The effects of dreissenid mussels on the survival and condition of burrowing mayflies (Hexagenia spp.) in western Lake Erie

Burrowing mayflies (Hexagenia limbata and H. rigida) are once again prominent members of the benthic community in western Lake Erie. However, this community is now dominated by dreissenid mussels. We conducted a laboratory experiment and field sampling to investigate whether survival and condition of Hexagenia were affected by the presence, density, and quality of dreissenid mussels. In a laboratory experiment, Hexagenia survival was higher in microcosms without dreissenid mussels. We also found Hexagenia density to be higher at field sites with low dreissenid density, suggesting that Hexagenia survival is higher in these areas as well. In microcosm treatments with low dreissenid density, Hexagenia survival was higher in treatments with live dreissenids than in treatments containing only dreissenid shells. These findings suggest that while dreissenid shells degrade the quality of soft sediments for Hexagenia, some of the negative effect is offset by the presence of live dreissenids. The positive effect of live dreissenids is likely due to additional food resources made available to Hexagenia by the filtering activity of dreissenids. Neither dreissenid density nor shell “type” (shells alone or live dreissenids in shells) had an effect on Hexagenia condition. Thus, the interactions between these dominant benthic invertebrates are complex. Recovery of Hexagenia populations in western Lake Erie is likely affected by both changing environmental conditions due to anthropogenic activities and the introduction of exotic species into the benthic community. The results are likely to be continued instability of the benthic food web and unpredictable consequences for human utilization of this ecosystem.     

Quantifying a shift in benthic dominance from zebra (Dreissena polymorpha) to quagga (Dreissena rostriformis bugensis) mussels in a large,inland lake

Dreissenid mussels are aggressive invasive species that are continuing to spread across North America and co-occur in the same waterbodies with increasing frequency, yet the outcome and implications of this competition are poorly resolved. In 2009 and 2015, detailed (700 + sample sites) surveys were undertaken to assess the impacts of invasive dreissenid mussels in Lake Simcoe (Ontario, Canada). In 2009, zebra mussels were dominant, accounting for 84.3% of invasive mussel biomass recorded. In 2015, quagga mussels dominated (88.5% of invasive mussel biomass) and had expanded into profundal (> 20 m water depth) sites and onto soft (mud/silt) substrates with a mean profundal density of 887 mussels/m² (2015) compared to ~ 39 mussels/m² in 2009. Based on our annual benthos monitoring, at a subset of ~ 30 sites, this shift from zebra to quagga mussels occurred ~ 2010 and is likely related to a population decline of zebra mussels in waterbodies where both species are present, as recorded elsewhere in the Great Lakes Region. As the initial invasion of dreissenid mussels caused widespread ecological changes in Lake Simcoe, we are currently investigating the effects this change in species dominance, and their expansion into the profundal zone, will have on the lake; and our environmental management strategies. Areas of future study will include: changes in the composition of benthos, fish, or phytoplankton communities; increased water clarity and reduction of the spring phytoplankton bloom; energy/nutrient cycling; and fouling of anthropogenic in-lake infrastructures (e.g. water treatment intakes) built at depths > 25 m to avoid previous zebra mussel colonization.     

Vulnerability of Dreissena polymorpha Larvae to Predation by Great Lakes Calanoid Copepods: the Importance of the Bivalve Shell

Dreissena polymorpha larvae were vulnerable to predation by three different species of calanoid copepods, Diaptomus sicilis, Limnocalanus macrurus, and Epischura lacustris, when presented to these copepods in bottle experiments. The degree of vulnerability was dependent upon the stage of the larva and the type of predator: trochophore larvae (without shells) were much more vulnerable than D-stage larvae (with shells). D. sicilis and L. macrurus were offered algae as alternate food, and each cleared trochophore larvae at a higher rate than algae. However, the clearance rate for D. sicilis feeding on D-stage larvae was not significantly different from zero, suggesting that this suspension-feeding omnivore-herbivore was not able to ingest D-stage larvae. Of the three species, the large cruising predator, L. macrurus, had the highest clearance rate for trochophore larvae (55.8 mL • animal⁻¹ • d⁻¹), but had a significantly lower clearance rate for D-stage larvae, only one eighth of that for trochophores. The smaller predator, E. lacustris, was more adept than L. macrurus or D. sicilis at preying on D-stage larvae: its clearance rate for D-stage larvae (17.9 mL • animal-1 • d⁻¹) was about one half of its clearance rate for trochophore larvae. Since bivalve larvae, including Dreissena, and copepods co-occur in many aquatic environments, our results suggest that copepod predation may have been a selective force for production of a protective shell early in the larval development of bivalves.     

Dreissenid colonization during the initial invasion of the quagga mussel in the largest Central European shallow lake,Lake Balaton,Hungary

The colonization progress of the invasive bivalve dreissenids, the formerly dominant Dreissena polymorpha and the recently (2008) introduced Dreissena rostriformis bugensis was studied between 2009 and 2013 in the largest Central European shallow lake, Lake Balaton, Hungary. The density of dreissenid planktonic veligers, new settlers (post-veligers and early juveniles), and the population structure (density, length frequency, relative abundance) of the two species were monitored on experimentally introduced natural stone substrata, on different time scales. Dreissenids started dynamic settling following a sudden veliger bloom. As substratum saturation progressed, competition between species for places was suggested, which, after two years, led to an increased number of large individuals (> 20 mm) and also recruits of D. r. bugensis. By contrast, the population of D. polymorpha was confined to middle size (11–18 mm) individuals of the first settler generation. On local substrata, where the benthic community was already established, the replacement of D. polymorpha by D. r. bugensis took longer, but it happened in a similar way. The invasion speed of D. r. bugensis in Lake Balaton resembled the speed obtained in other European water bodies where D. r. bugensis, similar to Lake Balaton, was introduced much later than D. polymorpha. However, a longer replacement process was found in North America, where both species invaded new habitats at the same time. This suggests that the speed, and probably the success, of D. r. bugensis invasion depends on new surface availability, and whether the two dreissenid species are introduced together or at different times.     

An alternative hypothesis to invasional meltdown in the Laurentian Great Lakes region: General facilitation by Dreissena

Invasional meltdown, where established non-indigenous species facilitate the establishment and spread of newly arriving non-indigenous species, may contribute to the increasing rate of biological invasions. The Laurentian Great Lakes have been used as an example of invasional meltdown, but our results suggest that this may not be the case. We propose that the increased numbers of facilitative interactions are not due to an invasion meltdown, but rather a strongly interacting species, such as Dreissena, promoting population level changes in both native and non-indigenous species. Dreissena are the facilitator in the majority of reported facilitations of non-indigenous species, and those non-indigenous species have not yet led to more invasions. Further, our results show that Dreissena facilitate non-indigenous and native species similarly. Literature reviews showed little evidence that Dreissena facilitate fish or are facilitated by phytoplankton. Consequently, the observed pattern of species interactions in the Great Lakes does not conform to the definition of invasional meltdown. We suggest that Dreissena cause strong interactions and change the benthos in a way that facilitates many organisms (native and non-indigenous), but that system-wide invasional meltdown is not occurring in the Great Lakes.     

Polymethylene-interrupted fatty acids: Biomarkers for native and exotic mussels in the Laurentian Great Lakes

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.     

Signs of the times: Isotopic signature changes in several fish species following invasion of Lake Constance by quagga mussels

Since the arrival of the invasive quagga mussels Dreissena rostriformis bugensis in Lake Constance, significant changes in the zooplankton and benthic invertebrate community were observed. Five years later the quagga mussel has become the dominating species of the benthic community. Its effects on other components of the food web, especially those at higher trophic levels such as fish, remain unclear around the world. To evaluate the actual impact of quaggas on the local food web of Lake Constance, the stable isotope compositions of pelagic whitefish and different benthic fish species from before and after the quagga invasion were compared. A significant increase in δ¹³C was detected in pelagic whitefish one year after the establishment of the quagga mussel in the lake. This change was most likely the consequence of an increase in benthic-derived nearshore primary production and a shift towards more littoral feeding, than a change in dietary composition. Stomach content analysis of contemporary samples revealed that pelagic whitefish (Coregonus wartmanni) still feed exclusively on pelagic zooplankton. In contrast, benthic whitefish (Coregonus macrophthalmus), roach (Rutilus rutilus) and tench (Tinca tinca) show today high levels of quagga consumption. However, this behaviour alone could not explain the observed differences in δ¹⁵N from periods before and after the quagga invasion. The results suggest that energy sources and pathways have changed considerably for both pelagic and benthic dwelling fish species in Lake Constance following the establishment of quaggas.     

Circular Rep encoding single stranded (CRESS) DNA virus-like sequences detected in quagga mussels (Dreissena rostriformis bugensis) and sediments from the central Lake Michigan benthos

Circular Rep Encoding Single Stranded (CRESS) DNA viruses are a diverse group of viruses that have been identified in both terrestrial and aquatic ecosystems. Recent work in the Laurentian Great Lakes characterized the ecology and diversity of CRESS-DNA viruses associated with amphipods, Diporeia spp. In the last 20 years the Lake Michigan benthos has changed considerably with drastic population declines of Diporeia spp. concurrent with an increase in invasive quagga mussel (Dreissena rostriformis bugensis) abundance. The purpose of this study was to characterize CRESS-DNA virus-like elements (VLES; which could represent complete, partial/defective, endogenized or satellite viruses) associated with both invasive quagga mussels and in sediment collected in central Lake Michigan. Viral metagenomic libraries were prepared from two size classes (>25 mm and <15 mm shell length) of Lake Michigan quagga mussels and for two different sediment layers (136 mm and 290 mm below the lake floor) in a sediment core extracted from the lake. Viral metagenomes were different between quagga mussels and sediment cores. Nine VLE sequences were present in both the quagga mussel tissues and the sediment core layers analyzed. Cs¹³⁷ radiometric dating results indicate that these VLEs were present in the sediment prior to arrival of quagga mussels in Lake Michigan. These data suggest quagga mussels may interact with CRESS-DNA VLEs and algal DNA VLEs historically present in the Lake Michigan benthos. Overall, these data suggest that quagga mussels interacted with CRESS-DNA VLES present in Lake Michigan benthos since at least 1952.     

Thiamine concentrations in lake whitefish eggs from the upper Great Lakes are related to maternal diet

Thiamine deficiency is responsible for reproductive impairment in several species of salmonines in the Great lakes, and is thought to be caused by the consumption of prey containing thiaminase, a thiamine-degrading enzyme. Because thiaminase levels are extremely high in dreissenid mussels, fish that prey on them may be susceptible to thiamine deficiency. We determined thiamine concentrations in lake whitefish Coregonus clupeaformis eggs from the upper Laurentian Great Lakes to assess the potential for thiamine deficiency and to determine if thiamine concentrations in lake whitefish eggs were related to maternal diet. Mean thiamine concentrations in lake whitefish eggs were highest in Lake Huron, intermediate in Lake Superior, and lowest in Lake Michigan. Some fish had thiamine concentrations below putative thresholds for lethal and sublethal effects in salmonines, suggesting that some larval lake whitefish may currently be at risk of at least sublethal effects of low thiamine concentrations, although thiamine thresholds are unknown for lake whitefish. Egg thiamine concentrations in lake whitefish eggs were statistically significantly related to isotopic carbon signatures, suggesting that egg thiamine levels were related to maternal diet, but low egg thiamine concentrations did not appear to be associated with a diet of dreissenids. Egg thiamine concentrations were not statistically significantly related to multifunction oxidase induction, suggesting that lower egg thiamine concentrations in lake whitefish were not related to contaminant exposure.     

The flathead catfish invasion of the Great Lakes

A detailed review of historical literature and museum data revealed that flathead catfish were not historically native in the Great Lakes Basin, with the possible exception of a relict population in Lake Erie. The species has invaded Lake Erie, Lake St. Clair, Lake Huron, nearly all drainages in Michigan, and the Fox/Wolf and Milwaukee drainages in Wisconsin. They have not been collected from Lake Superior yet, and the temperature suitability of that lake is questionable. Flathead catfish have been stocked sparingly in the Great Lakes and is not the mechanism responsible for their spread. A stocking in 1968 in Ohio may be one exception to this. Dispersal resulted from both natural range expansions and unauthorized introductions. The invasion is ongoing, with the species invading both from the east and the west to meet in northern Lake Michigan. Much of this invasion has likely taken place since the 1990s. This species has been documented to have significant impacts on native fishes in other areas where it has been introduced; therefore, educating the public not to release them into new waters is important. Frequent monitoring of rivers and lakes for the presence of this species would detect new populations early so that management actions could be utilized on new populations if desired.     

Quagga mussels (Dreissena bugensis) as biomonitors of metal contamination: A case study in the upper St. Lawrence River

In this study, the utility of quagga mussels (Dreissena bugensis) as biomonitors was investigated by measuring total concentrations of three trace metals, cadmium, copper, and zinc, in soft tissues. Quagga mussels were sampled from five sites along the upper St. Lawrence River, including one industrially influenced site, from 1999 through 2007. Mussels were collected from near-shore areas, divided into 5 size classes based on maximum shell length, and tissues were pooled for analysis of each size group. Two-way analysis of variance and a posteriori range tests were used to test for differences among sites along a distance gradient from the outflow of Lake Ontario and to examine inter-annual variability within and among sites. Cadmium concentrations were higher nearer the outflow of the lake. Copper concentrations varied among sites and years, but were generally highest near the industrial site. Zinc concentrations were relatively uniform, possibly reflecting internal regulation. Animal size measured as shell length was not an important factor in this section of the river, but warrants further consideration in a wider range of ecosystems and contaminant exposure levels. In general, concentrations of the three metals were not high compared to reports in the published literature for dreissenid mussels in contaminated environments. However, few studies have utilized quagga mussels rather than zebra mussels. The two species may differ in bioaccumulation patterns and may not be interchangeable as biomonitors. Further studies of bioaccumulation of contaminants by quagga mussels in a wider range of contaminant exposures would be useful particularly as quagga mussels displace zebra mussels in the Laurentian Great Lakes and the St. Lawrence River.     

Additional deepwater sampling of prey fish in Lake Michigan annual bottom trawl survey reveals new insights for depth distribution dynamics

Beginning in 2013, sites at the 128-m bottom depth contour were added to the sampling design of the annual Lake Michigan bottom trawl survey for prey fish, which has been conducted by the U.S. Geological Survey Great Lakes Science Center (GLSC) each fall since 1973, to better assess fish depth distributions in a changing ecosystem. The standard sampling design included bottom depths from 9 to 110 m, although the GLSC also sporadically conducted bottom trawl tows at the 128-m bottom depth contour during 1973–1988. Enactment of this new sampling design in 2013 revealed that mean biomass density of deepwater sculpins (Myoxocephalus thompsonii) at the 128-m depth exceeded the sum of mean biomass densities at shallower depths, indicating that the bulk of the deepwater sculpin population is residing in waters deeper than 110 m. Thus, our findings supported the hypothesis that the depth distribution of the deepwater sculpin population had shifted to deeper waters beginning in 2007, thereby explaining, at least in part, the marked decline in deepwater sculpin abundance since 2006 based on the standard sampling design. In contrast, our results did not support the hypothesis that the slimy sculpin (Cottus cognatus) population had shifted to deeper waters sometime after 2000. A portion of the burbot (Lota lota) population may have also shifted in depth distribution to waters deeper than 110 m after 2007, based on our results. Our findings have served as an impetus to further expand the range of depths sampled in our bottom trawl survey.     

Association of aerobic anoxygenic phototrophs and zebra mussels,Dreissena polymorpha,within the littoral zone of Lake Winnipeg

While nutrient loading has affected all levels of Lake Winnipeg’s ecology, its greatest influence has likely been on the microbial community. In addition to eutrophication, zebra mussels (Dreissena polymorpha) have recently invaded the ecosystem and threaten food web dynamics. Their filter-feeding predation and association with bacteria, specifically phototrophs, was investigated. A sampling trip to Lake Winnipeg in October 2017, focused on the isolation, enumeration, and identification of aerobic anoxygenic phototrophs in littoral water, sediment, and tissues of mussels. Gimli, Patricia, and Grand beaches, separated by >15 km across the South Basin, had similar bacterial counts when cultivated on rich organic, BG-11, purple non-sulphur, and K₂TeO₃-supplemented media. Culture-based enumeration on rich organic medium revealed 1.74% of heterotrophs from littoral waters were aerobic anoxygenic phototrophs, and represented 13.98% within sediments. In contrast, 0.48, 1.15, and 0.16% of cultured heterotrophs were aerobic anoxygenic phototrophs within zebra mussel gill, gut, and gonadal tissues, respectively. High-throughput sequencing of bacterial 16S V4 rDNA maintained similar trends in respective bivalve organs, where 0.22, 1.13, and 0.20% of total 16S genes belonged to these phototrophs. Several Sphingomonadaceae isolates were recovered from gut tissues, all with filamentous morphology large enough for predation. Bioaccumulation of metals was also studied in D. polymorpha. All tested associated aerobic anoxygenic phototrophs were capable of resisting the metalloid oxide tellurite. The consistent distribution of aerobic anoxygenic phototrophs within microbial communities across Lake Winnipeg, and their predominance in the gut tissues of zebra mussels suggested bacterial consumption by this invasive species.     

Comparison of thiaminase activity in fish using the radiometric and 4-nitrothiophenol colorimetric methods

Thiaminase induced thiamine deficiency occurs in fish, humans, livestock and wild animals. A non-radioactive thiaminase assay was described in 2007, but a direct comparison with the radioactive ¹⁴C-thiamine method which has been in use for more than 30 years has not been reported. The objective was to measure thiaminase activity in forage fish (alewife Alosa pseudoharengus, rainbow smelt Osmerus mordax, and slimy sculpin Cottus cognatus) consumed by predators that manifest thiamine deficiency using both methods. Modifications were made to the colorimetric assay to improve repeatability. Modification included a change in assay pH, enhanced sample clean-up, constant assay temperature (37 °C), increase in the concentration of 4-nitrothiophenol (4NTP) and use of a spectrophotometer fitted with a 0.2 cm cell. A strong relationship between the two assays was found for 51 alewife (R² = 0.85), 36 smelt (R² = 0.87) and 20 sculpin (R² = 0.82). Thiaminase activity in the colorimetric assay was about 1000 times higher than activity measured by the radioactive method. Application of the assay to fish species from which no thiaminase activity has previously been reported resulted in no 4NTP thiaminase activity being found in bloater Coregonus hoyi, lake trout Salvelinus namaycusch, steelhead trout Oncorhynchus mykiss or Chinook salmon Oncorhynchus tshawytscha. In species previously reported to contain thiaminase, 4NTP thiaminase activity was measured in bacteria Paenibacillus thiaminolyticus, gizzard shad Dorosoma cepedianum, bracken fern Pteridium aquilinum, quagga mussel Dreissena bugensis and zebra mussels D. polymorpha.     

Slimy sculpin depth shifts and habitat squeeze following the round goby invasion in the Laurentian Great Lakes

The collapse of Diporeia spp. and invasions of dreissenid mussels (zebra, Dreissena polymorpha; quagga, D. bugensis) and round goby (Neogobius melanostomus) have been associated with declines in abundance of native benthic fishes in the Great Lakes, including historically abundant slimy sculpin (Cottus cognatus). We hypothesized that as round goby colonized deeper habitat, slimy sculpin avoided habitat competition, predation, and aggression from round goby by shifting to deeper habitat. Accordingly, we predicted increased depth overlap of slimy sculpin with both round goby and deepwater sculpin (Myoxocephalus thompsonii) that resulted in habitat squeeze by both species. We used long-term bottom trawl data from Lakes Michigan, Huron, and Ontario to evaluate shifts in slimy sculpin depth and their depth overlap with round goby and deepwater sculpin. Lake Huron most supported our hypotheses as slimy sculpin shifted to deeper habitat coincident with the round goby invasion, and depth overlap between slimy sculpin and both species recently increased. Slimy sculpin depth trends in Lakes Michigan and Ontario suggest other ecological and environmental factors better predicted sculpin depth in these lakes.     

Updated invasion risk assessment for Ponto-Caspian fishes to the Great Lakes

Majority of invasive species discovered in the Great Lakes since 1985 are native to the Ponto-Caspian region, including species that have had strong negative impacts in the Great Lakes (for example, dreissenid mussels and the round goby). The rich biota of the Ponto-Caspian region coupled with a high volume of commercial shipping traffic strongly suggests that this region will continue to be a major source of invasive species to the Great Lakes. To assess invasion risk by Ponto-Caspian fishes that have not been included in previous studies, we reviewed English-language publications and untranslated European literature (published primarily in Russian), focusing on physiological and ecological traits that have proven useful in previous risk assessments. We then used discriminant analysis to identify fishes that had a high probability of becoming established, spreading, and having significant negative impacts in the Great Lakes. Our updated listing of high-risk Ponto-Caspian fishes includes five species identified previously (the Black and Caspian Sea sprat, Eurasian minnow, big-scale sand smelt, European perch, and monkey goby) and five additional species (the Black sea shad, Caspian tyulka, Volga dwarf goby, Caspian bighead goby, and black-striped pipefish). Of these ten species, four (the monkey goby, big-scale sand smelt, Caspian tyulka, and black-striped pipefish) are likely to survive ballast water exchange as eggs, larvae, or adults based on salinity tolerances. Our results can be used to focus ongoing surveillance and rapid response efforts by highlighting Ponto-Caspian fishes that pose the greatest risk of becoming established and having significant negative impacts in the Great Lakes.     

Monitoring metal and persistent organic contaminant trends through time using quagga mussels (Dreissena bugensis) collected from the Niagara River

Historically, the Niagara River received the discharge of persistent bioaccumulative and toxic chemicals from municipal and industrial outfalls and hazardous waste landfills. American and Canadian governments have coordinated investigations of chemicals entering the river and initiated remedial measures and monitoring programs with a goal to reduce loadings of toxic chemicals to the river. This study, a component of the Ontario Ministry of Environment Mussel Biomonitoring Program, compares contaminant concentrations in quagga mussels (Dreissena bugensis) collected from nine locations in the Niagara River in 1995 and 2003 to assess anticipated changes in tissue concentrations of contaminants in response to ongoing remedial efforts by government agencies and local industries. The concentrations of persistent organic compounds (e.g., PCBs, hexachlorobenzene, hexachlorobutadiene, octachlorostyrene) in quagga mussels in 2003 were lower than concentrations measured in 1995, consistent with a decrease in reported mean annual concentrations of these compounds in water. Significant differences in total PCB concentrations in mussels between stations (F = 4.6; P < 0.001) suggested sources of PCBs on the American side of the upper Niagara River. In general, highest concentrations of persistent organic compounds were found downstream of the Occidental Chemical Corporation Buffalo Avenue facility suggesting local sources of these contaminants notwithstanding remedial efforts. In contrast, metal concentrations in quagga mussels in 2003 were similar to concentrations found in 1995 and to values reported in the literature for mussels collected from industrialized areas in the Great Lakes. Overall, our results suggest that remedial efforts to improve water quality in the Niagara River have been successful.     

First evidence of grass carp recruitment in the Great Lakes Basin

We use aging techniques, ploidy analysis, and otolith microchemistry to assess whether four grass carp Ctenopharyngodon idella captured from the Sandusky River, Ohio were the result of natural reproduction within the Lake Erie Basin. All four fish were of age 1 +. Multiple lines of evidence indicate that these fish were not aquaculture-reared and that they were most likely the result of successful reproduction in the Sandusky River. First, at least two of the fish were diploid; diploid grass carp cannot legally be released in the Great Lakes Basin. Second, strontium:calcium (Sr:Ca) ratios were elevated in all four grass carp from the Sandusky River, with elevated Sr:Ca ratios throughout the otolith transect, compared to grass carp from Missouri and Arkansas ponds. This reflects the high Sr:Ca ratio of the Sandusky River, and indicates that these fish lived in a high-strontium environment throughout their entire lives. Third, Sandusky River fish were higher in Sr:Ca ratio variability than fish from ponds, reflecting the high but spatially and temporally variable strontium concentrations of southwestern Lake Erie tributaries, and not the stable environment of pond aquaculture. Fourth, Sr:Ca ratios in the grass carp from the Sandusky River were lower in their 2011 growth increment (a high water year) than the 2012 growth increment (a low water year), reflecting the observed inverse relationship between discharge and strontium concentration in these rivers. We conclude that these four grass carp captured from the Sandusky River are most likely the result of natural reproduction within the Lake Erie Basin.     

Investigating population genetics of invasive rainbow smelt in the Great Lakes Region

Increasing our understanding of invasive species is important because of the negative impacts they can have on the economies and ecosystems of invaded regions. There is growing interest in how environmental variability (e.g. temperature) and stochastic invasion events (e.g. founder effects) affect the genetic composition of populations of invasive species. Rainbow smelt (Osmerus mordax) are a cold-water, planktivorous fish that spread into the Great Lakes basin in the early 1900s. We performed genetic analyses using microsatellites (N = 10) to investigate the influence stochastic invasion events have had on the genetic composition of invasive rainbow smelt populations across a broad geographic range. Genetic analyses were conducted on rainbow smelt populations (N = 30/population) from Lake Ontario, Lake Michigan, Lake Superior, and four inland lakes in Northern Wisconsin. Populations from the Great Lakes were generally less differentiated than inland populations. Additionally, we found evidence of a significant bottleneck in two inland populations and evidence for two distinct genetic strains of rainbow smelt in Lake Ontario. We also performed genetic analyses using microsatellites to determine if a thermally-induced extreme mortality event had an effect on a population of rainbow smelt and found that there was no measurable genetic effect on the population. Overall, this study provides evidence that the genetic structure and diversity of introduced populations can vary significantly, and are likely influenced by factors such as the frequency and magnitude of introductions. Also the resiliency of an invasive species can be high despite a history of bottlenecks and low genetic diversity.     

Influences on Bythotrephes longimanus life-history characteristics in the Great Lakes

We compared Bythotrephes population demographics and dynamics to predator (planktivorous fish) and prey (small-bodied crustacean zooplankton) densities at a site sampled through the growing season in Lakes Michigan, Huron, and Erie. Although seasonal average densities of Bythotrephes were similar across lakes (222/m² Erie, 247/m² Huron, 162/m² Michigan), temporal trends in abundance differed among lakes. In central Lake Erie where Bythotrephes' prey assemblage was dominated by small individuals (60%), where planktivorous fish densities were high (14,317/ha), and where a shallow water column limited availability of a deepwater refuge, the Bythotrephes population was characterized by a small mean body size, large broods with small neonates, allocation of length increases mainly to the spine rather than to the body, and a late summer population decline. By contrast, in Lake Michigan where Bythotrephes' prey assemblage was dominated by large individuals (72%) and planktivorous fish densities were lower (5052/ha), the Bythotrephes population was characterized by a large mean body size (i.e., 37–55% higher than in Erie), small broods with large neonates, nearly all growth in body length occurring between instars 1 and 2, and population persistence into fall. Life-history characteristics in Lake Huron tended to be intermediate to those found in Lakes Michigan and Erie, reflecting lower overall prey and predator densities (1224/ha) relative to the other lakes. Because plasticity in life history can affect interactions with other species, our findings point to the need to understand life-history variation among Great Lakes populations to improve our ability to model the dynamics of these ecosystems.