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Food web theory predicts that one of the consequences of oligotrophication is the decline in fisheries following the establishment of nutrient-deficient conditions and low biogenic production. However, emerging evidence suggests that more complex trophic interactions, induced by plankton composition, dietary diversification, temporal and spatial heterogeneity, variations in fish age and size structure can lead to ecosystem responses that deviate from theoretical predictions; especially in the shallow littoral zone. In this study, two end-to-end modelling strategies are used to characterize potential food web structural shifts and overall ecosystem productivity in response to oligotrophication. Our modelling strategy draws upon empirical and theoretical evidence from the Bay of Quinte, a semi-enclosed embayment in northeastern Lake Ontario that has been subjected to phosphorus (P) abatement strategies in the 1970s. We first show that the role of food abundance has been a primary factor shaping fish production, but the fish community gradually responded to the reduced availability of pelagic staples of their diet by displaying greater reliance upon alternative sources, including benthic and other autochthonous (or even allochthonous) food items. Our analysis further suggests that degree of diet overlap between competing trophic guilds can significantly modulate realized biomass levels. The composition of the autotrophic community is another major confounding factor that can induce significant variations from the predicted food web productivity patterns in response to oligotrophication. Overall, our analysis suggests that the proposed reduction of the ambient total phosphorus levels may not necessarily trigger a significant decline in fish biomass in the Bay of Quinte. [ABSTRACT FROM AUTHOR]

The trophic ecology of juvenile salmonids in nearshore Lake Ontario is not well understood. We used stomach content and stable isotope diet and niche metrics, as well as condition metrics to understand the trophic ecology of juvenile Atlantic salmon (Salmo salar), brown trout (Salmo trutta), Chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhynchus mykiss), and lake trout (Salvelinus namaycush). Salmonids had comparable length-weight slopes and stomach mass, and allometric patterns existed between fork-length and both relative stomach content mass and energy density for all species. Interspecific differences existed with respect to diet, and the magnitude of difference generally increased with increasing body size. Small sized salmonid (fork length < 100 mm) diets mainly consisted of Diptera (% volume = 66 – 100; prey isotope %: 11.7 – 78.4), while large sized salmonids (fork length 200 – 300 mm) consumed fish (% volume = 20–100; prey isotope % = 21.5 – 42.7). Salmonids exhibited high interspecific niche overlap, with lake trout (SEA B = 22.9 ‰2) and brown trout (18.6 ‰2) having the largest isotopic niche size, and Atlantic salmon having the smallest (2.7 ‰2). Our study addressed a knowledge gap in trophic ecology between mostly stream-dwelling juvenile and open lake adult salmonid life stages, revealing differences in diet but comparable condition metrics which suggests different strategies to optimise performance in the nearshore environment. [ABSTRACT FROM AUTHOR]

Mysids (Mysis diluviana) and dreissenids (Dreissena polymorpha and mostly D. bugensis) are important invertebrate taxa in the food webs of the Laurentian Great Lakes but there are uncertainties about the seasonal and spatial variability in their stable isotope signatures. We quantified δ13C and δ15N in 304 mysid and 366 dreissenid samples across five spatial ecoregions, varying site depth, and three seasons (spring, summer, and fall) in Lake Ontario in 2012 and 2013. Particulate organic matter (POM) was also collected across site depth and season from the Deep Hole ecoregion for use as an isotopic baseline. Lipid normalization models for δ13C were generated for both taxa to reduce lipid bias in our statistical analysis. Season was a significant predictor of POM stable isotopes, with δ13C lower in the summer and δ15N decreasing from spring to summer before increasing into fall. Mysid lipid normalized δ13C varied by site depth and ecoregion while δ15N decreased across season and did not vary by site depth or ecoregion. Dreissenid stable isotopes varied significantly across season, depth, and ecoregion, with site depth having positive relationship with δ15N. Mysids and dreissenids were two trophic positions higher than POM based on δ15N; this comparison was restricted to the one region where POM was collected. Isotopic variability suggested selective feeding within POM and differing trophic pathways between mysids and dreissenids. Collecting an appropriate taxon across all observed variables to serve as an isotopic baseline, particularly in spatial and temporal studies, is critical to the correct interpretation of trophic relationships. [ABSTRACT FROM AUTHOR]

Understanding the spatial use of reintroduced fish is useful for fisheries management and evaluating restoration success. Atlantic salmon (Salmo salar) were reintroduced into Lake Ontario in the 1990s; however, the movement ecology of these land-locked fish is unknown. Using acoustic telemetry and Floy tag mark-recaptures, we examined seasonal home range and space use of Atlantic salmon in Lake Ontario. Hatchery-raised adult Atlantic salmon were tagged with acoustic transmitters (n = 14; 8 with depth sensors) or Floy tags (n = 1915) and released. Both acoustic telemetry and Floy tag recaptures (n = 90) indicated cross lake movements, and home ranges encompassed nearly the entire lake in summer but was smaller in winter. Movements were nearshore (<2 km from shore) from spring to summer at ∼20 m bathymetric depths, with movements closer to shore in the fall, and further offshore (∼5.5 km from shore and 45 m bathymetric depths) in winter. Depth use was relatively shallow (<4 m) with occasional deeper dives (max = 28.5 m), and small diel vertical movements (1–5 m), moving deeper during daytime, consistent with ocean movements of Atlantic salmon. There appears to be spatial segregation among Atlantic salmon and other Lake Ontario salmonids, however, overlap likely occurs in nearshore waters during the spring. Wide-ranging movements of Atlantic salmon in binational (Canada/USA) waters reflects the importance of government agencies collaborating to ensure sustainable fisheries and the coordination of species restoration activities. This is the first study to provide detailed spatial use of Lake Ontario Atlantic salmon to assist in the management of this reintroduced species. [ABSTRACT FROM AUTHOR]

Bloater, Coregonus hoyi , are deepwater planktivores native to the Laurentian Great Lakes and Lake Nipigon. Interpretations of commercial fishery time series suggest they were common in Lake Ontario through the early 1900s but by the 1950s were no longer captured by commercial fishers. Annual bottom trawl surveys that began in 1978 and sampled extensively across putative bloater habitat only yielded one individual (1983), suggesting that the species had been locally extirpated. In 2012, a multiagency restoration program stocked bloater into Lake Ontario from gametes collected in Lake Michigan. From 2012 to 2020, 1,028,191 bloater were stocked into Lake Ontario. Bottom trawl surveys first detected stocked fish in 2015, and through 2020 ten bloater have been caught (total length mean = 129 mm, s.d. = 44 mm, range: 96–240 mm). Hatchery applied marks and genetic analyses confirmed the species identification and identified stocking location for some individuals. Trawl capture locations and acoustic telemetry suggested that stocked fish dispersed throughout the main lake within months or sooner, and the depth distribution of recaptured bloater was similar to historic distributions in Lake Ontario and other Great Lakes. Predicted bloater trawl catches, based on modeled population abundance and trawl survey efficiency, were similar to observed catches, suggesting that post-stocking survival is less than 20% and contemporary bottom trawl surveys can quantify bloater abundance at low densities and track restoration. [ABSTRACT FROM AUTHOR]

Lake trout (Salvelinus namaycush) are an ecologically and economically important piscivore with reported differences in diet and feeding behaviour throughout its range. Eleven stomach content and stable isotope-based metrics were used to describe diets of 349 lake trout between two years (2013 and 2018) and among geographic zones (west, central, east, Kingston basin) in Lake Ontario. Using individual (e.g., volumetric, %V) and aggregate (e.g., index of relative importance, %IRI) diet metrics, we found an overwhelming dominance of alewife (Alosa pseudoharengus) in lake trout diets among some zones in 2013 (%V = 23.3 – 92.7; %IRI = 12.2 – 99.5) and all zones in 2018 (%V = 83.9 – 96.7; %IRI = 96.5 – 100). Round goby (Neogobius melanostomus) and rainbow smelt (Osmerus mordax) were secondary lake trout prey items with relative diet percentages only marginally reflected by spatial and temporal variation in prey abundance (round goby: %V = 1.0 – 33.3, %IRI = 0.1 – 13.2; rainbow smelt: %V = 2.5 – 54.0, %IRI = 0.1 – 54.0). Carbon (δ13C) and nitrogen (δ15N) isotopic niche areas and orientations were similar across all year-zone combinations reinforcing temporal and spatial consistency in lake trout diet. The findings of this study advance the time series in describing Lake Ontario lake trout diets and can be used to complement stock assessments and management decisions associated with carrying capacity for the diverse salmonid community. [ABSTRACT FROM AUTHOR]

Warming water temperatures present challenges for ectotherms in freshwater ecosystems through influence on metabolic rate and bioenergetics. Diet, such as the inclusion of high energy prey, can reduce these influences, but accurate temperature profiles are key to improved predictions. Here, using the Wisconsin bioenergetics approach, we modelled lake trout (Salvelinus namaycush) and Chinook salmon (Oncorhynchus tshawytscha) growth for two time periods (present [2010–2019] and future 30-year (2041–2070) averaged scenarios) in Lake Ontario with in-situ observed year-round temperatures (obtained through acoustic telemetry and pop-off data storage tags deployed between 2016 and 2019) occupied by the species. For the future, we considered two water temperature scenarios (low and high, where experienced temperature increased by a mean of 0.4 °C and 1.0 °C, respectively) and the effects of a hypothetical diet switch with the inclusion of higher energy prey (bloater, Coregonus hoyi) currently being reintroduced. Under all forecasted warming conditions, lake trout performed well and growth was 41.3 to 82.5 % above present when bloater was re-incorporated in the diet up to 40 %. Chinook salmon maximum attainable growth declined between 11.2 and 29.6 % under forecasted warming scenarios without diet change. However, when bloater comprised ∼ 33 % of their diet, Chinook salmon growth increased 3.7 % under the low future scenario compared to present. These results demonstrate that impacts of future lake warming on predatory fish will vary with life-history characteristics of species and composition and abundance of prey base, and highlights the need for effective management that diversifies and conserves forage fish species in the Great Lakes. [ABSTRACT FROM AUTHOR]

The North American Great Lakes have been experiencing dramatic change during the past half-century, highlighting the need for holistic, ecosystem-based approaches to management. To assess interest in ecosystem-based management (EBM), including the value of a comprehensive public database that could serve as a repository for the numerous physical, chemical, and biological monitoring Great Lakes datasets that exist, a two-day workshop was organized, which was attended by 40+ Great Lakes researchers, managers, and stakeholders. While we learned during the workshop that EBM is not an explicit mission of many of the participating research, monitoring, and management agencies, most have been conducting research or monitoring activities that can support EBM. These contributions have ranged from single-resource (-sector) management to considering the ecosystem holistically in a decision-making framework. Workshop participants also identified impediments to implementing EBM, including: 1) high anticipated costs; 2) a lack of EBM success stories to garner agency buy-in; and 3) difficulty in establishing common objectives among groups with different mandates (e.g., water quality vs. fisheries production). We discussed as a group solutions to overcome these impediments, including construction of a comprehensive, research-ready database, a prototype of which was presented at the workshop. We collectively felt that such a database would offer a cost-effective means to support EBM approaches by facilitating research that could help identify useful ecosystem indicators and management targets and allow for management strategy evaluations that account for risk and uncertainty when contemplating future decision-making. [ABSTRACT FROM AUTHOR]

Abstract Lake Ontario supports a diversity of native and non-native salmonids which are managed largely through stocking practices. Ecological changes (e.g., invasive species) altering the food web structure accompanied with shifts in prey abundance, necessitate understanding the trophic niches of Lake Ontario salmonids to aid in management. The objectives of this study were to quantify salmonid (5 species) trophic niches and dietary proportions using stable isotope ratios (δ13C and δ15N) of a large sample set (adult fish (>300 mm; n = 672) and key offshore prey (5 species, n = 2037)) collected across Lake Ontario in 2013. Estimates of prey based on stable isotope ratios were similar to stomach contents. Based on stable isotope ratios, non-native prey dominated salmonid diet; in particular alewife (Alosa pseudoharengus) constituted the majority (0.31 to 0.93) of all salmonid diets, and round goby (Neogobius melanostomus) contributed 0.26 and 0.19 of brown trout (Salmo trutta) and lake trout (Salvelinus namaycush) diets, respectively. Trophic niche overlap was high between all salmonids, except lake trout. The largest trophic niche overlap occurred between Chinook (Oncorhynchus tshawytscha), coho (Oncorhynchus kisutch), and Atlantic salmon (Salmo salar), and their reliance on alewife infers a strong pelagic foraging strategy. Lake, brown and rainbow (Oncorhynchus mykiss) trout had larger and/or more distinct trophic niches indicative of a more variable diet across individuals and utilizing different foraging strategies and/or habitats. Overall, Lake Ontario salmonids maintained a high reliance on alewife, and their potential for plasticity in diet provides important information to management regarding population sustainability. [ABSTRACT FROM AUTHOR]

Round goby ( Neogobius melanostomus ) and western tubenose goby ( Proterorhinus semilunaris ) invaded the Laurentian Great Lakes at approximately the same time and area yet have shown substantial differences in their post-invasion success with more rapid establishment and development of much larger abundances of round goby populations throughout the invaded habitat. In this study, we compared differences in physiological performance (thermal tolerance and standard metabolic rate) between round and tubenose goby collected from the Huron-Erie corridor. Tubenose goby were observed to have lower thermal tolerance but exhibited similar standard metabolic rate across environmental temperatures compared to round goby. At temperatures exceeding 31 °C, tubenose goby demonstrated significantly higher mortalities and shorter times to death relative to round goby. The observed differences in thermal tolerance were consistent with differences in the native geographic ranges observed for each species at their southern ranges. The observed differences in physiological performance combined with species differences in other life history traits such body size, reproduction, feeding ecology and habitat affiliation may also explain differences in the invasiveness experienced by these two Great Lakes invasive fish including a greater ability of round gobies to occupy extreme habitats with large water temperature fluctuations. [ABSTRACT FROM AUTHOR]

Grass carp ( Ctenopharyngodon idella ), native to eastern Asia, have established populations throughout the Mississippi River basin and are now reproducing naturally in the Great Lakes basin. As a large herbivorous fish, there is concern that an established grass carp population in the Great Lakes may threaten nearshore vegetated areas and wetlands. We parameterized a bioenergetics model for grass carp from the primary literature to quantify individual consumption levels and estimate the impacts of an established population on macrophytes in representative areas of Lakes Erie and Ontario. Individual life time consumption was estimated under average, cool and warm temperature conditions. Under average temperature conditions, a population of grass carp could consume up to 27.6 kg of vegetation per kg of fish per year, depending on energy density of the vegetation. When consumption was estimated for populations of various grass carp biomass densities, most simulated scenarios resulted in < 50% of vegetation remaining in an invaded wetland after one year, with the majority of consumption from pre-adult stages. Direct impacts will likely exceed these effects due to losses to vegetation production potential from grass carp feeding early in the growing season and grass carp foraging activity resulting in plant damage or uprooting. [ABSTRACT FROM AUTHOR]

Recent development of multi-dimensional stable isotope models for estimating both foraging patterns and niches have presented the analytical tools to further assess the food webs of freshwater populations. One approach to refine predictions from these analyses is to include a third isotope to the more common two-isotope carbon and nitrogen mixing models to increase the power to resolve different prey sources. We compared predictions made with two-isotope carbon and nitrogen mixing models and three-isotope models that also included sulphur ( δ 34 S) for the diets of Lake Ontario lake trout ( Salvelinus namaycush ). We determined the isotopic compositions of lake trout and potential prey fishes sampled from Lake Ontario and then used quantitative estimates of resource use generated by two- and three-isotope Bayesian mixing models (SIAR) to infer feeding patterns of lake trout. Both two- and three-isotope models indicated that alewife ( Alosa pseudoharengus ) and round goby ( Neogobius melanostomus ) were the primary prey items, but the three-isotope models were more consistent with recent measures of prey fish abundances and lake trout diets. The lake trout sampled directly from the hatcheries had isotopic compositions derived from the hatchery food which were distinctively different from those derived from the natural prey sources. Those hatchery signals were retained for months after release, raising the possibility to distinguish hatchery-reared yearlings and similarly sized naturally reproduced lake trout based on isotopic compositions. Addition of a third-isotope resulted in mixing model results that confirmed round goby have become an important component of lake trout diet and may be overtaking alewife as a prey resource. [ABSTRACT FROM AUTHOR]

Lake Ontario investigators discussed and interpreted published and unpublished information during two workshops to assess our current understanding of Lake Ontario ecosystem function and to identify research needs to guide future research and monitoring activities. The purpose of this commentary is to summarize key investigative themes and hypotheses that emerged from the workshops. The outcomes of the workshop discussions are organized under four themes: spatial linkages and interactions, drivers of primary production, trophic transfer, and human interactions. [ABSTRACT FROM AUTHOR]

The Lake Ontario ecosystem has undergone substantial ecological change over the past five decades. In this time, an economically important sport fishery developed around non-native salmon and trout species (i.e., Chinook and coho salmon ( Oncorhynchus tshawytscha and Oncorhynchus kisutch ) and rainbow trout ( Oncorhynchus mykiss )). While trying to maintain this economically important recreational fishery, fishery managers are also trying to restore native species to the ecosystem (i.e., lake trout ( Salvelinus namaycush ) and Atlantic salmon ( Salmo salar )). We describe the trophic niche space of five ecologically and socioeconomically important Lake Ontario salmonid species (Chinook and coho salmon and rainbow, brown ( Salmo trutta ) and lake trout) using stable isotopes of carbon and nitrogen ( 13 C and 15 N, respectively). Using a modified standard ellipse analysis, we found a high degree of stable isotope niche space overlap in Lake Ontario salmonid species. Lake trout had the largest trophic niche space and the smallest proportion of overlap relative to the other four salmonid species (14%–28%), whereas coho salmon had the smallest stable isotope niche space and exhibited the highest degree of overlap with the other species (66%–99%). This study identifies and quantifies dietary resource sharing between Lake Ontario salmonids and highlights the importance of other prey fish species to the restoration and sustainability of Lake Ontario salmonid fish stocks. [ABSTRACT FROM AUTHOR]

The diet of first-feeding fish larvae can influence their future growth and survival. The foraging ecology of yellow perch (YP) larvae was studied in western Lake Erie, a region characterized by habitat heterogeneity associated with the Maumee River Plume (MRP) and a high prevalence of invasive species. To determine the influence of the MRP on YP diet, water physical characteristics, zooplankton prey community, and larval YP diet and foraging selectivity were compared between MRP and non-MRP waters, 2006-2008. Water temperature was higher, while water clarity was lower in the MRP when compared to non-MRP waters. The zooplankton prey community (groups constituting >1% ofYP prey items) did not differ between MRP and non-MRP waters, being composed of small cladocerans, cyclopoid and calanoid copepods and non-native dreissenid mussel veligers. Ration increased with YP total length (TL), with no differences found between MRP and non-MRP waters. Diet composition also varied with larval YP length, with the smallest larvae preying more heavily on dreissenid veligers than larger fish, and no differences between MRP and non-MRP waters. Most sizes of larvae positively selected for cyclopoid copepods in MRP and non-MRP waters, while selection for other prey was neutral or negative. Our study reveals a similar zooplankton community and larval foraging ecology between MRP and non-MRP waters during spring season, despite habitat physical differences, and points to mechanisms unassociated with larval foraging (i.e., predation) as a reason for higher recruitment of YP residing inside versus outside of the MRP as larvae. [ABSTRACT FROM AUTHOR]

Abstract: Pheromone trapping is an increasingly viable strategy to reduce invasive fish populations, largely due to the pheromones'' function of evoking behavioral responses among conspecifics. Prior to attempting such population control techniques, the pheromones must be identified and their possible influences on non-target species addressed. The round goby (Neogobius melanostomus) is a species invasive to the Great Lakes region, and negatively impacts the ecosystem by interfering with local fish populations. At least two 5β-reduced 3α-hydroxyl steroids released by reproductive N. melanostomus (11-O-ETIO and 11-O-ETIO-3s) evoke olfactory sensory responses from the olfactory epithelium of conspecifics, and water conditioned by reproductive males (containing these steroids) attracts female round gobies. In this study, we examined whether these putative pheromones, along with simultaneously-released 11-O-ETIO-17s, stimulate olfactory sensory responses from alternative fish species sharing the same ecosystem as N. melanostomus in the Great Lakes region. Rock bass (Ambloplites rupestris), bluegill sunfish (Lepomis macrochirus), pumpkinseed sunfish (Lepomis gibbosus), smallmouth bass (Micropterus dolomieu), and yellow perch (Perca flavescens) were the targets of an electro-olfactogram experiment designed to record responses to odors. When compared to round goby responses from previous studies, amino acids and the bile acid consistently elicited electro-olfactogram responses across all species, but only round gobies showed a response to the putative pheromones. This study supports the concept of conducting a pheromone trapping trial in the field without adversely affecting the olfactory responses of non-target fish in the area. [Copyright &y& Elsevier]

Abstract: Hemimysis anomala, a littoral freshwater mysid native to the Ponto-Caspian region, is the newest invader to the Laurentian Great Lakes basin. Discovered in 2006, they have since been found in all of the Great Lakes (except Lake Superior) and have the potential to offset the dietary energy sink caused by invasive dreissenid mussels (Dreissena bugensis and D. polymorpha) in the littoral zone. We evaluated nearshore food web structure at four sites along Lake Ontario''s north shore spanning a gradient of Hemimysis density to determine: 1) if dominant nearshore food web pathways change seasonally, and 2) whether fish are exhibiting a dietary shift towards consumption of Hemimysis. No Hemimysis were found in any of the 431 fish (alewife Alosa pseudoharengus, round goby Neogobius melanostomus, and yellow perch Perca flavescens) stomachs analysed. We used stable isotopes of carbon (13C) and nitrogen (15N) collected from invertebrates and fish to characterise trophic linkages and fish dietary preference. Yellow perch and round goby exhibited significantly higher Δδ 15N at Bronte (high Hemimysis density) compared to Cobourg, Waupoos and the Bay of Quinte. Δδ 13C of alewife is more enriched at Bronte and is comparable to the Δδ 13C of Hemimysis. Our results suggest that Hemimysis are being incorporated into diets of round gobies, alewife and small yellow perch and their reliance on Hemimysis as a dietary component increases with Hemimysis density. As Hemimysis populations continue to establish and stabilize, fish may incorporate this species into their diets at a higher rate. [Copyright &y& Elsevier]

Abstract: The invasive Ponto-Caspian mysid Hemimysis anomala (hereafter Hemimysis) was first observed in south-eastern Lake Ontario in May 2006. In Lake Ontario, Hemimysis is most abundant at nearshore habitats characterised by cobble–rubble substrate, which it shares with the round goby Neogobius melanostomus (hereafter goby). To better understand the interaction if any between gobies and Hemimysis, gobies were collected throughout the summer and fall periods at three shallow (<3m) nearshore locations representing eastern, central, and western Lake Ontario where Hemimysis are relatively abundant. Goby stomach contents were examined for evidence of Hemimysis and other prey. In addition, gobies were collected at two locations in southwestern Lake Ontario in shallow- (3m) and deep- (11m) water nearshore habitat where stomach contents were also used to characterise diet. Despite higher Hemimysis abundance nearshore, Hemimysis were not consumed by any gobies collected from shallow nearshore locations, and only found in three of 23 gobies collected at the deep site. Goby abundance and consumption of Hemimysis by gobies at the deep site were unrelated to Hemimysis abundance in traps. At all locations, gobies fed on soft-bodied invertebrates and dreissenids although dreissenids were the dominant prey among large (>74mm) gobies. We suspect that the ability of Hemimysis to avoid predation by gobies is the result of a combination of anti-predator behaviours such as an evasive startle response, reduced activity and depth dependent segregation in interstitial spaces during the day, and a feeding mode used by gobies that is poorly suited for an evasive species like Hemimysis. [Copyright &y& Elsevier]

Abstract: Hemimysis anomala is one of the latest macroinvertebrates to invade the Laurentian Great Lakes. Since first reported in 2006, Hemimysis have been confirmed in several locations within the Great Lakes basin. However, little is known about the seasonal and spatial variation in demographics and dynamics of Hemimysis populations. We used a standardised pier-based methodology to describe the distribution of Hemimysis at 29 locations around the shoreline of Lake Ontario in 2009. Samples were collected in spring, summer, and fall at most locations, and bi-weekly at one site (Bronte Creek) over a 12-month period in 2009. For each site, we estimated abundance by sex and size. The more temporally intensive sampling at Bronte Creek enabled us to estimate production. Hemimysis were found at 83% of the sites visited, with densities generally highest in the northwest and lower at the other sites. Production estimates (2.67–14.09mg dry weight·m−2·d−1) were higher than that of other common zooplankton species in the Great Lakes. We provide important life history parameters that will help ecologists better understand the potential impacts of Hemimysis on Great Lakes ecosystems. [Copyright &y& Elsevier]

Abstract: Freshwater organisms synthesize a wide variety of fatty acids (FAs); however, the ability to synthesize and/or subsequently modify a particular FA is not universal, making it possible to use certain FAs as biomarkers. Herein we document the occurrence of unusual FAs (polymethylene-interrupted fatty acids; PMI-FAs) in select freshwater organisms in the Laurentian Great Lakes. We did not detect PMI-FAs in: (a) natural seston from Lake Erie and Hamilton Harbor (Lake Ontario), (b) various species of laboratory-cultured algae including a green alga (Scenedesmus obliquus), two cyanobacteria (Aphanizomenon flos-aquae and Synechococystis sp.), two diatoms (Asterionella formosa, Diatoma elongatum) and a chrysophyte (Dinobryon cylindricum) or, (c) zooplankton (Daphnia spp., calanoid or cyclopoid copepods) from Lake Ontario, suggesting that PMI-FAs are not substantively incorporated into consumers at the phytoplankton–zooplankton interface. However, these unusual FAs comprised 4–6% of total fatty acids (on a dry tissue weight basis) of native fat mucket (Lampsilis siliquoidea) and plain pocketbook (L. cardium) mussels and in invasive zebra (Dreissena polymorpha) and quagga (D. bugensis) mussels. We were able to clearly partition Great Lakes'' mussels into three separate groups (zebra, quagga, and native mussels) based solely on their PMI-FA profiles. We also provide evidence for the trophic transfer of PMI-FAs from mussels to various fishes in Lakes Ontario and Michigan, further underlining the potential usefulness of PMI-FAs for tracking the dietary contribution of mollusks in food web and contaminant-fate studies. [Copyright &y& Elsevier]

Abstract: We compared round goby (Neogobius melanostomus) population structure and biomass, estimated the quantity of prey consumed by round goby and assessed predation on round goby in two areas of the Bay of Quinte (upper and lower bay) that were invaded by this species two years apart. A bioenergetics approach was used to estimate round goby consumption and to illustrate the influence of changes in the round goby biomass, from May to October in both areas of the bay. Mean round goby biomass was 5tonnes/km2 for the upper bay and 11.2tonnes/km2 for the lower bay. Round goby populations were dominated by age-0 to age-2 individuals; older individuals were rare although they constituted a greater portion of total biomass in the earlier invaded lower bay than the upper bay. Estimated round goby consumption was 78.4tonnes/km2 for the upper bay and 127.1tonnes/km2 for the lower bay; dreissenids were the most important prey followed by chironomids, zooplankton and amphipods. Diet analysis showed that round gobies were consumed by all predator species analyzed, and constituted a high percentage of stomach content biomass in yellow perch (Perca flavescens), largemouth bass (Micropterus salmoides) and northern pike (Esox lucius). Age structure differences between the upper and lower bay are consistent with those expected between expanding and stable populations. [ABSTRACT FROM AUTHOR]

Abstract: Although lake whitefish Coregonus clupeaformis populations in the Laurentian Great Lakes have rebounded remarkably from the low abundance levels of the 1960s and 1970s, recent declines in fish growth rates and body condition have raised concerns about the future sustainability of these populations. Because of the ecological, economic, and cultural importance of lake whitefish, a variety of research projects in the Great Lakes have recently been conducted to better understand how populations may be affected by reductions in growth and condition. Based upon our participation in projects intended to establish linkages between reductions in growth and condition and important population demographic attributes (natural mortality and recruitment potential), we offer the following recommendations for future studies meant to assess the health of Laurentian Great Lakes lake whitefish populations: (1) broaden the spatial coverage of comparative studies of demographic rates and fish health; (2) combine large-scale field studies with direct experimentation; (3) conduct multi-disciplinary evaluation of stocks; (4) conduct analyses at finer spatial and temporal scales; (5) quantify stock intermixing and examine how intermixing affects harvest policy performance on individual stocks; (6) examine the role of movement in explaining seasonal fluctuations of disease and pathogen infection and transmission; (7) evaluate sampling protocols for collecting individuals for pathological and compositional examination; (8) quantify sea lamprey-induced mortality; and (9) enact long-term monitoring programs of stock health. [Copyright &y& Elsevier]

Abstract: Recent declines in growth and condition of several Great Lakes lake whitefish populations have raised concerns over potential impacts on juvenile physiological condition and ultimately recruitment. To test whether the condition of spawning adults influences juvenile condition via energy allocation dynamics, we partitioned the variation in age 0 juvenile physiological condition (i.e., growth in length and weight, whole-body moisture content, energy density, and protein content) among adult male and female (i.e., body condition, muscle moisture content, energy density, and protein content) and egg (i.e., wet and dry weight, moisture content, energy density, energy content per egg, and protein content) effects using redundancy analysis. Overall, a model that included sampling site, female condition, and egg quality explained 39% of the variation in juvenile physiological condition. After partitioning out the effects of females and eggs, site explained the most variation (23%). When other factors were accounted for, neither females (1.4%) nor eggs (2.7%) explained much variation in juvenile physiological condition. Of the variables studied, female muscle energy density, muscle moisture content, and egg moisture content were most closely associated with juvenile physiological condition. Our results suggest that parental effects, such as size, age, body condition, or body composition, may not be as important as extrinsic site-related effects or density-dependent effects in determining juvenile physiological condition. [Copyright &y& Elsevier]

Abstract: We identified an objective set of 25 commonly available ecosystem metrics applicable across the world''s large continental freshwater and brackish aquatic ecosystem. These metrics measure trophic structure, exploited species, habitat alteration, and catchment changes. We used long-term trends in these metrics as indicators of perturbations that represent an ecosystem not in homeostasis. We defined a healthy ecosystem as being in a homeostatic state; therefore, ecosystems with many changing trends were defined as more disturbed than ecosystems with fewer changing trends. Healthy ecosystems (lakes Baikal, Superior, and Tanganyika) were large, deep lakes in relatively unpopulated areas with no signs of eutrophication and no changes to their trophic structure. Disturbed ecosystems (lakes Michigan, Ontario, and Victoria) had shallow to moderately deep basins with high watershed population pressure and intense agricultural and residential land use. Transitioning systems had widely varying trends and faced increasing anthropogenic pressures. Standardized methodologies for capturing data could improve our understanding of the current state of these ecosystems and allow for comparisons of the response of large aquatic ecosystems to local and global stressors thereby providing more reliable insights into future changes in ecosystem health. [Copyright &y& Elsevier]

Abstract: We assessed round goby (Apollonia melanostoma) density and size structure in two sections of the Bay of Quinte (Lake Ontario) that had been invaded by this species two years apart. Round goby density was assessed with 50 m linear transects, recorded with an underwater video recording apparatus developed for this study that included a depth sounder for maintaining a fixed distance above the substrate. The highest mean round goby densities were observed in the shallowest depth zone (1.5–3 m) at both sites, but there were differences between the sites in the habitat types where the highest densities occurred and there were no significant density differences among habitat types at either site (rock with sparse vegetation, mud with sparse vegetation, sand/mud with moderate vegetation cover). In the upper bay, mean body length of round gobies declined with depth, whereas in the lower bay, mean round goby length was greatest in the deepest zone. Mean body length of round gobies did not differ significantly by habitat type in either section of the bay. [Copyright &y& Elsevier]

Patterns in abundance, growth, and condition of lake whitefish (Coregonus clupeaformis) from Lake Erie were compared with those from Lake Ontario. Discontinuous assessment data were available from 1972 to 2003 for each lake to describe abundance, growth, and condition, while a seasonally intensive field program was undertaken in 2003 to describe diet, energy density, and gonadosomatic index (GSI). Through time, abundance declined more in Lake Ontario than in Lake Erie. Length-at-age (growth) and condition both declined significantly in Lake Ontario but did not change in Lake Erie. Diet analysis revealed chironomids, dreissenid mussels and sphaeriids made up the bulk of lake whitefish diet in Lake Erie. Diet in Lake Ontario exhibited more seasonal variability with amphipods and gastropods comprising the bulk of the spring and fall diets, and dreissenid mussels dominating summer diets. Lake whitefish energy density (J/g wet mass) was significantly higher in Lake Erie than in Lake Ontario, increasing with body mass and strongly correlated with water content. Female gonadosomatic index was also significantly greater in Lake Erie than in Lake Ontario. Biological attributes of lake whitefish from Lake Erie did not change greatly from the late 1980s to 2003 while fish from Lake Ontario exhibited decreased size-at-age and condition likely due to decreased energy content of diets compared to predreissenid mussel conditions, and possibly lower availability of benthic prey compared to Lake Erie. [ABSTRACT FROM AUTHOR]

Burbot (Lota lota), are native Lake Superior piscivores that share similar habitat and food resources with other predators including lean and siscowet lake trout (Salvelinus namaycush). To better understand their ecological role in the fish community, we combined fisheries assessment information from 1970 to 2002 with a bioenergetics model to estimate their predatory impact in the Apostle Islands region of Lake Superior. Relative abundance declined in the Apostle Islands region from 3.41 fish/1,000 m in 1978 to 0.27 fish/1,000 m in 1998. Fishing mortality was minimal based on creel estimates and observed bycatch in the commercial fishery. Burbot < 400 mm consumed a higher fraction of small prey items such as Mysis relicta, fish eggs and sculpins (Cottidae) while larger burbot were almost exclusively piscivorous. Overall diet composition (by mass) was represented primarily by Coregonus spp. (64%) and rainbow smelt (Osmerus mordax) (17%). We estimated the burbot population size in the Apostle Islands between 1970 and 2000 at 56,541 to 1,585,035 age 1 + fish. This population consumed between 56 and 1,584 metric tonnes (0.13 to 3.54 kg/ha) of prey. Increases in both lean and siscowet lake trout abundance have likely contributed to the decline in burbot abundance through predation and/or competition for food resources. Given the current burbot population in the Apostle Islands, this species is unlikely to control production of prey fish or invertebrate taxa. [ABSTRACT FROM AUTHOR]

Round gobies, invasive fish that entered Lake Erie in 1994, are altering energy, contaminant, and nutrient pathways. Our objective was to quantify how they alter energy pathways in the central basin of Lake Erie by describing their diet and identifying the degree to which predatory fish feed upon round gobies. We used bioenergetic models parameterized with data collected in the central basin between 1995 and 2002 to estimate the type and amount of prey eaten, the biomass accumulation rate for the round goby population, and a partitioning of the food energy into ‘new’ energy derived from dreissenids as opposed to existing energy derived from zooplankton and non-dreissenid benthic prey. Mean (± SE) prey consumption peaked at 5.98 ± 2.17 × 104 tonnes wet mass in 1999 coincident with the maid- mum population size of 4.2 ± 1.5 billion round gobies. Zooplankton (40.2% by biomass) and dreissenid mussels (38.3%) dominated the prey consumed. Almost 90% of the zooplankton biomass was consumed by age-O round gobies, while over 80% of the dreissenids were eaten by older ages. Standing stock biomass of round gobies ranged between 203 and 4,803 tonnes y-¹ (interannual range), with an additional 475 to 8,943 tonnes of biomass accumulating through growth each year. Piscivorous fish showed an increasing reliance on round gobies as prey, with round gobies being the dominant prey fish in the diets of benthic-oriented predators. Hence, by being one of the few benthivores that exploit dreissenid mussels as prey, our analyses reveal that round gobies transfer new energy up the central Lake Erie food web. [ABSTRACT FROM AUTHOR]

Lake herring (Coregonus artedi) and rainbow smelt (Osmerus mordax) are a valuable prey resource for the recovering lake trout (Salvelinus namaycush) in Lake Superior. However, prey biomass may be insufficient to support the current predator demand. In August 1997, we assessed the abundance and spatial distribution of pelagic core gonines and rainbow smelt in western Lake Superior by combining a 120 kHz split beam acoustics system with midwater trawls. Core gonines comprised the majority of the midwater trawl catches and the length distributions for trawl caught fish coincided with estimated sizes of acoustic targets. Overall mean pelagic prey fish biomass was 15.56 kg ha-1 with the greatest fish biomass occurring in the Apostle Is lands region (27.98 kg ha-1), followed by the Duluth Minnesota region (20.22 kg ha-1), and with the lowest biomass occurring in the open waters of western Lake Superior (9.46 kg ha-1). Biomass estimates from hydroacoustics were typically 2-134 times greater than estimates derived from spring bottom trawl surveys. Prey fish biomass for Lake Superior is about order of magnitude less than acoustic estimates for Lakes Michigan and Ontario. Discrepancies observed between bioenergetics-based estimates of predator consumption of core gonines and earlier coregonine biomass estimates may be accounted for by our hydroacoustic estimates. [ABSTRACT FROM AUTHOR]

The round goby (Neogobius melanostomus) was introduced into the Great Lakes in the 1990s through ballast water transfer. Gobies are potential vectors for material transfer between the benthic and pelagic zones. A bioenergetics model was developed for round gobies to enable us to quantify the flow of energy, contaminants, and nutrients from the benthos to pelagic fishes. Weight-and temperature-dependent coefficients for metabolism and consumption were derived. Food consumption increased with temperature up to 26°C before sharply decreasing, and weight-specific consumption decreased with increasing fish weight (allometric coefficient = -0.256, SE = 0.160). Specific oxygen consumption was inversely related to body mass (allometric coefficient = -0.157, SE = 0.025) and increased exponentially with temperature. Estimated Q10 for respiration was 1.84. Additional parameters were obtained from the literature to describe specific dynamic action, egestion, excretion, and reproductive losses. The bioenergetics model explained growth of round gobies in Lake Erie and in their native range. [ABSTRACT FROM AUTHOR]

The round goby (Neogobius melanostomus) is a small, demersal fish that was introduced into the Great Lakes basin in 1990. Since their arrival, the round goby has been implicated in many ecological changes--most notably changes in the flow of energy from the benthic to the pelagic food web through their consumption of dreissenid mussels. However, methods for evaluating the density and size of round gobies across different substrates are lacking, preventing the true quantification of the effects of round gobies on invaded ecosystems. In our study, we evaluated catch efficiency of numerous passive and active sampling methods for capturing round gobies. We then applied the best techniques to estimate the distribution, density, and biomass of round gobies in western Lake Erie. Visual census (underwater video transects) proved the best technique for assessing round goby size and density across a wide range of substrates. A combination of angling and bottom trawling proved most effective for obtaining biological samples. We estimated 9.9 billion round gobies in western Lake Erie in 2002. Continued efforts to describe abundance and demographics of round gobies in invaded ecosystems will enable scientists and managers to fully understand the impacts of this invading species. [ABSTRACT FROM AUTHOR]

We assessed abundance, size, and species composition of forage fish and zooplankton communities of western Lake Superior during August 1996 and July 1997. Data were analyzed for three ecoregions (Duluth-Superior, Apostle Islands, and the open lake) differing in bathymetry and limnological and biological patterns. Zooplankton abundance was three times higher in the Duluth-Superior and Apostle Islands regions than in the open lake due to the large numbers of rotifers. Copepods were far more abundant than Cladocera in all ecoregions. Mean zooplankton size was larger in the open lake due to dominance by large calanoid copepods although size of individual taxa was similar among ecoregions. Forage fish abundance and biomass was highest in the Apostle Islands region and lowest in the open lake ecoregion. Lake herring (Coregonus artedi), rainbow smelt (Osmerus mordax) and deepwater ciscoes (Coregonus spp.) comprised over 90% of the abundance and biomass of fishes caught in midwater trawls and recorded with hydroacoustics. Growth and condition offish was good, suggesting they were not resource limited. Fish and zooplankton assemblages differed among the three ecoregions of western Lake Superior, due to a combination of physical and limnological factors related to bathymetry and landscape position. [ABSTRACT FROM AUTHOR]

We describe the diets of lake herring (Coregonus artedi) and rainbow smelt (Osmerus mordax) in western Lake Superior during the summers of 1996 and 1997. Both species consumed predominantly (> 71% by number) zooplankton, showing a preference for larger taxa. Diet overlap between the two species was low (Schoener's index = 0.42). Mysis was most important in rainbow smelt diets, whereas Diaptomus sicilis was most important in lake herring diets. Rainbow smelt selected larger taxa, and larger individuals within a taxon when compared to lake herring, although rainbow smelt tended to be smaller fish. Fish diets have changed relative to previous studies and may be reflecting changes in the zooplankton community. Continued changes in the fish and zooplankton community will alter predatorprey and energetic pathways, ultimately affecting growth and production of the ecosystem. [ABSTRACT FROM AUTHOR]