Rapid response of the nearshore round goby population to temperature declines associated with upwelling events in Lake Ontario

Upwellings occur in all the Great Lakes in response to prolonged wind events. Several studies have modeled or measured changes in abiotic conditions associated with upwellings, but few direct observations of fish responses have been documented. Video-capture techniques were used in conjunction with moored temperature loggers to link the disappearance of benthic round gobies to temperature declines during upwelling events along the southern Lake Ontario nearshore in summer of 2019. Benthic water temperatures declined by as much as 18 °C within 18 h over as many as nine events. Round goby density estimates were as high as 50/m² prior to upwellings, but declined to as low as 0/m² during the events. Using just nine observation dates, ARMA trend analysis suggested a relationship between benthic temperature and round goby abundance (AIC = 78.7, t = 2.21, P = 0.063). Although the actual response of gobies to rapidly declining temperatures was unknown, this population was unobservable in our sample area during upwellings, yet returned to pre-upwelling densities within days. Understanding the magnitude and frequency of fish responses to these events can improve our understanding of the potential for this non-native benthic fish to affect the nearshore environment.     

Diet partitioning, habitat preferences and behavioral interactions between juvenile yellow perch and round goby in nearshore areas of Lake Erie

Ecological interactions between native and non-indigeneous species depend on interspecies dietary and habitat overlap and species-specific behavior. In the Great Lakes, the exotic round goby (Apollonia melanostoma) is very abundant in littoral areas used by the native yellow perch (Perca flavencens). We examined yellow perch-round goby interactions using multiple approaches. Field surveys analyzing dietary overlap among three size classes of yellow perch and round goby detected significant overlap only between juvenile perch (< 95 mm TL) and gobies (< 60 mm TL). Laboratory experiments using juvenile stages tested for habitat preference differences (open sand, macrophytes and dreissenids) in solitary, intraspecific (2 perch) and interspecific (1 perch, 1 goby) treatments. In macrophyte and dreissenid habitats, we tested for treatment differences in fish behavior (intraspecific vs. interspecific) and yellow perch growth (solitary, intraspecific and interspecific). Round goby consistently preferred complex habitats. Yellow perch showed diurnal preference of complex habitats, but increased nocturnal use of sand in the solitary and interspecific treatments. Activity was greater in dreissenid than macrophyte habitat, but prey attacks showed the opposite trend. Activity and prey attacks were greater in the intraspecific than interspecific treatments. The trend was due to lower prey attacks executed by round goby. In macrophytes, individual yellow perch growth was lower in the intraspecific than in the solitary and interspecific treatments. In dreissenids, intraspecific and interspecific competitors equally decreased yellow perch growth. Our results suggest differences in diet, habitat preference and behavior between juvenile round goby and yellow perch may allow their coexistence in nearshore areas.     

Density and habitat use by the round goby (Apollonia melanostoma) in the Bay of Quinte,Lake Ontario

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.     

Double-crested Cormorant (Phalacrocorax auritus) chick bioenergetics following round goby (Neogobius melanostomus) invasion and implementation of cormorant population control

As the fish community changed in the Beaver Archipelago (northern Lake Michigan), so has the diet of breeding Double-crested Cormorants (Phalacrocorax auritus). In 2000, the energetically dense alewife (Alosa pseudoharengus) dominated the diet, but more recently, the relatively low quality round goby (Neogobius melanostomus) has become the most common prey item. Additionally, cormorant control measures have been underway in the archipelago. This study investigated decreases in cormorant chick numbers and changes in chick bioenergetics in response to this change in diet, as well as the influence of control efforts, and compared these results to an earlier study. Two colonies, one actively controlled the other only controlled following completion of this study, were investigated to determine changes in colony size throughout the breeding season, document chick diet, measure chick growth, and determine chick survivorship to fledging. The bioenergetics model estimated that in 2010, chicks consumed a greater biomass of prey to reach fledging due to the change in diet, but lower biomass overall due to fewer chicks in the system. Control efforts in combination with the change in diet reduced chick numbers. Overall, the impact of cormorant chicks on available fish biomass has declined in recent years due to control efforts. However, the birds have the potential to consume a greater biomass of round goby in the absence of control. The impact of cormorants on fisheries resources is complicated by the abundance and effects of non-native species.     

Multiple male reproductive morphs in the invasive round goby (Apollonia melanostoma)

Alternative male reproductive tactics are taxonomically widespread. In such species, parental, or conventional, males express secondary sexual characteristics, court females and guard offspring, while smaller parasitic or sneaker males avoid the costs of courtship and parental care by performing sneak fertilizations. Theory predicts that sneakers will invest more in testes mass and produce more competitive ejaculates than parentals because sneakers always experience sperm competition while parental males experience sperm competition only when a sneaker is present. Here we present convergent lines of evidence supporting the existence of alternative male reproductive tactics in round gobies (Apollonia melanostoma, formerly Neogobius melanostomus), a recent invader in the Great Lakes. Dark morph males exhibited secondary sexual characteristics, were larger and had higher plasma 11-ketotestosterone concentrations than light morphs, while light morph males invested more in ejaculates (both testes mass and sperm density). Both male morphs had enlarged urogenital papillae, but papillae were relatively longer in light morph males. Sperm tail length did not differ between morphs, and sperm from dark morphs swam faster than sperm from light morphs. Our data strongly argue for the presence of alternative tactics in round gobies, support some predictions from sperm competition theory and align with empirical observations in other taxa. For species of concern like the invasive round goby, it is critical to consider such evidence of alternative male mating tactics when constructing population growth models and assessment of invasion success and impacts.     

Piscivory in recovering Lake Michigan cisco (Coregonus artedi): The role of invasive species

Contemporary conditions in Lake Michigan where cisco (Coregonus artedi) populations are expanding are vastly different from those encountered by the historic fish community. Invasive species introductions have substantially altered the Lake Michigan ecosystem in the last half century. Successful management efforts for cisco in Lake Michigan hinge on our ability to understand their contemporary ecology, especially diet. We collected 725 cisco stomachs opportunistically from commercial fisheries (2%) and in agency surveys (98%) over six years (2014–2019). The majority (70%) of stomachs were from East Grand Traverse Bay and 96% of these were collected at Elk Rapids. Additional samples were collected from Charlevoix (8%), Little Traverse Bay (11%), other sites in northern Lake Michigan (4%), Central Lake Michigan (6%), and Green Bay (1%). Our results indicated a high degree of piscivory, in contrast to historical and contemporary accounts of planktivory for cisco in the other Laurentian Great Lakes. The top three prey items by mass were not native to the Great Lakes and these accounted for 87% of all observed prey mass consumed: round goby (Neogobius melanostomus) (58%), Bythotrephes longimanus (15%), and alewife (Alosa pseudoharengus) (14%). Round goby dominated the prey in the spring and summer, while B. longimanus and alewife occurred more in summer and fall diets. The contemporary population of cisco in Lake Michigan has been able to uniquely capitalize on abundant invasive prey resources, which may be less limiting and more energy-rich than a more typical planktivorous cisco diet.     

Evaluating gonadosomatic index as an estimator of reproductive condition in the invasive round goby, Neogobius melanostomus

Using gonadosomatic index cut-off scores has become a standard protocol for selecting reproductive fish in studies on the reproductive biology of round goby, Neogobius melanostomus, a significant invader of the Laurentian Great Lakes, but the validity of this practice has not been validated with histological staging. The goal of the current study was to evaluate the effectiveness of using gonadosomatic index (GSI) cut-off scores to classify reproductive status in male and female round goby by documenting associations between GSI, sex steroids, and gonad development. Gonadal stage was determined in both sexes using hematoxylin and eosin histology. Plasma 11-ketotestosterone and testosterone were measured in males, and testosterone and 17β-estradiol were measured in females. Gonadosomatic index cut-off scores were effective in selecting spawning capable individuals at higher GSI values, but GSI values were limited in the ability to make further distinctions of gonadal stage and missed many spawning capable females. In females, testosterone levels were highest during vitellogenic growth and declined prior to ovulation. 17β-estradiol displayed a similar, but non-statistically significant pattern. Males with developing testes had higher levels of 11-ketotestosterone —but not testosterone —than reproductively immature males, although levels of these androgens were overall positively correlated in males. The findings indicate that the conventional GSI cut-off scores (1% in males, 8% in females) accurately assign spawning capable condition in both sexes; however, they may also exclude some spawning capable females with lower GSIs.     

Salinity tolerance of the invasive round goby: Experimental implications for seawater ballast exchange and spread to North American estuaries

The Eurasian round goby (Neogobius melanostomus) invaded the freshwater North American Great Lakes in ~ 1990 via accidental introduction from ballast water discharge. Its genotypes in the Great Lakes traced to estuaries in the northern Black Sea, where the round goby flourishes in a variety of salinities to 22 parts per thousand (ppt). To prevent further introductions, U.S. and Canadian Coast Guard regulations now require that vessels exchange ballast water at sea before entering the Great Lakes. Since salinity tolerance of the invasive round goby population is poorly understood, we tested 230 laboratory-acclimated fish in three experimental scenarios: (1) rapid salinity increases (0–40 ppt), simulating ballast water exchange, (2) step-wise salinity increases, as during estuarine tidal fluxes or migration from fresh to saltwater, and (3) long-term survivorship and growth (to 4 months) at acclimated salinities. Almost all gobies survived experiments at 0–20 ppt, whereas none survived ≥ 30 ppt, and at 25 ppt only 15% withstood rapid changes and 30% survived step-wise increases. Ventilation frequencies were lowest at 10–15 ppt in step-wise experiments, in conditions that were near isotonic with fish internal plasma concentrations, reflecting lower energy expenditure for osmoregulation. Growth rates appeared greatest at 5–10 ppt, congruent with the larger sizes reached by gobies in Eurasian brackish waters. Thus, we predict that the Great Lakes round goby would thrive in brackish water estuaries along North American coasts, if introduced. However, oceanic salinities appear fatal to the invasive round goby, which likely cannot withstand complete seawater ballast exchanges or oceanic habitats.     

First documentation of spawning and nest guarding in the laboratory by the invasive fish,the round goby (Neogobius melanostomus)

Although the round goby, Neogobius melanostomus, is widespread in the Great Lakes and has an extended breeding season with a high reproductive rate; its spawning behaviour remains elusive. We present the first reported accounts of spawning by the round goby in the laboratory. By simulating winter conditions and restoring spring conditions, we induced round gobies to spawn in October 2007, March 2008, May 2008, and January 2009. In one case, fanning by the nest-holding male began 10 days before egg deposition and, during this period, the male rubbed secretions along the ceiling of the nest. Males were choosy about which gravid females entered the nest and prevented entry by some females. Spawning involved repeated inversions by females and males releasing gametes on the ceiling of the nest. Males guarded the nest by blocking the entrance, producing agonistic vocalizations and chasing intruders. Inside the nest, eggs were regularly inspected by the males and constantly ventilated using pectoral and caudal fins. Up to three gravid females spawned sequentially in a nest. Peak ventilation occurred after egg deposition and declined with time until the parental male ate the eggs. The decline of parental care and egg cannibalism was likely an artifact of laboratory conditions and small brood size. Our findings offer new information on the reproductive habits of the invasive round goby. Because the reproductive sequence in the laboratory seems easy to disrupt, the procedures may lead to a management tool to control the spread of the species into new areas.     

Line transect distance sampling in aerial surveys for double-crested cormorants in coastal regions of Lake Huron

Line transect distance sampling was employed in aerial surveys of double-crested cormorants (Phalacrocorax auritus) along the coasts of Georgian Bay and the North Channel, Lake Huron. A double-observer method was used to estimate detection probability near the transect line (g(0) = 0.724). Detection of cormorants was not consistent but varied based on group size, location (water, land, flying), and season. Probability of detection in the area covered by the survey was often below 0.5. Incorporating both lack of detection on the flight line along with lack of detection over the covered area inherent in distance sampling provided defensible density estimates of free-ranging double-crested cormorants. Most cormorants were detected loafing on shore (land) among the many islands defining this area of the Lake Huron coast. Land detections exceeded the combined detections of birds on the water and flying. Density in 2004 ranged from a peak of 2.30 cormorants per km² (95% CI = 1.72–3.03) in late July to 1.21 cormorants per km² (95% CI = 0.78–1.70) in late August in the sampled areas extending from shore to approximately 20 km offshore. Aerial surveys employing distance sampling can be useful tools in monitoring the distribution and abundance of free-ranging double-crested cormorants and other waterbirds in the Laurentian Great Lakes.     

Response to putative round goby (Neogobius melanostomus) pheromones by centrarchid and percid fish species in the Laurentian Great Lakes

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.     

Two decades since first invasion: Revisiting round goby impacts on nearshore aquatic communities in the Upper St. Lawrence River

Time Since Invasion (TSI) at invaded sites is an important factor that can moderate invasive species abundances and their ecological impacts on resident communities, and yet it remains rarely addressed by invasion studies. We revisit the ecological impacts of round goby invasion in the Upper St. Lawrence River (Canada) on macroinvertebrate and fish communities, by taking into consideration the effects of TSI since initial invasion 20 years ago. Our objectives were to: 1) test the effect of TSI and invasion status on round goby abundance, and 2) investigate the effects of TSI, invasion status, and round goby abundance on macroinvertebrate and fish communities. Round goby abundance displayed a strong positive relationship with TSI. Environmental conditions, especially substrate and water conductivity were found to reduce round goby TSI and characterize uninvaded sites and >12 years TSI sites. However, we detected first potential signals of a reduction in round goby abundance at sites with TSI of 17–19 years compared with sites having 13–16 years TSI which had similar environmental conditions. Nonetheless, round gobies continued to have enduring impacts on total macroinvertebrate diversity, even at sites with TSI of 17–19 years, likely as a result of ongoing predation. This observation was supported by a negative association of round goby TSI on the abundance of dominant macroinvertebrate taxa at sampling sites in the Upper St Lawrence River, as well as dietary stomach content data. The fish community was less sensitive to round goby TSI than macroinvertebrate communities.     

Seasonal and annual patterns in density of double-crested cormorants in two coastal regions of Lake Huron

Aerial line transect distance sampling surveys of coastal areas in the North Channel and Georgian Bay, Lake Huron, were conducted to estimate density of double-crested cormorants (Phalacrocorax auritus). Surveys were approximately every two weeks through summer from 2000 to 2005. In each year, density of cormorants declined towards late summer indicating a large scale outward migration of cormorants from Lake Huron. The seasonal pattern of decline differed between the two regions based on a mixed model analysis. Seasonal patterns in apparent fledging patterns may reflect differences between the North Channel and Georgian Bay in nesting phenology, nesting synchrony or some combination of these factors. Density was generally higher in the North Channel relative to Georgian Bay seasonally and during the period 2000–2002 likely reflecting higher per unit area productivity in the North Channel. In the years 2003–2005, density was lower in both regions and similar compared to earlier years of the survey likely reflecting a regime shift that occurred in Lake Huron at that time. The effect of this change was greater in the North Channel than in Georgian Bay as indicated by a greater decline in cormorant density in the North Channel after 2002.     

Benthic invertebrate assemblage changes in an urban bay of Lake Ontario: 1990 to 2012

Invasions by dreissenid mussels and the round goby have altered the makeup of benthos in many areas of the Great Lakes complicating the use of benthic invertebrate composition as an indicator of environmental conditions. The Ontario Ministry of the Environment, Conservation and Parks (MECP) has been monitoring the composition of benthos at nearshore stations since the 1990s. The interactive influences of environmental change and invasive species have been evident in areas of the Toronto Waterfront such as Humber Bay (HB), an area of intensifying urbanization. Dreissenid mussels were first detected in HB in 1994, became abundant in 2000, and were followed by the round goby in 2005. Both invasions overlapped with increasing population density in the watershed and efforts to reduce pollutant and nutrient inputs. Benthic invertebrate diversity increased post-dreissenids, with densities of gastropods and amphipods increasing during peak mussel abundance (2000–2003). Dreissenid abundance declined after 2005, and snails (Valvatidae) disappeared, corresponding with increasing goby abundance. Abundance of sphaeriid clams declined over the monitoring period. Water clarity increased and the chlorophyll a level declined in the mid-1990s, before dreissenids were abundant at the site. Trophic conditions inferred from invertebrate composition shifted from oligotrophic in 1990 to mesotrophic and back to oligotrophic in 2012. There was little change in sediment quality other than a subtle increase in clay and fine silt at the primarily silty site. The trajectory of changes in the benthos was more strongly correlated with the dreissenid and goby invasions than changes in environmental quality.     

Ontogenetic changes in pharyngeal morphology correlate with a diet shift from arthropods to dreissenid mussels in round gobies (Neogobius melanostomus)

The potential of predators to regulate populations of dreissenid mussels (Dreissena polymorpha and Dreissena rostriformis bugensis) has been addressed since early in the dreissenid invasion of North America. Round gobies (Neogobius melanostomus) larger than approximately 60 mm have been shown to prey extensively on dreissenids, whereas smaller round gobies feed mainly on aquatic insects and crustaceans. We propose that ontogenetic changes in pharyngeal morphology may contribute to this diet shift in round gobies. Pharyngeals of 69 round gobies ranging from 31 to 164 mm total length were investigated using light microscopy and scanning electron microscopy. Areas of lower pharyngeals and pharyngobranchial 2 increased allometrically with fish length. Pharyngeals of round gobies smaller than 50 mm contained narrow (< 0.1 mm diameter) papilliform teeth that are consistent with eating soft-bodied prey. By the time round gobies reached approximately 80 mm in length, pharyngeals contained larger diameter (0.3-0.5 mm) molariform teeth typical of those found in molluscivorous fish. Pharyngeal teeth of the largest round gobies also showed considerable wear. Although changes in pharyngeal morphology may contribute to the previously described diet shift in round gobies as they age, genetic and environmental factors both likely influence pharyngeal remodeling and therefore merit further investigation.     

A biological regime shift in the Bay of Quinte ecosystem (Lake Ontario) associated with the establishment of invasive dreissenid mussels

Thirty-two biological variables (taxonomic and/or functional groups) representing the four major communities, phytoplankton, zooplankton, benthos and fish, characterizing the upper Bay of Quinte (Lake Ontario) ecosystem, were assembled for the 27-year period, 1982-2008. Coincident regime shifts were detected in phytoplankton, benthos, and fish in 1995, which was just after invasive zebra mussels (Dreissena spp.) became established in the bay in 1993-1994. Two independent methods were used to detect these shifts: 1) principal components analysis followed by a Regime Shift Detector test for a change point in the running mean of the first principal component scores and (2) measurements of significant difference between pre- and post-Dreissena ecosystem structure based on measures of Bray-Curtis community similarity. Although a statistically significant shift was not detected in the zooplankton community by itself, zooplankton variables were instrumental in the overall ecosystem shift, determined for the combined four communities. All 32 variables were ranked for their individual contribution to the difference between the pre- and post-Dreissena ecosystem structures. The resolution of two distinct ecosystem structures, pre- and post-Dreissena, was greatly improved after employing a novel method of variable optimization that involved a selective and sequential removal of variables contributing least to the statistical difference between pre- and post-Dreissena ecosystem structures. The resultant 20-variable subset defined a 1995 ecosystem regime shift at very high level of statistical confidence (ANOSIM-R = 0.970).     

Laboratory evaluation of spawning substrate type on potential egg predation by round goby (Neogobius melanostomus)

Spawning habitat structure may protect demersal eggs of broadcast spawning species from depredation. Egg predation by round goby (Neogobius melanostomus) is commonly referenced as a concern associated with their invasion of the Laurentian Great Lakes. Whereas nest-building species have received some attention in egg predation studies, broadcast spawning species may be particularly vulnerable as they do not guard their eggs. This study used a 2?×?7 factorial experiment to investigate how substrate characteristics influenced the rate that fertilized eggs are lost when exposed to round goby. Eggs for two recreationally important broadcast spawning species, northern pike (Esox lucius) and muskellunge (Esox masquinongy), were placed within spawning habitat treatments (bare, silt, sand, rubble, gravel, filamentous algae, and submerged aquatic vegetation) representing a range (least to most complex) of habitat complexity. Regardless of substrate type, egg loss was similar between the two esocid species. Across all substrates and species, the number of eggs lost varied two-fold over the 24-hour experiment, with the lowest rate of egg loss observed in the most complex substrate (submerged aquatic vegetation) and the highest over the least complex substrates (bare and silt). Both northern pike and muskellunge are known to spawn over structurally complex submerged aquatic vegetation and filamentous algae. Although eggs spawned over these substrates likely offer some protection from predation, eggs that settle over less complex ancillary habitats may face higher predation risk.     

First report of tubenose goby (Proterorhinus semilunaris) in the eastern basin of Lake Erie

The tubenose goby (Proterorhinus semilunaris), native to the Ponto-Caspian region, was first discovered in the Laurentian Great Lakes in 1990 after it was introduced through ballast water discharge. Compared with Neogobius melanostomus, another exotic gobiid from the Ponto-Caspian, colonization of the Great Lakes by P. semilunaris has been slow, with reports of the species being largely confined to the Huron-Erie Corridor (HEC) and western portions of Lake Erie and Lake Superior. This is the first report of P. semilunaris in the Great Lakes east of the western basin of Lake Erie. Between 28 June and 27 July, 2012, 176 P. semilunaris were collected from shallow (< 1.2 m) water of Marina Lake, a 40 ha embayment in Presque Isle State Park (Erie, PA). The large number of P. semilunaris collected at the site and the presence of individuals as small as 17 mm total length suggest an established population. However, the mechanism by which P. semilunaris was introduced to Presque Isle Bay is not clear.     

Impacts of predation by the Eurasian round goby (Neogobius melanostomus) on molluscs in the upper St. Lawrence River

An invasive Eurasian fish, the round goby Neogobius melanostomus, has recently spread from the Great Lakes into the St. Lawrence River. We quantified prey preferences of this benthivore and determined whether its predatory impacts on molluscs in the river are similar to those in the Great Lakes. We measured the size structure of gastropods and dreissenid mussels at 13 St. Lawrence River sites where round goby densities ranged from 0 to 6 m^− 2. For four of these sites, data were available for multiple years before and after invasion. Contrary to studies in the Great Lakes, there were no consistent effects of round goby density on the size structure of dreissenids, although there was an ontogenetic diet shift toward dreissenids. However, the abundance and richness of small gastropods (≤ 14 mm) was negatively correlated with round goby density across all sites, and declined over time at three of four sites sampled before and after invasion. Median gastropod size also declined across sites with increasing round goby density. Gastropods (as well as chironomid larvae, caddisfly larvae, and ostracods) were consistently among the most preferred prey items consumed by gobies, whereas dreissenids (as well as leeches and freshwater mites) were consistently avoided. These results indicate the major role of the round goby in structuring gastropod populations in the St. Lawrence River, and highlight large-scale spatial variation in its predatory impact on dreissenid populations.