We estimated isotopic niche size and trophic position of native lake whitefish (Coregonus clupeaformis (Mitchill, 1818)) decades after cisco (Coregonus artedi Lesueur, 1818) introduction into two lakes of Algonquin Park, Ontario, Canada. Lake whitefish residing in a set of neighboring lakes without native cisco were used for comparison. We found lake whitefish isotopic niche size was larger and trophic position higher in lakes with abundant introduced cisco compared to a lake with a low density of introduced cisco, consistent with findings for European coregonines. At sufficient densities, planktivorous coregonines introduced into lake food webs will shift the niche space of native coregonines. We then compared isotopic niche size and trophic position for native pairings of lake whitefish and cisco to assess if isotopic niche and trophic patterns resulting from cisco introductions are features of native parings of cisco and lake whitefish that have co-existed post-glacially for thousands of years. Introductions of cisco match to some degree niche partitioning and niche size between lake whitefish and cisco co-existing since glacial retreat. The range of trophic niche size and partitioning was large in native pairings indicating niche expansion, compression and divergence characterize lake whitefish coexisting with cisco for thousands of years.

We explored patterns in muscle sulphur to nitrogen ratio (S:N) and sulphur stable isotope ratios (δ34S) and their relationships with muscle total mercury concentrations ([THg]) for fishes of 13 coastal rivers and 18 inland lakes (n = 1370 fish) from northern Ontario, Canada. Muscle S:N decreased and δ34S increased with increasing latitude across populations but neither showed consistent ontogenetic variation within populations. Relative differences among species were similar between rivers and lakes for S:N, but not for δ34S. The piscivore walleye (Sander vitreus) had distinctly higher S:N than other species. In coastal rivers, muscle δ34S was highest in purported anadromous species, whereas in lakes muscle δ34S was most frequently highest in walleye. Within populations, [THg] was positively related to δ15N and δ34S, particularly in lakes. Within food webs, [THg] was positively related to δ15N in both coastal rivers and inland lakes, and positively related to S:N in coastal rivers, but not related to δ34S in either habitat. Our results further our understanding of sulphur in freshwater food webs and its association with mercury dynamics.

Concentrations of bioaccumulative contaminants in fish increase with their size and age; thus, research and monitoring of these contaminants in fish across space and time can be confounded by size covariation. To account for this, size-standardization of contaminant concentrations within fish samples is a common practice. Standardized concentrations are often estimated using within-sample regression models, also known as power series regression (referred to here as sampling event regressions, or SERs). This approach requires higher sample sizes than mixed effect models (MEMs), which are suited for this application but are not as commonly used. Herein we compare SERs to three MEM approaches; restricted maximum likelihood, Bayesian inference via Markov chain Monte Carlo (MCMC), and approximate Bayesian inference with nested Laplace approximation (INLA). We did this for two contaminants: mercury (Hg), a contaminant known to bioaccumulate, and arsenic (As), where the bioaccumulative potential is less understood. The MEM approaches generated size-standardized concentrations for small populations (e.g., <5 fish) and/or populations that lacked the range of sizes required for SER estimates, with comparable residual and root mean squared error to SER estimates. INLA was determined to be the best method in most cases because it was computationally less intensive than other approaches and showed consistent performance across a range of scenarios with sample-size limitations. Additionally, we provided example code for prediction using the R-INLA package to enable use and application in fisheries' contaminant monitoring and research.

Freshwater ecosystems are affected by fluctuations in terrestrially derived dissolved organic carbon (DOC). Increased DOC runoff from watersheds can contribute to the “browning” of freshwater systems, altering water properties and disrupting food web dynamics, with potential impacts on fish. This study examined how carbon sources and the trophic ecology of temperate freshwater fishes varied with DOC concentrations. We used stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) to trace carbon sources in organisms at low trophic levels and to assess the trophic ecology of six fish species from 70 lakes in Sweden, Germany, Canada, and the United States. We found that both pelagic and benthic δ13C decreased as DOC increased, suggesting higher reliance on terrestrial carbon by lower trophic level organisms. Fish responses to elevated DOC were limited to certain species. Mid-trophic level European perch (Perca fluviatilis) increased pelagic diet proportions, while top predators like northern pike (Esox Lucius) and walleye (Sander vitreus) showed nonlinear shifts. Except for walleye, no effect of DOC was detected on fish trophic position. Our findings suggest that DOC alters energy use and diet but not trophic position in some freshwater fish.

Food web theory has illustrated that mobile top predators, such as lake trout (Salvelinus namaycush), can be potent stabilizers of food webs due to their ability to shift foraging behaviors in response to changing conditions. Consistent with this, research has demonstrated that mean lake trout food web attributes (i.e., trophic position and nearshore coupling) structurally change across environmental gradients; however, intraspecific variation in these attributes across gradients has not been fully explored. Here, we used stable isotope-based food web metrics to investigate how both mean and intraspecific variation in trophic structure changes in Canadian boreal shield lakes across gradients in ecosystem size, temperature, and competition. Consistent with earlier findings, we find nearshore coupling decreases and trophic position increases with warmer summer climate. In contrast to previous findings, increasing lake area predicted increased nearshore coupling and was not associated with lake trout trophic position. Our results show that warmer temperatures and smaller ecosystem sizes reduce the expression of intraspecific variation in food web structures. Specifically, larger lakes increased variation in nearshore coupling and trophic position, resulting in larger niche areas, and warmer lakes reduced variation in nearshore coupling and tended to generate smaller niche areas. Interestingly, we found little evidence for the relative abundance of lake trout or other predator taxa (surrogates of intra- and interspecific competition) influencing mean and variance in lake trout trophic structure. Intraspecific variation can promote ecosystem resilience by enabling diverse individual responses that help buffer populations against environmental change. Therefore, reduction in intraspecific variation in smaller, warmer lakes may have undesirable consequences for lake trout and the biota in these Canadian boreal shield lakes, leaving these ecosystems less able to adjust to future perturbations.

Arsenic can accumulate in freshwater biota, sometimes reaching potentially harmful levels. However, the toxicity of arsenic strongly depends on which arsenic species are present. Although organic species are considered less harmful than inorganic ones, they have not been extensively studied in freshwater environments, and drivers of variation in arsenic speciation among sites and taxa remain unclear. We assessed concentrations of two organic arsenic species, arsenobetaine (AsB) and dimethylarsinic acid (DMA), in fish and invertebrates from three lakes near Sudbury, Ontario, Canada-a region with widespread mining impacts. Both AsB and DMA were detected in most samples (n = 212), varying across a wide range of concentrations (<0.001-30.144 and <0.006-5.262 mg/kg dry wt, respectively). The lake with the most severe mining impacts typically had the highest concentrations (designated by square brackets []) of AsB and DMA. In contrast, the percentage of total arsenic made up by AsB (%AsB) and DMA (%DMA) did not vary significantly between lakes. Arsenic speciation in fish muscle varied with fish size, selenium concentrations, and trophic elevation (inferred from nitrogen stable isotope ratios δ15N), but relationships with dietary carbon source (inferred from carbon stable isotope ratios δ13C) were more varied. Within all three lake food webs, [AsB] and [DMA] typically underwent biodilution, decreasing with trophic elevation (i.e., δ15N). Although the aforementioned factors explained some variation in arsenic speciation, there remains considerable unexplained variation. Further studies on arsenic speciation in freshwater biota should target a wider diversity of taxa to better understand drivers of variation in arsenic speciation. In addition, research emphasizing the percentage of inorganic arsenic and other organic arsenic species is needed to improve environmental and human health risk assessments. Environ Toxicol Chem 2024;43:833-846. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

The heart is an essential organ for survival and reproduction across all animals. In particular, salmonine fishes display vast variability in heart morphology across and even within populations, which often reflects their functional requirements for cardiac output and aerobic capacity. However, changes in heart morphology are not always adaptive, and other factors such as diet can influence heart morphology. A growing concern is the consumption of the thiamine (vitamin B1)-degrading enzyme thiaminase found in some prey species such as rainbow smelt ( Osmerus mordax (Mitchell, 1814)). Here, we investigate the association between rainbow smelt consumption and heart morphology in wild lake trout ( Salvelinus namaycush (Walbaum in Artedi, 1792)) found in the Sudbury Basin (Ontario, Canada). We found that rainbow smelt consumption is associated with increases in ventricular mass, reductions in ventricular fluid content, and alterations in the allometry of myocardia. We discuss the potential impacts of these changes in heart morphology for the conservation of the species.

Water resource development can alter the movement and ecology of sturgeons. We studied total (THg) and methylmercury concentrations in whole blood sampled non-lethally from namew (Moose Cree L-dialect, lake sturgeon, Acipenser fulvescens), an endangered and culturally important subsistence fish. Namew were sampled from two tributaries within the Moose Cree Homeland: the Lower Mattagami River (an impacted system with four hydroelectric generating stations) and the North French River (a reference system that is free-flowing system with no development). Results indicated namew from the North French River had higher blood [THg] than those from the Mattagami River. Further modeling showed that trophic position was the primary driver of these differences, with North French namew having the highest nitrogen isotope ratios. Based on further isotope modeling, crayfish were major components of namew diet at all sites, while other prey items differed between sites. Specifically, namew with unobstructed access to the lower watershed had notably more enriched isotope values when compared to the freshwater benthic macroinvertebrates sampled, implying that other prey not captured herein may contribute to their diets and [THg]. Overall, we found differences in namew’s trophic ecology but no elevation in blood mercury levels at a site impacted by hydroelectric operations 60+ years post-impoundment.

To better understand the spatial patterns in arsenic (As) bioaccumulation in freshwater systems, we investigated ecological, physical, and chemical factors associated with total arsenic concentrations ([As]) in lacustrine and riverine fish across Ontario, Canada, using a dataset of 3200 fish across 152 waterbodies. Assembled data of water chemistry, landscape characteristics, and stable carbon and nitrogen isotope ratios in muscle tissue were then used to assess factors related to As bioaccumulation. Results show that [As] were generally low across most species and waterbodies (i.e., <1 µg·g−1 wet in many inland fish). However, fish from northern coastal rivers had up to 23-fold higher [As] when compared with fish from landlocked sites. As concentrations increased slightly with the proportion of pelagic carbon in a fish's diet, although relationships varied among species and sites. Furthermore, principal component scores, representing landscape and water chemistry variables, were related to [As] in fish, but these relationships varied among species. These results will help improve the efficacy of fish contaminant monitoring by further identifying key physical and ecological variables related to higher [As] in fish.

Elevated concentrations of total arsenic (As) have been reported in boreal freshwater fish in both human -impacted and relatively pristine areas. We assessed the arsenic speciation profiles in muscle tissue of six fish species (n = 300) sampled from nine locations across a remote freshwater watershed in northern Ontario, Canada, extending from inland headwater lakes to the coastal marine confluence. Of the five arsenic species measured, only arsenobetaine (AsB) and dimethylarsinic acid (DMA) were detected in these fish. Riverine fish had up to 10-fold higher total [As] when compared to lacustrine fish. On average, these riverine fish also had higher percentages of AsB (%AsB, 60 & PLUSMN; 26%) and lower percentages of unmeasured arsenic (%UNM, 20 & PLUSMN; 21%), compared to lacustrine fish (28 & PLUSMN; 18% and 52 & PLUSMN; 21% %AsB and %UNM, respectively). DMA percentages (% DMA) were relatively consistent across the watershed, averaging 20 & PLUSMN; 21% across all fish. We examined ecological drivers of As speciation and found that %AsB increased slightly with fish weight in large-body predatory fish, but not in forage fish or insectivores. Furthermore, %AsB was positively related to trophic elevation (inferred from delta 15N) in lacustrine fish across 3 out of 4 communities and within some populations. Lastly, riverine fish with a more marine-based diet had markedly higher %AsB when compared to fish with more freshwater-based diets, indicating an effect of anadromy on arsenic speciation. Overall, knowledge on arsenic speciation in freshwater fish has been limited and these results indicate potential drivers that can be considered in future studies. Furthermore, the absence of toxic inorganic As species in these boreal fish is an important consideration for future environmental monitoring practices and risk assessments, some of which assume 10-20% of total [As] in fish is present as toxic inorganic As. Additional studies on As bioaccumulation and biotransformation are needed in freshwater systems, particularly at the base of aquatic food webs.

Mercury (Hg) and arsenic (As) contamination of fish can be toxic and limit safe human consumption, whereas selenium (Se) can potentially protect fish and consumers from the adverse effects of Hg and As. We assembled datasets of the above-mentioned elements in Canadian freshwater fish and compare them with risk assessment thresholds. We further assessed linkages between the elemental concentrations and anthropogenic activities and ecozones. Mercury concentrations exceeded the retail fish Canadian threshold (0.5 & mu;g/g wet weight) in 31% of all Walleye; this proportion rose to 64% in reservoirs. Reservoirs and lakes impacted by logging and urbanization had higher fish [Hg] than other types of impacted systems. Se and As concentrations exceeded Canadian guidelines in 5% (aquatic life) and 0.2% of all fish, respectively. In mining areas, fish [Hg] were low and negatively correlated with [Se], and fish [Se] were positively correlated with [As]. In all areas, we observed an important overall and previously unpublished negative relationship between mean fish [As] and [Hg], suggesting an inverse consumption risk for these two elements. The ratio Se/Hg was lower than the protective value of 1 for 14% of all fish and was negatively correlated with fish length. However, the benefit-risk value (BRV) threshold, which accounts for the Se intake from other food products, did not suggest any fish consumption limitations, except for few very contaminated top predators ( > 2 & mu;g/g ww). More studies need to assess the role of Se against Hg toxicity and adjust fish consumption guidelines accordingly.

Fish total mercury concentration ([THg]) has been linked to various fish attributes, but the relative importance of these attributes in accounting for among-individual variation in [THg] has not been thoroughly assessed. We compared the contributions of ontogeny (age, length), growth (growth rate, body condition), and food web position (δ13C, δ15N) to among-individual variability in [THg] within populations of seven common fishes from 141 north temperate lakes. Ontogenetic factors accounted for most variation in [THg]; age was a stronger and less variable predictor than length for most species. Adding both indices of growth and food web position to these models increased explained variation (R2) in [THg] by 6–25% among species. Fish [THg] at age increased with growth rate, while fish [THg] at length decreased with growth rate, and the effect of body condition was consistently negative. Trophic elevation (inferred from δ15N) was a stronger predictor than primary production source (inferred from δ13C) for piscivores but not benthivores. Fish [THg] increased with δ15N in all species but showed a more variable relationship with δ13C. Among-individual variation in [THg] is primarily related to age or size in most temperate freshwater fishes, and effects of growth rate and food web position need to be considered in the context of these ontogenetic drivers.

Differences in reproductive strategies of male and female fishes are presumably accompanied by differences in nutrient allocation and predicted to lead to divergence in body composition between the sexes. We compared patterns of variation in fatty acid profiles of lipids extracted from ova, liver, muscle and visceral fat between mature male and female walleye (Sander vitreus) sampled from two wild spawning stocks. Fatty acid profiles differed significantly among body tissues in both males and females, with the strongest contrast between muscle and visceral fat. Significant differences in fatty acid composition between the sexes were found in liver, muscle and visceral fat tissues. Variation among sexes and populations was greater in liver than in the other tissues. Female livers had lower relative abundances of palmitic acid (PA, 16:0) and oleic acid (OA, 18:1(n-9)), and higher relative abundances of arachidonic acid (ARA, 20:4(n-6)), eicosapentaenoic acid (EPA, 20:5(n-3)) and docosahexaenoic acid (DHA, 22:6(n-3)) compared to male livers. In addition, female muscle had higher relative abundance of OA and lower relative abundance of DHA compared to male muscle. Our results illustrate the differential effects of reproductive demands on the biochemical composition of males and females and have implications for the analysis of fatty acid profiles in studies of wild fish populations.

Mercury pollution is a global environmental problem that threatens ecosystems, and negatively impacts human health and well-being. Mercury accumulation in fish within freshwater lakes is a complex process that appears to be driven by factors such as individual fish biology and water chemistry at the lake-scale, whereas, climate, and land-use/land-cover conditions within lake catchments can be influential at relatively larger scales. Nevertheless, unravelling the intricate network of pathways that govern how lake-scale and large-scale factors interact to affect mercury levels in fish remains an important scientific challenge. Using structural equation models (SEMs) and multiple long-term databases we identified direct and indirect effects of lake-scale and larger-scale factors on mercury levels in Walleye and Northern Pike - two species that are valued in inland fisheries. At the lake-level, the most parsimonious path models contained direct effects of fish weight, DOC, and pH, as well as an indirect effect of DOC on fish mercury levels via fish weight. Interestingly, lakeshed-, climate-, and full-path models that combine the effects of both lakeshed and climate revealed indirect effects of surrounding landscape conditions and latitude via DOC, pH, and fish weight but no direct effects on fish mercury levels. These results are generally consistent across species and lakes, except for some differences between stratified and non-stratified lakes. Our findings imply that understanding climate and land-use driven alterations of water chemistry and fish biology will be critical to predicting and mitigating fish mercury bioaccumulation in the future.

Eating fish provides numerous health benefits, but it is also a dominant pathway for human exposure to contaminants. Many studies have examined mercury (Hg) accumulation in fish, but fewer have considered other elements, such as arsenic (As) and chromium (Cr). Recently, freshwater fish from several pristine boreal systems across northern Ontario, Canada, have been reported with elevated concentrations of As and Cr for reasons that are not well understood. Our goal was to investigate the ecological and environmental influences over concentrations of As, Cr, and other elements in these fish to better understand what affects metal uptake and the risk to consumers. We measured 10 elements (including As, Cr, Hg) as well as carbon (delta C-13), nitrogen (delta N-15), and sulfur (delta S-34) stable isotopes in 388 fish from 25 lake and river sites across this remote region. These data were used to determine the effect of: 1) trophic ecology; and 2) watershed geology on piscine elemental content. Overall, most element concentrations were low, often below provincial advisory benchmarks (ABs). However, traces of Hg, As, Cr, and selenium (Se) were detected in most fish. Based on their exceedance of their respective ABs, the most restrictive elements on fish consumption in these boreal systems were Hg > As > Cr. Arsenic and Se, but not Cr concentrations were related to fish size and trophic ecology (inferred from delta C-13 and delta N-15), suggesting bioaccumulation of the former elements. Fish with enriched delta S-34 values, suggestive of anadromous behaviour, had marginally lower Hg but higher Se concentrations. Modeling results suggested a strong effect of site-specific factors, though we found weak trends between piscine elemental content and geological features (e.g., mafic intrusions), potentially due to the broad spatial scale of this study. Results from this study address gaps in our understanding of As and Cr bioaccumulation and will help to inform fish consumption guidelines. (C) 2020 Elsevier Ltd. All rights reserved.

We compared the trophic niches of freshwater sculpins Cottus spp. with those of other co-habiting forage fishes in two groups of boreal lakes with distinct habitats and fish communities. Near North Lakes (45° 00' to 47° 30' N) were deeper, stratified and contained lake trout Salvelinus namaycush as the apex piscivore, whereas Far North Lakes (51° 10' to 52° 20' N) were shallower, did not stratify and contained pike Esox lucius and walleye Sander vitreus as the apex piscivores. Trophic niches of sculpins and other forage fishes were compared based on niche metrics calculated from muscle stable carbon (δ13 C) and nitrogen (δ15 N) isotope ratios. In Near North Lakes, sculpins were found almost exclusively in deep, offshore waters and their niche positions reflected a greater reliance on pelagic production (lower δ13 C) and a higher trophic elevation (higher δ15 N) compared with most other forage fishes. Furthermore, sculpins in Near North Lakes tended to have larger trophic niches (occupied greater area in δ13 C- δ15 N space), particularly in the food chain (δ15 N) dimension, than other cohabiting forage fishes. In contrast, sculpins in Far North Lakes were commonly found in the nearshore and had trophic niche positions and sizes that were similar to those of the other cohabiting forage fishes. This study illustrates the flexibility in the realised trophic niches of sculpins in relation to habitat availability and fish community composition in boreal lakes.

Egg quality influences early life survival in fishes, but drivers of egg quality variation remain poorly understood. I examined egg quality of a long-lived, iteroparous salmonid (lake trout (Salvelinus namaycush)) with respect to maternal traits and environmental conditions in wild and captively reared populations. Variation was stronger and more consistent for egg size than for lipid content. Among females, egg size was most strongly related to maternal age, in both wild and captive populations, and faster growing females tended to produce larger eggs. Among wild populations, maternal growth was more strongly associated with indices of ecosystem size or productivity than climate, whereas the opposite was observed for egg size. Egg size and, to a lesser extent, maternal growth in captive populations were positively correlated with egg size and growth, respectively, in their wild source populations. However, captive females grew faster and produced larger eggs at age, but smaller eggs at length, than their wild counterparts. Lake trout egg quality variation has both maternal and environmental components, and captive rearing appears to alter the ontogenetic progression of egg size.

It is commonly assumed that most (>95%) of the mercury (Hg) found in fish muscle is the toxic form, methylmercury (MeHg), due to its efficient assimilation and retention in biotic tissue. However, this assumption is largely based on studies examining the percentage of MeHg (%MeHg [the fraction of total Hg as MeHg]) in muscle from mostly large‐bodied predatory fish; less is known about the %MeHg in smaller bodied individuals or those of different trophic guilds. The present study analyzed MeHg and total Hg concentrations in the muscle of 2 large‐bodied piscivores (walleye and northern pike), one large‐bodied benthivore (white sucker), and 2 small‐bodied forage fish (sculpins and shiners) across a broad size range. We found substantially lower %MeHg than the commonly assumed 95% in several fish (e.g., 17 individuals had <70% MeHg). Muscle %MeHg significantly increased with size and age in all species except walleye, which had significantly higher %MeHg than pike or suckers, particularly in smaller and younger fish (e.g., 18–21% higher at 10 g; 5–11% higher at 500 g). Results of predictive modeling suggest that muscle %MeHg is higher in pelagic‐feeding fish and those with lower lipid content, although model results varied significantly among species. According to our findings, total Hg measurement in muscle is not an appropriate proxy for MeHg in smaller fish from all species, an important consideration for future piscine Hg studies and monitoring. Environ Toxicol Chem 2018;37:2682–2691. © 2018 SETAC

Information related to the analysis of Lake Erie walleye.

Burbot (Lota lota) are northern freshwater gadoid fish that spawn under ice-cover, making their reproductive behavior largely unknown to science. Some members of the cod family vocalize as part of their mating system. These calls are produced by rapidly contracting drumming muscles on their swim bladders. Burbot also possess drumming muscles, like their marine counterparts, which may enable them to vocalize. To assess the potential for burbot to make calls, pre-spawning adult burbot were collected and placed in a large under-ice enclosure in Great Slave Lake, Northwest Territories, Canada, along with a recorder that monitored low frequency sound over their spawning period. The recorded acoustic data revealed that burbot called coincident with the onset of their spawning period and that the call signatures were stereotypical of swim bladder generated vocalizations made by other gadoid fishes. Burbot showed a wide repertoire of calls, from slow knocks to fast buzzing, similar to the closely related haddock (Melanogrammus aeglefinus). Although never-before documented, calling by fish under ice-cover is likely an important part of the mating system of under-ice spawning gadoids because light limitation would reduce the usefulness of visual cues. These under-ice communications may be affected by anthropogenic noise from increasing resource development in northern regions.