Predation of zebra mussels by round gobies, Neogobius melanostomus

Preliminary gut analysis of a recent Great Lakes invader, the round goby, Neogobius melanostomus (7.0–8.4 cm), collected from the Detroit River, showed that they ate zebra mussels (58%), snails (6%), and other invertebrates (36%), including aquatic insects (Hexagenia), softshelled crayfish, and zooplankton. Because zebra mussels, Dreissena polymorpha, predominated as prey, we investigated the ability of round gobies to consume different size classes of zebra mussels. In laboratory experiments, we examined feeding preferences of three size classes of round gobies (5.5–6.9 cm; 7.0–8.4 cm; 8.5–10.3 cm standard length) on four different size classes of zebra mussels (6.0–9.9 mm, 10.0–12.9 mm, 13.0–15.9 mm, 16.0–18.9 mm). All sizes of round gobies ate zebra mussels < 10.0 mm. Only the largest size class of round gobies ate larger zebra mussels (10.0–12.9 mm) when all prey sizes were presented. The association between the total mass of zebra mussels available and the amount consumed by round gobies increased positively up to about 6.5 g of available mussels and then levelled off. Round gobies consumed an average of 1.0 g of mussels in 24 h. There was a significant positive relationship between gape size and standard length of round gobies. Although larger round gobies (over the size range of fish in our study) are able to consume larger zebra mussels, small mussels were preferred. Our findings suggest that the preference of small zebra mussels by round gobies has the potential to alter the size structure of zebra mussel populations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Diet composition and consumption rate in round goby (<Emphasis Type="BoldItalic">Neogobius melanostomus</Emphasis>) in its expansion phase in the Trent River,Ontario

We assessed density, gut fullness and prey composition of round goby (Neogobius melanostomus) from three areas in the Trent River (Ontario) representing areas of initial introduction and subsequent expansion. Round goby had been present at the area of original introduction since 2003, and by 2007/2008, their range had expanded upstream and downstream into the outermost reaches sampled in the study. Catch per unit angling effort in nearshore sites indicated that round goby density in the area of original introduction was more than double their density in the upstream expansion area and nearly three times the density in the downstream expansion area. Gut fullness index was lower in gobies from the area of original introduction than for those at the upstream and downstream edges of their expanded range. The most dramatic difference in diet composition was with dreissenids, where large gobies (≥70 mm) occupying the area of original introduction had almost no dreissenid biomass in their guts, whereas dreissenids were the predominant prey type in gobies occupying the two expansion areas. Post-hoc zebra mussel density in the area of original introduction was an order of magnitude lower than in the two expansion areas which, combined with the differences in stomach fullness and prey composition, suggest that local, density-related reduction of this prey type was occurring in the river.     

Increased parental care cost for nest-guarding fish in a lake with hyperabundant nest predators

Although parental care increases offspring survival, providingcare is costly, reducing parental growth and survival and, thereby,compromising future reproductive success. To determine if anexotic benthic predator might be affecting parental care bynest-guarding smallmouth bass (Micropterus dolomieu), we comparednest-guarding behavior and energy expenditures in two systems,one with a hyperabundant recently introduced predator, the roundgoby (Neogobious melanostomus). In Lake Erie, USA, smallmouthbass vigorously defended their nests from benthic round gobies.In Lake Opeongo, Canada, smallmouth bass were exposed to fewerand predominantly open-water predators and were less activein their nest defense. From scuba and video observations, wedocumented that nest-guarding smallmouth bass chased predators(99% of which were round gobies) nine times more frequentlyin Lake Erie than in Lake Opeongo. This heightened activityresulted in a significant decline in weight and energetic contentof guarding males in Lake Erie but no change in Lake Opeongomales. Bioenergetic simulations revealed that parental careincreased smallmouth bass standard metabolic rate by 210% inLake Erie but only by 28% in Lake Opeongo. As energy reservesdeclined and offspring became increasingly independent, malesin both lakes consumed more prey and spent more time foragingaway from their nests; however, nest-guarding smallmouth bassconsumed few prey and, in Lake Erie, rarely consumed round gobies.Therefore, increased parental care costs owing to the presenceof round gobies could affect future growth, reproduction, andsurvival if smallmouth bass approach critically low energy reserves.     

Terrestrial feeding in the Mudskipper Periophthalmus (Pisces: Teleostei): A cineradiographic analysis

Mudskipping gobies (Periophthalminae) are among the most terrestrial of amphibious fishes. Specializations associated with terrestrial prey capture and deglutition have been studied in Periophthalmus koelreuteri by light and X-ray cinematography which permits direct visualization of pharyngeal jaw movement during deglutition. Anatomical specializations of the pharyngeal jaws are described and include depressible teeth, a large ventral process on ceratobranchial five, and muscular modifications.
Multiple terrestrial feedings occur by Periophthalmus without a return to the water, and cineradiography reveals that the buccal cavity is often filled with air during terrestrial excursions in contrast to some previous hypotheses. Transport of the prey into the oesophagus occurs primarily by anteroposterior movement of the upper pharyngeal jaw. The lower pharyngeal jaw plays a limited role in food transport and may serve primarily to hold and position prey. The bite between upper and lower pharyngeal jaws occurs between the anterior teeth, and both jaws are protracted together during raking of food into the oesophagus. Functional specializations correlated with terrestrial feeding include obligatory use of pharyngeal jaws for swallowing even small prey items and positioning of the prey in the pharynx by pharyngeal jaw and hyoid movements alone.
This analysis of terrestrial feeding allows hypotheses of design constraints imposed by the aquatic medium on fishes to be raised and tested.     

Expansion of tubenose gobies <Emphasis Type="Italic">Proterorhinus semilunaris</Emphasis> into western Lake Erie and potential effects on native species

The Eurasian freshwater tubenose goby Proterorhinus semilunaris (formerly Proterorhinus marmoratus) invaded the Laurentian Great Lakes in the 1990s, presumably via ballast water from transoceanic cargo ships. Tubenose gobies spread throughout Lake St. Clair, its tributaries, and the Detroit River system, and also are present in the Duluth-Superior harbor of Lake Superior. Using seines and bottom trawls, we collected 113 tubenose gobies between July 2007 and August 2009 at several locations in western Lake Erie. The number and range of sizes of specimens collected suggest that that tubenose gobies have become established and self-sustaining in the western basin of Lake Erie. Tubenose gobies reached maximum densities in sheltered areas with abundant macrophyte growth, which also is their common habitat in native northern Black Sea populations. The diet of tubenose gobies was almost exclusively invertebrates, suggesting dietary overlap with other benthic fishes, such as darters (Etheostoma spp. and Percina sp.), madtoms (Noturus spp.), and sculpins (Cottus spp.). A single mitochondrial DNA haplotype was identified, which is the most common haplotype found in the original colonization area in the Lake St. Clair region, suggesting a founder effect. Tubenose gobies, like round gobies Neogobius melanostomus, have early life stages that drift owing to vertical migration, which probably allowed them to spread from areas of colonization. The Lake St. Clair-Lake Erie corridor appears to have served as an avenue for them to spread to the western basin of Lake Erie, and abundance of shallow macrophyte-rich habitats may be a key factor facilitating their further expansion within Lake Erie and the remainder of the Laurentian Great Lakes.     

Soluble nitrogen and phosphorus excretion of exotic freshwater mussels (Dreissena spp.): potential impacts for nutrient remineralisation in western Lake Erie

1. Recent increases in phytoplankton biomass and the recurrence of cyanobacterial blooms in western Lake Erie, concomitant with a shift from a community dominated by zebra mussels (Dreissena polymorpha) to one dominated by quagga mussels (D. bugensis), led us to test for differences in ammonia‐nitrogen and phosphate‐phosphorus excretion rates of these two species of invasive molluscs. 2. We found significant differences in excretion rate both between size classes within a taxon and between taxa, with zebra mussels generally having greater nutrient excretion rates than quagga mussels. Combining measured excretion rates with measurements of mussel soft‐tissue dry weight and shell length, we developed nutrient excretion equations allowing estimation of nutrient excretion by dreissenids. 3. Comparing dreissenid ammonia and phosphate excretion with that of the crustacean zooplankton, we demonstrated that the mussels add to nitrogen and phosphorus remineralisation, shortening nitrogen and phosphorus turnover times, and, importantly, modify the nitrogen and phosphorus cycles in Lake Erie. The increased nutrient flux from dreissenids may facilitate phytoplankton growth and cyanobacterial blooms in well‐mixed and/or shallow areas of western Lake Erie.     

Morphological convergence of pharyngeal jaw structure in durophagous perciform fish

This study investigated the ecomorphology of pharyngeal jaw structure and durophagy in three families of marine teleosts: the Sciaenidae, Haemulidae and Carangidae. Regressions of the bone and muscle mass of pharyngeal jaws were generated to elucidate the differences associated with eating hard-bodied and soft-bodied prey; within-family comparisons revealed significant differences in masses of bones and muscles involved with processing the former. Generally, the durophagous species − Trachinotus carolinus (Carangidae), Pogonias cromis (Sciaenidae) and Anisotremus surinamensis (Haemulidae) − had heavier and stronger pharyngeal toothplates and larger protractor pectoralis muscles, with masses of these musculoskeletal elements ranging from five times to nearly an order of magnitude larger than those of their soft-prey feeding relatives. Pogonias cromis and T. carolinus demonstrate convergence in the ontogeny and morphological modification of the pharyngeal toothplates and protractor pectoralis muscles that enhance crushing ability. In the Haemulidae, moderate size increases in a few pharyngeal jaw elements (and larger overall body size in A. surinamensis ) are sufficient for durophagy. Morphospace analysis of six species from the three families illustrates the strong functional association between the biomechanical properties of prey and the relative sizes of biting and transport mechanisms.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 80 , 147−165.     

Functional morphology of durophagy in black carp,Mylopharyngodon piceus

The black carp, Mylopharyngodon piceus (Osteichthyes: Cyprinidae), crushes its snail and other molluscan prey with robust pharyngeal jaws and strong bite forces. Using gross morphology, histological sectioning, and X‐ray reconstruction of moving morphology (XROMM), we investigated structural, behavioral, and mechanical aspects of pharyngeal jaw function in black carp. Strut‐like trabeculae in their pharyngeal jaws support large, molariform teeth. The teeth occlude with a hypertrophied basioccipital process that is also reinforced with stout trabeculae. A keratinous chewing pad is firmly connected to the basioccipital process by a series of small bony projections from the base of the pedestal. The pharyngeal jaws have no bony articulations with the skull, and their position is controlled by five paired muscles and one unpaired median muscle. Black carp can crush large molluscs, so we used XROMM to compare pharyngeal jaw postures as fish crushed ceramic tubes of increasing sizes. We found that black carp increase pharyngeal jaw gape primarily by ventral translation of the jaws, with ventral rotation and lateral flaring of the jaws also increasing the space available to accommodate large prey items. A stout, robust ligament connects left and right jaws together firmly, but allows some rotation of the jaws relative to each other. Contrasting with the pharyngeal jaw mechanism of durophagous perciforms with fused left and right lower pharyngeal jaws, we hypothesize that this ligamentous connection may serve to decouple tensile and compressive forces, with the tensile forces borne by the ligament and the compressive forces transferred to the prey. J. Morphol. 276:1422–1432, 2015. © 2015 Wiley Periodicals, Inc.     

Predicting habitat use and trophic interactions of Eurasian ruffe,round gobies,and zebra mussels in nearshore areas of the Great Lakes

The Laurentian Great Lakes have been subject to numerous introductions of nonindigenous species, including two recent benthic fish invaders, Eurasian ruffe (Gymnocephalus cernuus) and round gobies (Neogobius melanostomus), as well as the benthic bivalve, zebra mussel (Dreissena polymorpha). These three exotic species, or “exotic triad,” may impact nearshore benthic communities due to their locally high abundances and expanding distributions. Laboratory experiments were conducted to determine (1) whether ruffe and gobies may compete for habitat and invertebrate food in benthic environments, and (2) if zebra mussels can alter those competitive relationships by serving as an alternate food source for gobies. In laboratory mesocosms, both gobies and ruffe preferred cobble and macrophyte areas to open sand either when alone or in sympatry. In a 9-week goby–ruffe competition experiment simulating an invasion scenario with a limited food base, gobies grew faster than did ruffe, suggesting that gobies may be competitively superior at low resource levels. When zebra mussels were added in a short-term experiment, the presence or absence of mussels did not affect goby or ruffe growth, as few zebra mussels were consumed. This finding, along with other laboratory evidence, suggests that gobies may prefer soft-bodied invertebrate prey over zebra mussels. Studies of interactions among the “exotic triad”, combined with continued surveillance, may help Great Lakes fisheries managers to predict future population sizes and distributions of these invasive fish, evaluate their impacts on native food webs, and direct possible control measures to appropriate species.     

Diet-shifts by an estuarine goby (Pomatoschistus microps) in the face of variable prey availability

Ontogenetic shifts in diet provide a mechanism for maximising fitness throughout development and are common where predators exhibit large increases in size. In order to maximise their fitness throughout development, benthic feeding fish can show diet-shifts that centre on the transition from meiofaunal to macrofaunal prey. Here we assessed whether such a shift was influenced by natural variation in prey-size availability by comparing the sizes of prey consumed by naturally foraging common gobies (Pomatoschistus microps). We tested explicitly for the presence of an ontogenetic shift by analysing the length of prey consumed and an index for prey importance for gobies of different lengths. We also tested the match between actual diets and those predicted by a foraging model. The goby size at which the diet-shift occurred was consistent among locations that differed in their availability of prey and through temporal changes in densities and types of prey. The mean sizes of ingested prey increased for gobies > 35 mm in length and the relative importance of macrofauna increased at 30 mm. The foraging model predicted that gobies > 30 mm would eat larger prey than would smaller gobies which differed from the observed changes in prey-size at 35 mm. Availability of prey did not appear to influence the lengths at which gobies changed diet but did affect the size of prey taken after the diet-shift. A relatively large abundance of large-bodied chironomids at two sites was reflected in the mean size of prey consumed by gobies > 30 mm at these sites. Our study indicates that intrinsic mechanisms can be more important than fluctuating environments in determining prey-choice and shifts in diet, although for the common goby, variability in prey-size may have implications for prey-choice later in ontogeny.     

Diel interactions between prey behaviour and feeding in an invasive fish, the round goby, in a North American river

1. We studied the diet of the invasive round goby (Neogobius melanostomus) on a diel basis in the Flint River, a warmwater stream in Michigan, U.S.A. Diet and available prey samples were collected seven times over a 24 h period in four consecutive months. The section of river studied lacked zebra mussels (Dreissena polymorpha), the primary prey of adult round gobies elsewhere in the Great Lakes region. 2. Diet changed on a diel basis with hydropsychid caddisfly and chironomid larvae predominating during the day, chironomid pupae dominating in the evening and heptageniid mayflies dominating at night. Simultaneous study of macroinvertebrate drift suggested that caddisfly and chironomid larvae were most likely picked from submerged rocks, chironomid pupae were most likely taken during their emergent ascent and mayflies were either captured from the drift or picked from rocks. 3. The Flint River lacks a diverse darter (Family: Percidae) and sculpin (Family: Cottidae) fauna and it appears that the round goby has occupied a generalised darter/sculpin niche. Our results indicate that round gobies have the potential to invade successfully riverine systems, particularly those lacking a diverse benthic fish assemblage.     

Ecological and evolutionary consequences of the trophic polymorphism in Cichlasoma citrinellum (Pisces: Cichlidae)

The neotropical cichlid fish Cichlasoma citrinellum is polymorphic in the structure of its pharyngeal jaw apparatus and external morphology. The pharyngeal jaws are either gracile and bear slender, pointed teeth (papilliform) or robust with strong, rounded teeth (molariform). Molariform morphs have a ‘benthic’, and papilliform morphs a ‘limnetic’ body form. Furthermore, this species is also polychromatic, with yellow and black morphs. The molariform morphology of the pharyngeal jaw apparatus adapts the fish for cracking and feeding on snails. Based on analysis of stomach contents, 94% of the molariform morph ate snails whereas only 19%, of the papilliform morph did so. This result suggests that the morphs occupy different ecological niches. The morphology of the pharyngeal jaw apparatus does not correlate significantly with sex, but it does with body colouration (P<0.005). Cichlasoma citrinellum mate assortatively with their own colour; therefore a mating preference for colour may lead to genetic isolation of trophic morphs. The frequency of the molariform morph differs strikingly among populations of five Nicaraguan lakes and its abundance is correlated with the abundance of snails, the fishes' principal prey item. Among populations the frequency of molariform morphs decreases in the dry season. Morphology possibly changes reversibly within particular individuals between seasons. These results suggest that phenotypic plasticity and polymorphisms may be an adaptive characteristic of cichlid fishes. Patterns of intraspecific morphological variation match patterns of interspecific morphological diversification which suggests that universal developmental mechanisms canalize the possible expressions of morphology. The ability to respond morphologically to environmental shifts, in conjunction with genetically determined trophic polymorphisms and sexual selection via mate choice, could be the basis for speciation through intermediate stages of polymorphism of the impressive adaptive radiation of cichlid fishes.     

Has the invasive round goby caused new links in Baltic food webs?

The Ponto-Caspian round goby (Neogobius melanostomus, Pallas 1814) most probably was established in the Gulf of Gdańsk, Baltic Sea, in the late 1980’s and has since become one of the dominant species in the region. In this study we assess the role of round gobies as prey for two important fish species in the Gulf of Gdańsk, cod (Gadus morhua) and perch (Perca fluviatilis). We compared their present diet with stomach analyses from the area prior the round goby establishment, as well as with diet analysis from Baltic regions where round gobies are absent. There were large differences in the diet between cods from the Gulf of Gdańsk 2003–2006 compared to cods in earlier studies (1977–1981) from the Southern Baltic Sea. There were also large differences in cod and perch diets from areas with and without round goby. Presently, round goby constitutes the most important prey for medium sized cods in Gulf of Gdańsk, and perch from the same area almost exclusively feed on gobiids. Stomach analysis, trophic level estimates, and stable isotope analyses all indicated that cod and perch in Gulf of Gdańsk after the round goby establishment belonged to a similar trophic level. Beside round goby, no mussel feeding fish contributed much to the diet of cod or at all to the diet of perch. Thus, it is likely that round gobies constitute a new energetic pathway from mussels to top predators. However, due to the short time elapsed after round goby establishment, we can only speculate on the species future impacts on Baltic food webs.     

Habitat Shift in Invading Species: Zebra and Quagga Mussel Population Characteristics on Shallow Soft Substrates

Unexpected habitat innovations among invading species are illustrated by the expansion of dreissenid mussels across sedimentary environments in shallow water unlike the hard substrates where they are conventionally known. In this note, records of population characteristics of invading zebra (Dreissena polymorpha) and quagga (Dreissena bugensis) mussels from 1994 through 1998 are reported from shallow (less than 20m) sedimentary habitats in western Lake Erie. Haphazard SCUBA collections of these invading species indicated that combined densities of zebra and quagga mussels ranged from 0 to 32,500 individuals per square meter between 1994 and 1998, with D. polymorpha comprising 75–100% of the assemblages. These mixed mussel populations, which were attached by byssal threads to each other and underlying sand-grain sediments, had size–frequency distributions that were typical of colonizing populations on hard substrates. Moreover, the presence of two mussel cohorts within the 1994 samples indicated that these species began expanding onto soft substrates not later than 1992, within 4 years of their initial invasion in western Lake Erie. Such historical data provide baselines for interpreting adaptive innovations, ecological interactions and habitat shifts among the two invading dreissenid mussel species in North America.     

Occupation,body size and sex ratio of round goby (<Emphasis Type="Italic">Neogobius melanostomus</Emphasis>) in established and newly invaded areas of an Ontario river

Invasive species represent a challenge because the particular characteristics of a species’ invasion are often unknown before the invasion. To provide some clarity as to how invasive species demographic structure might change as a population advances its range, we compared the proportion of occupied sites, size structure and sex ratio of round gobies in the area where they first invaded with more recently invaded areas at the extent of their range in a river in south-eastern Ontario. We used a standardized angling technique to sample gobies larger than 45-mm total length in the summer and early autumn of 2007. Round goby at the upstream and downstream extent of their range occupied a lower proportion of randomly selected sites, and contained a wider distribution of sizes as well as significantly larger individuals. Sex ratios in all areas were male-biased and the male-to-female ratio was significantly higher in the upstream segment of the river (2.2:1) compared to the area of first introduction (1.4:1). The difference between the newly invaded and the established sites suggests that round goby population structure may be affected by density. The results of this study help us further describe the demographic characteristics of biological invasions whilst examples of population structure and behaviour in gobies and other species provide a basis for generating hypotheses for range expansion.     

Ten polymorphic microsatellite markers in the invasive round goby (Neogobius melanostomus) and cross‐species amplification

Ten tetranucleotide, dinucleotide and compound microsatellite loci were isolated and characterized for the round goby, Neogobius melanostomus. Analysis of 64 gobies from one nonindigenous population in Lake Erie, Ontario, Canada, indicated that allele number varied from three to 12 per locus, while observed heterozygosity ranged between 0.33 and 0.86. Eight of these primers showed some amplification in other species in four genera. These newly developed microsatellite markers are a powerful tool that will provide insights into population structure and dispersal of the round goby in their novel environment.     

Rapid dispersal and establishment of a benthic Ponto-Caspian goby in Lake Erie: diel vertical migration of early juvenile round goby

The round goby, Apollonia melanostoma, a molluscivore specialist, was introduced to the Great Lakes in the early 1990s and rapidly expanded its distribution, especially in Lake Erie. Adult round goby morphology suggests low dispersal and migration potential due to the lack of a swim bladder and benthic life style. Given that the larval stage occurs inside the benthic egg, and juveniles have adult morphologies, it has been suspected that dispersal and invasion potential is low for early life stages also. However, we identified early juvenile round gobies in the nocturnal pelagic in Lake Erie and thus we conducted a sampling study to determine the extent to which this life stage uses the nocturnal pelagic. Replicate ichthyoplankton samples were collected at 3-h intervals (1900–0700 h) at three depths (2 m, 5 m, 8 m) in western Lake Erie (water depth = 10 m) in July and August 2002 and June 2006. Early juvenile round gobies (6–23 mm TL) were present almost exclusively in the nocturnal samples (2200 h, 0100 h, 0400 h) with peak densities approaching 60 individuals per 100 m³ of water sampled. Nocturnal density was also significantly greater at 8-m depth versus 2-m and only the smallest fish (6–8 mm TL) migrated to the surface (2-m). Analyses of diet clearly demonstrated that these fish are foraging on plankton at night and thus may not be light limited for foraging in ship ballast tanks. In ships that take on thousands of tonnes of water for ballast, nocturnal ballasting could easily result in transport of thousands of young round gobies at a time. Additionally, within-lake dispersal at this lifestage is likely common and may facilitate downstream passage across barriers designed to limit range expansion.     

Secondary invasion of the round goby into high diversity Great Lakes tributaries and species at risk hotspots: potential new concerns for endangered freshwater species

The round goby (Neogobius melanostomus) first invaded North America in 1990 when it was discovered in the St. Clair River. Despite more than 15 years of potential invasion, many Great Lakes’ lotic systems remained uninvaded. Recently, we captured the round goby from several Great Lakes tributaries known as species-at-risk hotspots. With a combination of field sampling of round gobies and literature review of the impact of round gobies on native taxa, we assess the potential impacts of the secondary invasion to native species using three mechanisms: competition; predation; and indirect impacts from the loss of obligate mussel hosts. We estimate that 89% (17/19) of benthic fishes and 17% (6/36) of mussels that occur in these systems are either known or suspected to be impacted by the secondary invasion of round goby. In particular, we note that the distribution of potential impacts of round goby invasion was largely associated with species with a conservation designation, including seven endangered species (1 fish, 6 mussels). As these recent captures of round goby represent novel occurrences in high diversity watersheds, understanding the potential impacts of secondary invasion to native biota is fundamental to prevent species declines and to allow early mitigation.     

Ontogenetic variability in the external morphology of monkey goby, Neogobius fluviatilis (Pallas, 1814) and its relevance to invasion potential

In the previous decade, four species of non-native gobies have invaded the middle section of the river Danube and its tributaries. An effective tool for understanding biological invasions is the evaluation of various biological traits (morphological, life history, ontogenetic) within an epigenetic context. The present study examines the external morphology of monkey goby Neogobius fluviatilis (Pallas, 1814) from the mouth of the River Hron, the morphological differences among three goby species (monkey, bighead and round) and the relevance of these differences for invasive potential. Monkey goby reach their definite phenotype very early in their ontogeny and thus represent a strongly precocial (specialized) species with direct development. The morphological differences between monkey and two other goby species also reflect its strong specialization for sandy substrata and smaller prey types. Thus, monkey goby are not expected to spread to new areas as fast as the round and bighead gobies, and their distribution is likely to be limited to sandy and/or sandy-gravel substrata. If this assumption is correct, then the potential adverse impact of monkey goby on native fauna or even ecosystem is likely to be less than that of the bighead and round gobies. Handling editor: K. Martens     

Living at the edge of the front; reduced predation risk to invasive round goby in a Great Lakes tributary

The objective of this research was to determine if the highly invasive round goby (Neogobius melanostomus) experiences lower predation risk during early stages of invasion. We compared round goby predation rates between a recently invaded area (occupied for ≈1 year) and a longer established area (≈7 years) of the Trent River, Ontario, Canada. Tethering trials were conducted in three habitat types, and comparable habitats in the two areas were similar in water temperature, velocity and depth. Predation rates of tethered round gobies were on average 27% lower in the recently invaded area. Reduced predation in the recently invaded area may be due to the short duration of round goby occupancy and/or differences in predator communities between the two study areas. Data before the round goby invasion suggest that predator communities were similar between the two range areas, but differences in predator abundance cannot be ruled out as a potential mechanism. Other possible mechanisms include a numerical or learned response by predators over time to a novel prey item. Reduced predation rate during the initial stages of invasion may contribute to the fitness of individuals that migrate into areas not previously occupied, and thus facilitate successful range expansion.