In situ assessment of lampricide toxicity to age-0 lake sturgeon

The lampricides 3-trifluoromethyl-4-nitrophenol (TFM) and 2′, 5-dichloro-4′-nitrosalicylanilide (niclosamide) are used to control sea lamprey (Petromyzon marinus), an invasive species in the Great Lakes. Age-0 lake sturgeon (Acipenser fulvescens), a species of conservation concern, share similar stream habitats with larval sea lampreys and these streams can be targeted for lampricide applications on a 3- to 5-year cycle. Previous laboratory research found that lake sturgeon smaller than 100 mm could be susceptible to lampricide treatments. We conducted stream-side toxicity (bioassay) and in situ studies in conjunction with 10 lampricide applications in nine Great Lakes tributaries to determine whether sea lamprey treatments could result in in situ age-0 lake sturgeon mortality, and developed a logistic model to help predict lake sturgeon survival during future treatments. In the bioassays the observed concentrations where no lake sturgeon mortality occurred (no observable effect concentration, NOEC) were at or greater than the observed sea lamprey minimum lethal concentration (MLC or LC99) in 7 of 10 tests. We found that the mean in situ survival of age-0 lake sturgeon during 10 lampricide applications was 80%, with a range of 45–100% survival within streams. Modeling indicated that in age-0 lake sturgeon survival was negatively correlated with absolute TFM concentration and stream alkalinity, and positively correlated with stream pH and temperature. Overall survival was higher than expected based on previous research, and we expect that these data will help managers with decisions on the trade-offs between sea lamprey control and the effect on stream-specific populations of age-0 lake sturgeon.     

Implications of tagging effects for interpreting the performance of sea lamprey traps in a large river

Abundance estimates can be crucial for managing species of economic concern. The accuracy of these estimates can depend on the methods used to track animals and to estimate abundance from tracking data. We tested experimentally if disparate estimates of trapping efficiency calculated for sea lamprey (Petromyzon marinus) in the St. Marys River near Sault Ste. Marie, Canada could be explained by effects related to the invasiveness and handling involved in tagging or the tag size used in the marking procedures. Trapping is used to gauge adult abundance, trapping efficiency, and success of a binational sea lamprey control program in the Laurentian Great Lakes, North America. Our experiment compared nightly catches of sea lamprey marked with external fin clips, surgically-implanted passive integrated transponder tags (PIT-only), and surgically-implanted PIT and acoustic tags (PIT+acoustic). We found no evidence that the probability of being trapped was affected by the added invasiveness and handling of internal tagging. Nightly recaptures of PIT-only tagged sea lamprey, relative to fin-clipped sea lamprey, were not different from expectations based on the numbers of individuals released from each treatment group. Conversely, there was evidence of effects related to tag size. Nightly recaptures of PIT+acoustic tagged sea lamprey, relative to PIT-only tagged sea lamprey, were lower than expected based on numbers of individuals released from each treatment group. Effects related to tag size partially explain the disparate estimates in trapping efficiency observed for sea lamprey.     

Diets,population structure,and seasonal activity patterns of mudpuppies (Necturus maculosus) in an urban,Great Lakes coastal habitat

Freshwater species native to the Laurentian Great Lakes region face numerous environmental stressors, and the conservation status and ecological relationships of many remain poorly understood. One such species, the mudpuppy (Necturus maculosus), is declining, but better information on their natural history and development of more effective population monitoring techniques is needed. We assessed seasonal variation in capture success, biases in capture techniques, and feeding ecology of mudpuppies in Wolf Lake, a highly perturbed and urban former estuarine wetland complex to Lake Michigan. Trapping periods of ≥ 3 consecutive nights occurred from January to May 2015, and October 2015 to March 2016. Overall trapping success differed among trapping periods (p = 0.01) and declined precipitously at water temperatures above 14.1 °C (p < 0.001). Mudpuppies in traps (mean 26.9 ± 0.5 cm) were larger than those caught with hand nets (mean 14.7 ± 0.8 cm, p < 0.0001), suggesting that multiple methods may be needed to accurately assess demographics. Stomach contents obtained through gastric lavage included mollusks, leeches, insects, isopods, amphipods, crayfish, fish, a frog, and a juvenile conspecific. Invasive species, including rusty crayfish (Orconectes rusticus), round gobies (Neogobius melanostomus), and zebra/quagga mussels (Dreissena spp.) were present in guts, suggesting mudpuppy foraging has changed along with aquatic communities in the region. Prey community analyses revealed differences in overall diet among size classes of mudpuppies (p = 0.001), but relatively weak similarity within size classes. Results suggest that mudpuppies in lake ecosystems occupy a broad niche that changes as they grow.     

Warmer waters increase the larval sea lamprey's (Petromyzon marinus) tolerance to the lampricide 3-trifluoromethyl-4-nitrophenol (TFM)

Invasive sea lampreys (Petromyzon marinus) in the Great Lakes are controlled by applying the pesticide (lampricide) 3-trifluoromethyl-4-nitrophenol (TFM) to waters infested with larval lamprey. However, treatment effectiveness can be undermined by “residual” larval sea lamprey that survive TFM exposure, and subsequently complete metamorphosis into parasitic juvenile sea lamprey that prey on culturally and economically important fishes. We investigated how season and temperature influenced the TFM tolerance of larval sea lamprey. Acute toxicity tests on lamprey collected from the Au Sable River, Michigan, revealed that the 12-h LC₅₀ and LC_99.9 were 2.0- to 2.5-fold greater in late spring and summer, than in early spring and fall. Subsequent toxicity tests indicated that greater TFM tolerance in summer was due to warmer temperatures, based on an almost 2-fold greater 12-h LC₅₀ and LC_99.9 in warm (24 °C) compared to cool (6 °C) water. Variations in energy stores (glycogen, lipid, protein) or condition did not appear to affect TFM sensitivity. We conclude that higher water temperature is the primary factor driving the larval sea lamprey's greater tolerance to TFM during the summer, possibly due to an increase in their capacity to detoxify TFM. Considering seasonal variations in temperature may be prudent when selecting and treating sea lamprey infested streams with TFM to minimize treatment residuals. In the longer term, increases in average and peak water temperatures due to climate change could result in greater TFM requirements and costs due to the greater tolerance of larval sea lamprey to TFM at warmer temperatures.     

Researcher disciplines and the assessment techniques used to evaluate Laurentian Great Lakes coastal ecosystems

The Laurentian Great Lakes of North America have been a focus of environmental and ecosystem research since the Great Lakes Water Quality Agreement in 1972. This study provides a review of scientific literature directed at the assessment of Laurentian Great Lakes coastal ecosystems. Our aim was to understand the methods employed to quantify disturbance and ecosystem quality within Laurentian Great Lakes coastal ecosystems within the last 20 years. We focused specifically on evidence of multidisciplinary articles, in authorship or types of assessment parameters used. We sought to uncover: 1) where Laurentian Great Lakes coastal ecosystems are investigated, 2) how patterns in the disciplines of researchers have shifted over time, 3) how measured parameters differed among disciplines, and 4) which parameters were used most often. Results indicate research was conducted almost evenly across the five Laurentian Great Lakes and that publication of coastal ecosystems studies increased dramatically ten years after the first State of the Great Lakes Ecosystem Conference in 1994. Research authored by environmental scientists and by multiple disciplines (multidisciplinary) have become more prevalent since 2003. This study supports the likelihood that communication and knowledge-sharing is happening between disciplines on some level. Multidisciplinary or environmental science articles were the most inclusive of parameters from different disciplines, but every discipline seemed to include chemical parameters less often than biota, physical, and spatial parameters. There is a need for an increased understanding of minor nutrient, toxin, and heavy metal impacts and use of spatial metrics in Laurentian Great Lakes coastal ecosystems.     

A Practical Method for Obtaining Useful Quantities of Pheromones from Sea Lamprey and Other Fishes for Identification and Control

Pheromonally-mediated trapping is currently being developed for use in sea lamprey control in the Laurentian Great Lakes. To identify and test lamprey pheromones a practical procedure was needed to isolate relatively large quantities of pheromone from lamprey holding water. The present study developed such a technique. It employs Amberlite XAD7HP, an adsorbent resin which we found can extract over 80% of the sea lamprey migratory pheromone from larval holding water at low cost and with relative ease. This technique allowed us to collect tens of milligrams of all three components of the sea lamprey migratory pheromone, eventually permitting both identification and successful field testing. This technique might also be used to collect pheromones released by other species of fish.     

Growth and behavioral effects of the lampricide TFM on non-target fish species

Although the lampricide 3-trifluoromethyl-4-nitrophenol (TFM) has been used for over 60 years to control sea lamprey Petromyzon marinus in the Laurentian Great Lakes, its potential non-lethal impacts on non-target species have not been fully evaluated. We exposed juveniles of two species of fish (lake sturgeon Acipenser fulvescens and rainbow trout Oncorhynchus mykiss) and one adult fish species (fathead minnows Pimephales promelas) to various concentrations of TFM (0.25–7.5 mg/L) in three sets of experiments examining TFM effects on growth, avoidance of TFM treated water, and predation susceptibility. Lake sturgeon and rainbow trout were monitored for two weeks after a 12 hour exposure to TFM to observe differences in instantaneous growth among four treatment levels (0, 2.5, 5, and 7.5 mg/L TFM). Growth rates did not differ significantly among control and treated fish of either species. Next, potential avoidance of TFM by rainbow trout was evaluated in a test tank where half the water was contaminated with TFM (0, 0.25, or 2.5 mg/L). No avoidance behavior was observed as rainbow trout spent equal amounts of time in TFM and control water. Finally, fathead minnows were exposed at three different concentrations of TFM (0, 2.5, and 7.5 mg/L) and placed in mesocosms with a non-exposed largemouth bass Micropterus salmoides predator. Two separate trials were performed, both with no significant differences due to treatments. In summary, results indicate that for the conditions tested, TFM has no detectable sub-lethal effects on growth, avoidance behavior, or predation mortality on the fish species tested.     

Contribution of manipulable and non-manipulable environmental factors to trapping efficiency of invasive sea lamprey

We identified aspects of the trapping process that afforded opportunities for improving trap efficiency of invasive sea lamprey (Petromyzon marinus) in a Great Lake's tributary. Capturing a sea lamprey requires it to encounter the trap, enter, and be retained until removed. Probabilities of these events depend on the interplay between sea lamprey behavior, environmental conditions, and trap design. We first tested how strongly seasonal patterns in daily trap catches (a measure of trapping success) were related to nightly rates of trap encounter, entry, and retention (outcomes of sea lamprey behavior). We then tested the degree to which variation in rates of trap encounter, entry, and retention were related to environmental features that control agents can manipulate (attractant pheromone addition, discharge) and features agents cannot manipulate (water temperature, season), but could be used as indicators for when to increase trapping effort. Daily trap catch was most strongly associated with rate of encounter. Relative and absolute measures of predictive strength for environmental factors that managers could potentially manipulate were low, suggesting that opportunities to improve trapping success by manipulating factors that affect rates of encounter, entry, and retention are limited. According to results at this trap, more sea lamprey would be captured by increasing trapping effort early in the season when sea lamprey encounter rates with traps are high. The approach used in this study could be applied to trapping of other invasive or valued species.     

Lateral and vertical distribution of downstream migrating juvenile sea lamprey

Sea lamprey is considered an invasive and nuisance species in the Laurentian Great Lakes, Lake Champlain, and the Finger Lakes of New York and is a major focus of control efforts. Currently, management practices focus on limiting the area of infestation using barriers to block migratory adults, and lampricides to kill ammocoetes in infested tributaries. No control efforts currently target the downstream-migrating post-metamorphic life stage which could provide another management opportunity. In order to apply control methods to this life stage, a better understanding of their downstream movement patterns is needed. To quantify spatial distribution of downstream migrants, we deployed fyke and drift nets laterally and vertically across the stream channel in two tributaries of Lake Champlain. Sea lamprey was not randomly distributed across the stream width and lateral distribution showed a significant association with discharge. Results indicated that juvenile sea lamprey is most likely to be present in the thalweg and at midwater depths of the stream channel. Further, a majority of the catch occurred during high flow events, suggesting an increase in downstream movement activity when water levels are higher than base flow. Discharge and flow are strong predictors of the distribution of out-migrating sea lamprey, thus managers will need to either target capture efforts in high discharge areas of streams or develop means to guide sea lamprey away from these areas.     

Benthic Macroinvertebrates as Indicators of Environmental Degradation in the Southern Nearshore Zone of the Central Basin of Lake Erie

The benthic macroinvertebrates of the central basin of Lake Erie were sampled with a Ponar grab in the summers of 1978 and 1979 along a 155-km reach of the nearshore zone (≤ 12 km offshore) in Ohio, U.S.A., at depths of less than 20 m. The major groups and their most abundant species were, in order of overall abundance, Oligochaeta (Limnodrilus hoffmeisteri, L. cervix-L. claparedeianus group, L. maumeensis), Sphaeriidae (Pisidium casertanum, P. henslowanum, Sphaerium corneum, Musculium transversum), and Chironomidae (Procladius sp., Chironomus spp.). The average abundance of oligochaetes in the harbors was 21,000 individuals m^?2 in 1978 and 12,700 m^?2 in 1979, compared with 1,500 m^?2 and 1,200 m^?2, respectively, in the areas outside of harbors. Comparison of the macrobenthic assemblages with those in other regions of the Great Lakes, using several numerical indices as well as indicator species distributions, indicated that the general area of the nearshore zone outside of harbors possesses a moderate degree of organic enrichment, with a gradient of decreasing pollution in an offshore direction. The harbors appeared to be severely degraded, as reflected by the high densities of oligochaetes and the almost complete absence of all but the most pollution-tolerant species. The documentation of species distributions will enable future assessments of changes in the nearshore benthic communities.     

Seasonal Patterns in Growth,Blood Consumption,and Effects on Hosts by Parasitic-Phase Sea Lampreys in the Great Lakes: An Individual-Based Model Approach

An individual-based model (IBM) was developed for sea lamprey (Petromyzon marinus) populations in the Laurentian Great Lakes. The IBM was then calibrated to observed growth, by season, for sea lampreys in northern Lake Huron under two different water temperature regimes: a regime experienced by Seneca-strain lake trout (Salvelinus namaycush) and a regime experienced by Marquettestrain lake trout. Modeling results indicated that seasonal blood consumption under the Seneca regime was very similar to that under the Marquette regime. Simulated mortality of lake trout directly due to blood removal by sea lampreys occurred at nearly twice the rate during August and September under the Marquette regime than under the Seneca regime. However, cumulative sea lamprey-induced mortality on lake trout over the entire duration of the sea lamprey's parasitic phase was only 7% higher for the Marquette regime compared with the Seneca regime. Thus, these modeling results indicated that the strain composition of the host (lake trout) population was not important in determining total number of lake trout deaths or total blood consumption attributable to the sea lamprey population, given the sea lamprey growth pattern. Regardless of water temperature regime, both blood consumption rate by sea lampreys and rate of sea lamprey-induced mortality on lake trout peaked in late October. Elevated blood consumption in late October appeared to be unrelated to changes in water temperature. The IBM approach should prove useful in optimizing control of sea lampreys in the Laurentian Great Lakes.     

In vivo and in vitro phenotypic differences between Great Lakes VHSV genotype IVb isolates with sequence types vcG001 and vcG002

Viral hemorrhagic septicemia virus (VHSV) is an aquatic rhabdovirus first recognized in farmed rainbow trout in Denmark. In the past decade, a new genotype of this virus, IVb was discovered in the Laurentian Great Lakes basin and has caused several massive die-offs in some of the 28 species of susceptible North American freshwater fishes. Since its colonization of the Great Lakes, several closely related sequence types within genotype IVb have been reported, the two most common of which are vcG001 and vcG002. These sequence types have different spatial distributions in the Great Lakes. The aim of this study was to determine whether the genotypic differences between representative vcG001 (isolate MI03) and vcG002 (isolate 2010-030 #91) isolates correspond to phenotypic differences in terms of virulence using both in vitro and in vivo approaches. In vitro infection of epithelioma papulosum cyprini (EPC), bluegill fry (BF-2), and Chinook salmon embryo (CHSE) cells demonstrated some differences in onset and rate of growth in EPC and BF-2 cells, without any difference in the quantity of RNA produced. In vivo infection of round gobies (Neogobius melanostomus) via immersion exposure to different concentrations of vcG001 or vcG002 caused a significantly greater mortality in round gobies exposed to 10² plaque forming units ml^− 1 of vcG001. These experiments suggest that there are phenotypic differences between Great Lakes isolates of VHSV genotype IVb.     

Legacy polychlorinated organic pollutants in the sediment of the Great Lakes

Legacy, organic pollutants, including polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), naphthalenes (PCNs), and diphenyl ethers (PCDEs) were quantified in sediments of the Laurentian Great Lakes of North American. A total of 40 cores (939 core segments) and 198 Ponar surface grab samples were collected from the five Great Lakes between 2010 and 2014. Median concentrations in Ponar grab samples were 8.4, 0.27, 0.05, 0.19 and 0.01 ng/g dry weight (dw) for total-PCBs, ∑₇PCDDs, ∑₁₀PCDFs, ∑₁₂PCNs, and ∑₇PCDEs, respectively. By using Geographic Information Systems Analysis with the inverse distance weight (IDW) interpretation of the spatial distribution of the chemical inventory at coring sites, total mass loads in the five lakes combined were estimated to be 511, 15.3, 5.3, 20.7 and 2.9 t for total-PCBs, ∑₇PCDDs, ∑₁₀PCDFs, ∑₁₂PCNs, and ∑₇PCDEs, respectively. Patterns of spatial distributions revealed pollution hotspots and provided evidence for historical local sources. Concentrations of residues in Ponar grabs and inventories at coring sites, when normalized to concentrations of organic carbon, exhibited statistically significantly correlations with latitude and longitude of the sampling sites for all five chemical groups. At most coring sites, concentrations have been decreasing towards the sediment surface. At locations relatively close to known or suspected sources, estimated half-times for all classes of chemicals were approximately 20 years. The declining trends of PCDDs and PCDFs were unclear at some locations, suggesting the presence of currently active emission sources.     

Comparison of thermal tolerance and standard metabolic rate of two Great Lakes invasive fish species

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.     

Quantification of Octachlorostyrene and Related Compounds in Great Lakes Fish by Gas Chromatography - Mass Spectrometry

Residues of octachlorostyrene (OCS) and related polychlorinated compounds including isomers of heptachloro-, hexachloro-, and pentachlorostyrene; hexachlorobenzene, pentachlorobenzene, isomers of tetrachloro- and trichlorobenzene; and hexachlorobutadiene have been quantitated by multiple-ion-detection gas chromatography-mass spectrometry in Great Lakes fish collected between 1974 and 1980. The results show that the two upper lakes, Superior and Michigan, do not appear to have residues of OCS greater than 5 ng/g, while residues in the lower lakes, Huron, Ontario, and Erie, are as high as 400 ng/g. A selected tributary to Lake Erie has been shown to contain very high levels of all of the chemicals studied which suggests one possible source of chlorostyrenes in the Great Lakes.     

Organochlorine Residues in Suspended Solids Collected from the Mouths of Canadian Streams Flowing into the Great Lakes 1974–1977

Suspended solids were collected from 105 streams on the Canadian side of the Great Lakes during snow melt and spring flow between 1974 and 1977. Analyses were performed for organochlorine and organophosphorus insecticides and three industrial chemicals. All streams sampled contained suspended solids that were contaminated with DDT, its metabolites, and polychlorinated biphenyl (PCB). The highest mean concentrations of ΣDDT (356 μg/kg) occurred in 1974 from streams draining the Niagara Peninsula and entering Lake Ontario; these were correlated to areas of high use on fruit in the past. The highest loadings by lake section, however, were along the north shore of Lake Erie involving a very much larger delivery of suspended solids. The highest concentrations of PCB were on suspended solids entering Lake Ontario in 1974 along the northern (191 μg/kg) and southwestern (172 μg/kg) shore and were associated with the high urban-industrial activity in the megalopolis of Hamilton-Burlington-Oakville-Mississauga-Toronto. However, the highest loadings occurred along Lake St. Clair where two large rivers, the Thames and Sydenham, discharge a much larger volume of suspended solids. The highest individual concentrations of HEOD (dieldrin) were found on suspended solids in two streams entering along the southwestern shoreline of Lake Ontario. However, the highest mean residues on solids for all streams was in waters entering Lake Erie (3 μg/kg) and the highest loadings were into Lake St. Clair. Endosulfan, chlordane, and heptachlor epoxide were detected in some streams entering all five lakes. No aldrin, endrin, mirex, heptachlor, or organophosphorus insecticides were detected.     

Organochlorine Contaminant Residues in Young-of-the-Year Spottail Shiners from Lakes Ontario,Erie, and St. Clair

The spottail shiner (Notropis hudsonius) was found to be a useful biological integrator of organochlorine contaminants in near-shore habitats. During the fall of 1975 spottail shiners were collected from nine sampling sites on Lakes Ontario, Erie and St. Clair. The restricted home range of the young-of-the-year spottail and its relatively short exposure to contaminants facilitated localized residue level assesment and reflected on recent contaminant loadings. Contaminant types and residue levels in fish generally reflected the regional land-practices. All samples analyzed contained PCB and DDT residues, while some also contained heptachlor epoxide, dieldrin, endrin and chlordane. Mean PCB concentrations for theyoung-of-the-year spottails ranged from 59 ng/g - 844 ng/g wet weight, while total DDT residues ranged from 32 ng/g - 244 ng/g. The mean PCB residue concentrations in seven of the nine fish collections tested exceeded the 100 ng/g body burden level recommended for protection of wildlife by the I.J.C.     

Spatial and Ontogenetic Variability of Sea Lamprey Diets in Lake Superior

Invasive sea lamprey (Petromyzon marinus) remain an important source of fish mortality in the Laurentian Great Lakes, yet assessing their impact is hindered by lack of quantitative diet information. We examined nitrogen and carbon stable isotope ratios (δ¹⁵N and δ¹³C) of sea lamprey and host species in six ecoregions of Lake Superior, mainly in 2002–2004. Data implied that most sea lamprey fed primarily on upper trophic level species, including forms of lake trout (Salvelinus namaycush). However, in Ontario waters, particularly semi-enclosed Black Bay, sea lamprey relied heavily on lower trophic levels, such as coregonines (Coregonus spp.) and suckers (Catostomus spp.). Sea lamprey δ¹⁵N and δ¹³C generally increased with sea lamprey size, implying dependence on higher trophic levels later in life. Most parasitic sea lamprey that we captured were attached to either lean lake trout (35% of observed attachments), lake whitefish (Coregonus clupeaformis; 25%), or cisco (C. artedii; 25%); the latter sea lamprey were typically < 15 g. Survey- and fishery-dependent wounding rate data compiled from 1986–2005 suggest that lean and siscowet lake trout were selectively parasitized by sea lamprey, which is consistent with our stable isotope data. Our results largely support the notion that lake trout are the principal host species in Lake Superior. However, stable isotope evidence that sea lamprey feed at lower trophic levels in some regions argues for comprehensive monitoring of sea lamprey impacts throughout the fish community in systems that sea lamprey have invaded.     

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.