seasonal abundance and mirex concentrations of Cercopagis pengoi (Ostroumov) and the planktivorous alewife (Alosa pseudoharengus). Abundance, stable isotope, and alewife stomach data indicate that Cercopagis pengoi has become an established portion of the Lake Ontario food web. Cercopagis, a predaceous cladoceran, feeds on the lower portion of the trophic web and is clearly fed upon by the planktivorous alewife. Cercopagis is a link in the Lake Ontario food web, in which energy and materials are being passed from one level of the trophic web to another. However, mirex levels of the planktivorous alewife did not increase during the period of highest Cercopagis abundance. The annual load of mirex (mass of Cercopagis times concentration) transferred from one level of the trophic web to the next is low. In the summer, when Cercopagis is abundant, alewives were not feeding on them.     

The Influence of the Niagara River on Contaminant Burdens of Lake Ontario Biota

Top predator and forage fish species, netplankton (> 153 μm), zooplankton, and benthic macroinvertebrates from Lake Erie and Lake Ontario were analyzed for whole body levels of trace metals and organic contaminants. Comparison of contaminant concentrations in similar aquatic food chains from both lakes indicated that levels of PCB, DDT, mirex, and mercury are significantly greater (P <0.05) in the biota of Lake Ontario. The Niagara River, the single largest tributary to Lake Ontario, was confirmed as a major source of organic contaminants and trace metals. Organic contaminants are adsorbed to the particulate load of the river and dispersed throughout Lake Ontario by the circulating currents. There was no significant regional difference (P<0.05) in the degree of contaminant accumulation by the pelagic food chain of Lake Ontario. Conversely, both inorganic and organic contaminant levels in the demersal amphipod, Pontoporeia affinis, were significantly different (P<0.05) between the eastern and western basins of Lake Ontario. The uptake and concentration of contaminants at the sediment-water interface is suggested as a possible mechanism to explain this observed difference.     

Assessment of Maternal Contaminant Burden by Analysis of Snapping Turtle Eggs

Snapping turtles (Chelydra serpentina serpentina) have been extensively used as an ecosystem and localized indicator of tissue contaminant bioaccumulation, and for monitoring the geographic and temporal trends of contaminants. The purpose of this study was to determine if any relationship exists in the pattern and quantity of select contaminants found in turtle tissues relative to egg contamination. Six gravid snapping turtle females were collected from within and outside the Great Lakes basin and analyzed for congener-specific polychlorinated biphenyls (PCBs), dichlorodiphenyl-dichloroethylene (DDE), mirex and hexachlorobenzene (HCB) distributions in the fat, liver, and eggs. Preliminary data collected from five sites indicates that the pattern of congener-specific PCBs and homologs, average chlorine/biphenyl, and total PCBs are significantly correlated among snapping turtle eggs and tissues. Significant correlations were also observed for DDE, mirex, and HCB. All six turtles in this study revealed a consistent, and enhanced lower-chlorinated PCB pattern in the eggs relative to maternal adipose and liver tissues. Although these preliminary results are based on a relatively small sample size (n = 6), the significance of the relationships observed indicate that the utilization of snapping turtle eggs provides a reliable indicator of maternal contaminant burdens found in the highest environmental trophic levels.     

Maternal Transfer of Organochlorines to Eggs of Walleye (Stizostedion vitreum) in Lake Manitoba and Western Lake Superior

Gravid walleye were sampled from Lake Manitoba and western Lake Superior (St. Louis River) to measure the concentrations of organochlorine contaminants (OCs) in eggs and muscle and to assess the influence of maternal age and size on the transfer of OCs from mother to egg. Concentrations of most OCs in Lake Superior walleye eggs were 1 to 3 orders of magnitude greater than in Lake Manitoba eggs. Toxaphene (mean concentration (wet weight) ± 1 SE, 1580 ± 462 ng/g) and polychlorinated biphenyls (PCBs) (240 ± 24 ng/g) were the predominant OCs in Lake Superior walleye eggs, whereas DDT and metabolites (eggs 16 ± 1.5 ng/g, muscle 2.1 ± 0.36 ng/g) and PCBs (eggs 9.2 ± 0.83 ng/g, muscle 2.0 ± 2.4 ng/g) were the most common OCs in Lake Manitoba walleye eggs and female muscle. Egg size (dry mass) and the concentration of most OCs in Lake Manitoba walleye eggs were positively correlated with female length and age. This relationship was strongest for more hydrophobic OCs (e.g., PCBs) but was not significant for less hydrophobic OCs (e.g., hexachlorocyclohexanes (HCHs)). Neither egg size nor egg OC concentration of Lake Superior walleye were significantly correlated with female length or age. There was no relationship between OC concentrations in muscle tissue and female length or age of Lake Manitoba walleye. OC concentrations in Lake Manitoba walleye eggs were not correlated with concentrations in the muscle tissue of the mothers, suggesting that OCs in walleye eggs are derived from various tissues. A positive relationship between the egg:muscle ratio of PCB concentrations and the egg:muscle ratio oflipid in freshwater fish suggests that the maternal transfer of PCBs in freshwater fish is related to the relative amounts of lipid in the eggs and mother. The transfer of hydrophobic OCs from mother to eggs in freshwater fish appears to vary within and among fish species and with the hydrophobicity of the OC.     

Thiamine and Fatty Acid Content of Lake Michigan Chinook Salmon

Nutritional status of Lake Michigan Chinook salmon (Oncorhynchus tshawytscha) is inadequately documented. An investigation was conducted to determine muscle and liver thiamine content and whole body fatty acid composition in small, medium and large Chinook salmon. Muscle and liver thiamine concentrations were highest in small salmon, and tended to decrease with increasing fish size. Muscle thiamine was higher in fall than spring in large salmon. The high percentage of Chinook salmon (24–32% in fall and 58–71% in spring) with muscle thiamine concentration below 500 pmol/g, which has been associated with loss of equilibrium and death in other Great Lake salmonines, suggest that Chinook appear to rely less on thiamine than other Great Lakes species for which such low concentrations would be associated with thiamine deficiency (Brown et al. 2005b). A positive correlation was observed between liver total thiamine and percent liver lipids (r = 0.53, P < 0.0001, n = 119). In medium and large salmon, liver lipids were observed to be low in fish with less than 4,000 pmol/g liver total thiamine. In individuals with greater than 4,000 pmol/g liver thiamine, liver lipid increased with thiamine concentration. Individual fatty acids declined between fall and spring. Essential omega-3 fatty acids appear to be conserved as lipid content declined. Arachidonic acid (C20:4n6), an essential omega-6 fatty acid was not different between fall and spring, although the sum of omega-6 (Sw6) fatty acids declined over winter. Elevated concentrations of saturated fatty acids (sum) were observed in whole body tissue lipid. In summary, thiamine, a dietary essential vitamin, and individual fatty acids were found to vary in Lake Michigan Chinook salmon by fish size and season of the year.     

Sex difference in polybrominated diphenyl ether concentrations of walleyes

Polybrominated diphenyl ether (PBDE) concentrations were determined for mature male and mature female walleyes (Sander vitreus) sampled from the Saginaw Bay population during 2007. PBDE concentrations in prey fish caught in the Saginaw River, the primary tributary to Saginaw Bay, and in Saginaw Bay during 2005 and 2007 also were determined. Mature male and mature female walleyes averaged 70.3 ng/g and 24.8 ng/g, respectively, in ΣPBDE, which was equal to the sum of concentrations of six PBDE congeners (BDE-28, BDE-47, BDE-99, BDE-100, BDE-153, and BDE-154). This sex difference was likely due to males spending more time in the Saginaw River system than females. Prey fish captured in the Saginaw River were roughly ten times higher in ΣPBDE than those caught in Saginaw Bay. BDE-47 was the predominant congener in both walleyes and prey fish, and this congener contributed about 50%, on average, to ΣPBDE. Congener profiles differed significantly between the two sexes of walleyes. In contrast, congener profiles of the prey fish did not differ significantly between the river-caught fish and the bay-caught fish. One plausible explanation for these congener profile results was that net trophic transfer efficiencies of PBDEs to walleyes from their prey were similar for all congeners except BDE-28, and that diet composition differed between the two sexes of walleyes.     

Polychlorinated biphenyls and organochlorine pesticides concentration patterns and trends in top predator fish of Laurentian Great Lakes from 1999 to 2014

Concentration patterns and temporal trends of legacy persistent, bioaccumulative and toxic (PBT) contaminants were determined using the Great Lake Fish Monitoring and Surveillance Program (GLFMSP) top predator fish data from 1999 to 2014 and applying Kendall-Theil robust regression after cluster-based age normalization. For most Great Lakes sites, significant decreasing concentration trends ranging from ?4.1% to ?21.6% per year (with the only exception being mirex in Lake Erie walleye) were found for PBTs including polychlorinated biphenyls (PCBs), dichloro-diphenyl-trichlorethane (DDTs), dieldrin, endrin, chlordane, oxychlordane, nonachlor, mirex, and hexachlorobenzene (HCB) reflecting the successful historical and ongoing reduction of fugitive releases and remediation efforts in the U.S. and Canada including physical removal (dredging) coupled with sediment sequestration. Generally, lower concentrations and faster decreasing trends are observed in western/northern sampling sites compared to eastern/southern sites as the former sites are generally more remote from population centers and industrial activities. PCBs, which can be released from ongoing sources, have the highest concentration, the second slowest decreasing trend, and increasing mass fractions of the contaminants studied suggesting that they will continue to be the legacy contaminant of greatest concern into the future.     

Contaminant Residues in Tissues of Adult and Prefledged Herring Gulls from the Great Lakes in Relation to Diet in the Early 1990s

In the early 1990s, herring gulls (Larus argentatus) were collected in 15 breeding colonies throughout the Great Lakes basin and in two reference colonies on Lake Winnipeg and the Bay of Fundy. Organochlorine and metal concentrations, and stable isotope ratios (¹⁵N/¹⁴N and ¹³C/¹²C) were measured in their tissues, and we qualitatively assessed their diet. Breast muscle δ¹⁵N suggested that adults fed on planctivorous or insectivorous fish at six colonies, on piscivorous fish at four, and at a lower trophic level at the remaining three. The concentrations of Co, Ni, Al, Cr, Sn, Fe, and Pb in kidneys of adults suggested anthropogenic enrichment in the Great Lakes basin. Concentrations of contaminants were highest most often in tissues of gulls from Lake Ontario and northern Lake Michigan colonies. Concentrations of Pb in adults from Hamilton Harbour and the Detroit River, and of Se in adults from the southern Lake Huron colony, were similar to published toxicity thresholds. Tissue levels of Cd have increased, while those of Pb have decreased markedly since 1983. DDE, dieldrin, mirex, and ΣPCB concentrations in livers collected from nine of these colonies revealed declines of 16 to 87% at most locations since the early 1980s. 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents were highest in adults from the offshore colonies in western Lake Erie and northern Lake Michigan, where gulls feed on piscivorous fish, and were driven by non-ortho PCBs. There was evidence of an unusually high bioavailablity of organochlorines, especially dieldrin, near the northern Lake Michigan colony during the period of chick growth, and of an ongoing loading of mercury to eastern Lake Ontario. Tissues of adult gulls from colonies on Lakes Ontario, Erie, and Michigan best reflect local conditions whereas those from Lake Superior and northern Lake Huron reflect contaminants accumulated from time spent on the lower lakes.     

Spatial trends and temporal declines in tissue metals/metalloids in the context of wild fish health at the St. Clair River Area of Concern

The St. Clair River, which flows from Lake Huron to Lake St Clair, receives discharges and emissions from numerous petroleum refineries, petrochemical facilities, and other sources. We used a multi-factor ANOVA to test for spatial and temporal changes in concentrations of trace metals/metalloids in homogenates of fish from Lake Huron (reference site) and two river sites - an industrialized region (Stag Island) and downstream Walpole Island/Chenal Écarte. In 2002/2003 and in 2014, we sampled 20 adult fish of each sex of the following species: shorthead redhorse (Moxostoma macrolepidotum), yellow perch (Perca flavescens); we added emerald shiner (Notropis atherinoides; unsexed) in 2014. There was a temporal decline of most metals/metalloids in shorthead redhorse at the river sites. Linear Discriminant Analysis separated the 2014 shorthead redhorse by site. Emerald shiners had higher concentrations of metals/metalloids, apart from mercury, than the other species; those concentrations were highest at Chenal Écarte, which warrants further research. Tissue concentrations of mercury, which declined temporally at Stag Island and Walpole Island, were above the protective guidance for wildlife consumers at all sites, but were lower than predicted to affect fish health. Apart from increased liver size (ANCOVA) in shorthead redhorse at Walpole Island (2014), the health variables in the riverine fish were stable or differed slightly from reference, which indicated good health and reproductive potential. In shorthead redhorse, tissue metals correlated inconsistently with health indices. Further research could ascertain if temporal declines in metals/metalloids in riverine SHRH are benefits of remediation actions or fluctuations with other causes.     

The behavioural response of migratory sea lamprey (Petromyzon marinus) to potential damage-released larval and migratory chemical alarm cues

We investigated the presence of damage-released alarm cues and the reactions they may cause in landlocked migratory sea lamprey (Petromyzon marinus) using semi-natural laboratory conditions during the day. In two separate experiments, migratory sea lampreys were exposed to stimuli prepared from the skinless carcasses and skin tissue of larval sea lamprey and from the skin tissue and muscle tissue of migratory sea lamprey. Migratory female sea lamprey swam significantly longer after being exposed to the stimulus prepared from the skinless carcass of larval sea lamprey. No significant changes were seen in the behaviour of the migratory female lamprey in response to larval skin extract or in the behaviour of male sea lamprey to any experimental extract in the larval extract experiment. In the second experiment that utilised migratory lamprey stimuli, neither male and nor female migratory sea lamprey showed a significant difference in their behavioural response among different treatments and controls. Our findings indicate that adult female sea lamprey respond strongly to damage-released alarm cues from larval sea lamprey; hence, the latter holds promise for sea lamprey behavioural manipulations for control purposes during the day. Further research is needed to examine responses to damage-released alarm cues at night, when migratory sea lampreys are more active.     

Evaluation of adult and offspring thiamine deficiency in salmonine species from Lake Ontario

Thiamine Deficiency Complex (TDC) is an ongoing problem impacting salmonine health in various waterbodies, including Lake Ontario. The prevalence of TDC has been variable and explanations for differences are limited. In the current study, thiamine concentrations were measured in eggs, liver tissue, and muscle tissue sampled from brown trout (Salmo trutta), Chinook salmon (Oncorhynchus tshawytscha), coho salmon (O. kisutch), lake trout (Salvelinus namaycush), and steelhead trout (O. mykiss) that were collected from Lake Ontario and its surrounding tributaries. The occurrence of TDC was measured for each species based on TDC-induced offspring mortality rates under laboratory conditions. TDC-induced offspring mortality was observed for all species except brown trout. For affected species, egg free thiamine (Th) was consistently low compared to lake trout collected from Lake Superior that are considered thiamine replete. In addition, species with the lowest percentages of Th in their eggs were the most susceptible to TDC, suggesting that limited thiamine reserves in the form of Th may cause TDC-induced offspring mortality. Lastly, our results show that egg thiamine concentrations have yearly variation and increased for all species throughout the study. Reasons for such variation are undetermined; but, if egg thiamine concentrations continue to increase, the impacts of TDC on these salmonine species may lessen. Future monitoring is needed for determining if thiamine concentrations are increasing and the potential impacts that may have on the entire Lake Ontario fishery.     

Trophic structure and mercury transfer in the subarctic fish community of Great Slave Lake,Northwest Territories,Canada

In recent decades, mercury concentrations have increased in fish of Great Slave Lake (GSL), a subarctic great lake in northern Canada with important recreational, subsistence, and commercial fisheries. This study characterized habitat use and trophic position of common fish species in GSL near the City of Yellowknife (Northwest Territories, Canada), measured mercury concentrations in water and in taxa from lower trophic levels of the food web, and examined trophic and biological influences on mercury concentrations within and among fish species. Northern pike (Exos lucius) and lake whitefish (Coregonus clupeformis) fed predominantly nearshore, cisco (Coregonus artedi) and longnose sucker (Catostomus catostomus) fed predominantly offshore, and burbot (Lota lota) fed roughly equally in both habitats. Habitat-specific feeding did not influence mercury bioaccumulation in fish, in contrast with published studies of smaller lakes. Water concentrations of total mercury and methylmercury were low and showed little spatial variation among sites or depths. Zooplankton (>200 μm) had similarly low methylmercury concentrations as littoral and profundal amphipods, suggesting little habitat-variation of mercury exposure near the base of the food web. Age, size, and trophic position were significant explanatory variables for muscle total mercury concentrations within populations of fish species. Among fish species, size and trophic position explained 80% of the variation in muscle total mercury concentrations. This study generated the most comprehensive dataset to date on mercury bioaccumulation in the food web of GSL, which will serve as a baseline for future studies of this great lake.