Round Goby Research Articles

Municipal wastewater effluent affects fish communities: A multi-year study involving two wastewater treatment plants

Although effluent from municipal wastewater treatment plants (WWTPs) is a major stressor in receiving environments, relatively few studies have addressed how its discharge affects natural fish communities. Here, we assessed fish community composition over three years along a gradient of effluent exposure from two distinct WWTPs within an International Joint Commission Area of Concern on the Great Lakes (Hamilton Harbour, Canada). We found that fish communities changed with distance from both WWTPs, and were highly dissimilar between sites that were closest to and furthest from the wastewater outfall. Despite differences in the size and treatment technology of the WWTPs and receiving habitats downstream, we found that the sites nearest the outfalls had the highest fish abundances and contained a common set of signature fish species (i.e., round goby Neogobius melanostomus, green sunfish Lepomis cyanellus). Non-native and stress tolerant species were also more abundant near one of the studied WWTPs when compared to the reference site, and the number of young-of-the-year fish collected did not vary along the effluent exposure gradients. Overall, we show that fish are attracted to wastewater outfalls raising the possibility that these sites may act as an ecological trap.

Have a break or keep going - behavioral and metabolic overwintering strategies of two invasive species of the river Rhine, Germany

Winter conditions, extended periods of low temperature and limited food availability, impose pressures to develop strategies to cope with such unfavorable environmental conditions. Overwintering strategies may embrace both behavioral and physiological adaptations and result in differential responses. To address the effects of low temperature and feeding treatments (feeding or starving) in two common invasive goby species of the River Rhine, the round goby (Neogobius melanostomus) and the monkey goby (Neogobius fluviatilis) we monitored locomotor activity and measured oxygen consumption at 5 °C in monthly intervals for 90 days. Under still warm water conditions at 15 °C as a baseline, locomotor activity did not differ between the two species, but N. melanostomus had a higher metabolic rate. During the course of the 5 °C cold overwintering period, the behavioral responses were strikingly different between the two species: N. melanostomus maintained higher activity levels regardless of the feeding treatment, while N. fluviatilis showed a lower activity with a strong response to the feeding treatment. After 90 days of simulated winter condition, the starved groups of both species increased their locomotor activity. Oxygen consumption on the other hand showed no differences between fed and starved N. fluviatilis. In contrast, N. melanostomus had a stronger response to temperature per se and for starving individuals consistently lower metabolic rates. We conclude that the two locally most abundant fish species adopted two different overwintering strategies. N. fluviatilis applies a low energy expenditure strategy resulting in a high survival rate even under starvation. N. melanostomus applies a more risky high energy expenditure strategy with increased activity levels under extended periods of starvation.

Oocyte hydration in round goby Neogobius melanostomus from the Gulf of Gdańsk: another invasive strategy?

Summary The round goby (Neogobius melanostomus) is one of the most abundant non-native species in the Gulf of Gdansk that use various strategies to increase fecundity and achieve invasive success. Oocyte hydration appears to be a specific osmotic adaptation of round goby's gametes for successful fertilization and hatching. The aim of this study was to demonstrate details of oocyte final maturation and hydration during the spawning-capable phase in round goby and also compare various indices such as 17β-estradiol (E2), hepatosomatic index (HSI) and gonadosomatic index (GSI) among oocyte final maturation stages. E2 fluctuated according to the development of oocyte batches. HSI did not show significant differences throughout the oocyte maturation. GSI was an inaccurate estimator of oocyte maturation in round goby. This study, for the first time, shows that hydration does accompany oocyte final maturation in this species. These findings are important to the cognition of biology and the adaptive strategies of this invasive species to environmental factors and can be essential components of the protection and preservation of native living resources in the Gulf of Gdansk.

Assessment of the Impact of Oil and Heavy Metal Pollution in the Azov Sea on the Condition of the Neogobius Melanostomus (Pallas, 1814)

This article presents the results of long-term monitoring of the oil pollution in the Azov Sea. Areas with long-term chronic pollution (Arabat and Kazantip bays) are identified. The Azov sea transport routes and the location of natural gas fields can be the reason of the oil pollution. In addition to chronic oil pollution, this area is contaminated with heavy metals in particular in bottom sediments. The biological indicator was chosen for a more accurate analysis - the condition of the bottom-dwelling fish Round goby (Neogobius melanostomus (Pallas, 1814). Comparison of the size-mass characteristics, biochemical indicators and morphophysiological parameters of Neogobius melanostomus in different years has shown a decrease in the quality of habitat conditions. This decrease may occur due to contamination of the territory with oil products and heavy metals.

Competition fluctuates across years and seasons in a 6-species-fish community: empirical evidence from the field

Competition not only occurs between species, but (probably more importantly) also among individuals within species, especially, for species occurring in high densities and when resources are limiting. Fluctuating densities and dietary overlap may account for massive ecological and evolutionary changes in fish communities. We sampled a mixed juvenile (0+ stages, being younger than 1 year of age) fish community at the Lower Rhine over three consecutive years, including native and newly established species (~ 64,000 captured individuals and ~ 4200 dissected fish). Our aim was to develop a new approach in which data on density, individual consumption rates, and diet overlaps (both intra- and interspecific) solely originating from the natural environment, were used and combined to estimate species-specific competitive strengths within and between seasons and species. The juvenile fish community at the food-limited Rhine was dominated by invasive gobies. Species-specific consumption rates decreased depending on the dietary overlap with other juveniles. Two invasive gobies (Neogobius melanostomus and N. fluviatilis) showed the greatest competitive abilities, followed by a third invasive Gobiid species (Ponticola kessleri) and the native Aspius aspius, a species utilizing an exclusive food source. Both other native species (Perca fluviatilis and Sander lucioperca) had the lowest estimates of food-related competitive strength, indicating that these species are forced into a juvenile competitive bottleneck by the invasive gobies, before they finally can develop into successful predators on gobies later in life. Our new analytical approach to measure dietary competitive strength among individuals provides a powerful tool to empirically study eco-evolutionary feedback dynamics in the field.

Fatty acids differentiate consumers despite variation within prey fatty acid profiles

Abstract Techniques that biochemically trace foraging habits of predators rely on the assumption that intra‐specific variation in prey species is smaller than variation among them. At the same time, these techniques often show that diets can induce drastic changes in the biochemical profiles of prey species, especially across different ecosystems. We tested if intra‐specific variation in fatty acid profiles of prey species added enough noise to confound quantitative fatty acid signature analysis (QFASA) using a controlled feeding experiment. Steelhead trout (Oncorhynchus mykiss) were fed either alewife (Alosa pseudoharengus) or round goby (Neogobius melanostomus) from either Lake Ontario or Cayuga Lake for a period of 8 weeks. Fatty acid profiles were significantly different between prey species and between lake of origin within each species. Differences in fatty acid profiles of steelhead trout strongly reflected prey species differences, whereas differences related to prey origin (lakes) were noted at a much lesser extent. QFASA performed remarkably well given the differences observed between the lakes prey originated from. Our results indicate that QFASA models for steelhead trout are probably not specific to one lake, and could provide estimates for other freshwater systems where alewife and round goby serve as the primary forage.

Shelter competition between mature non-indigenous western tubenose goby (Proterorhinus semilunaris) and immature invasive round goby (Neogobius melanostomus) for plants and rocks

Non-indigenous western tubenose goby Proterorhinus semilunaris and round goby Neogobius melanostomus have rapidly increased in numbers and displaced native ichthyofauna in invaded ecosystems. This displacement has been predominantly linked to competition for resources and shelters. Similarly, P. semilunaris was displaced in many localities after introduction of N. melanostomus but the mechanisms of displacement are still not clear. Within the present study shelter competition laboratory studies were completed with P. semilunaris as resident fish before the introduction of either a sized matched P. semilunaris or N. melanostomus intruder. The fish competed for shelters which resembled a rock or plant. Time in shelter and accounts of direct aggression, indirect aggression and the guarding of shelter were also recorded for each fish. Shelter competitions showed P. semilunaris spent more time in plant shelters but N. melanostomus spent similar time periods in both shelter types. In intraspecific pairings, resident fish spent significantly more time in the shelters and exhibited significantly higher direct aggression and shelter guarding, especially for plant shelters. In interspecific pairings, there was no significant difference in the time spent in shelter by resident P. semilunaris and intruder N. melanostomus in either shelter type at day or night. However, in interspecific pairings, the intruder fish showed significantly higher levels of all aggression types. In intraspecific pairing, the concept of ‘resident wins’ was observed. However, in interspecific pairings, N. melanostomus was able to displace the resident fish. The higher aggression of N. melanostomus in shelter competition could account for greater invasive success and the reduction of P. semilunaris observed in the wild.

Documentation of Freshwater Mussels (Unionidae) in the Diet of Round Gobies (Neogobius melanostomus) within the French Creek Watershed, Pennsylvania

The round goby (Neogobius melanostomus) was discovered in 2013 in the French Creek (Allegheny River Drainage) watershed and is the first documented invasion outside of the Lake Erie basin in Pennsylvania. The round goby throughout the Laurentian Great Lakes is known to eat dressenid mussels (Dreissenidae), but consumption of either dressenids or native mussels (Unionidae) in tributaries to Lake Erie is minimal based on low populations of any bivalves. The French Creek watershed, on the other hand, harbors 29 species of native freshwater mussels as well as introduced fingernail clams (Sphaeriidae; Corbicula fluminea). The objectives of this study were to document the diet of round gobies in the French Creek watershed to determine whether consumption of native freshwater mussels was occurring. Round gobies were collected in the summer months (May–Sept.) of 2016 via kick seine in four locations, dissected, and their stomach contents identified to lowest possible taxa. We separated the gobies into categories based on length classes, in order to determine if diet changed with increased size and age. Unionid mussels were consumed by all length classes, particularly in length class one (30–44 mm), but diet shifted to a dominance of sphaeriids in length class four (≥75 mm). Round gobies also consumed benthic aquatic insects, a large percentage of which were chironomids (greater than 24% in all size classes). This is the first documentation of unionid consumption by the round goby in Pennsylvania that poses possible threats to native mussels in the French Creek watershed.

Investigation of round goby viral haemorrhagic septicaemia outbreak in New York

Eleven viral haemorrhagic septicaemia virus (VHSV) genotype IVb isolates were sequenced, and their genetic variation explored to determine the source of a VHS outbreak on the eastern shore of Cayuga Lake. An active fish kill of round gobies (Neogobius melanostomus, Pallas) was intensively sampled at King Ferry, NY and nearby Long Point State Park in May 2017. Gross lesions observed on 67 moribund round gobies and two rock bass (Ambloplites rupestris, Rafinesque) included moderately haemorrhagic internal organs and erythematous areas on the head, flank, and fins. RT-qPCR tests for VHSV were positive for all 69 fish. Viral isolation on epithelioma papulosum cyprinid cells showed cytopathic effect characteristic of VHSV for six round goby samples from King Ferry. The complete nucleotide sequence of the VHSV IVb genomes of five Cayuga Lake round goby isolates were derived on an Illumina platform along with 2017 VHSV IVb isolates from round gobies collected from the following: Lake Erie near Dunkirk, NY; the St. Lawrence River near Clayton and Cape Vincent, NY; and Lake St. Lawrence near Massena, NY. The phylogenetic tree created from these aligned sequences and four other complete VHSV IVb genomes shows Cayuga Lake isolates are closely related to the Lake Erie isolates.

Diversity and distribution of larval and juvenile fish in nearshore waters of the Southeastern Caspian Sea and Gorgan Bay

Occurrence and distribution of larval and juvenile fish was investigated in nearshore waters of the southeastern Caspian Sea including the Gorgan Bay. Samples were collected during seven surveys using a fine mesh beach-seine in five sampling sites from July 2014 to June 2015. A total of 15500 individuals mostly juvenile were captured, representing 18 species from 8 families. Most juveniles belonged to Liza saliens, Atherina caspia, Neogobius pallasi and Neogobius melanostomus. The juveniles of Alburnus chalcoides, Vimba persa, Syngnathus caspius and gobies Neogobius caspius and Ponticola syrman occurred in low number (relative abundance 0.1% - 9.3%). Highest number of juveniles were caught in summer season. The occurrence of larval fish in coastal waters of southern Caspian Sea was recorded for the first time (Atherina caspia with minimum length of 12 mm in Caspian and 9.5 mm in Gorgan Bay). In spring, early juveniles of mullet, Liza aurata, increased in the Caspian sites but they moved up to deeper waters by the end of the season. Based on results, the density of juvenile fish (specially Liza saliens) in Gorgan Bay was several times more than Caspian sites so it could be considered an important nursery ground in the region.

Coalescent models characterize sources and demographic history of recent round goby colonization of Great Lakes and inland waters.

The establishment and spread of aquatic invasive species are ecologically and economically harmful and a source of conservation concern internationally. Processes of species invasion have traditionally been inferred from observational data of species presence/absence and relative abundance. However, genetic‐based approaches can provide valuable sources of inference. Restriction site‐associated DNA sequencing was used to identify and genotype single nucleotide polymorphism (SNP) loci for Round Gobies (Neogobius melanostomus) (N = 440) from 18 sampling locations in the Great Lakes and in three Michigan, USA, drainages (Flint, Au Sable, and Cheboygan River basins). Sampled rivers differed in size, accessibility, and physical characteristics including man‐made dispersal barriers. Population levels of genetic diversity and interpopulation variance in SNP allele frequency were used in coalescence‐based approximate Bayesian computation (ABC) to statistically compare models representing competing hypotheses regarding source population, postcolonization dispersal, and demographic history in the Great Lakes and inland waters. Results indicate different patterns of colonization across the three drainages. In the Flint River, models indicate a strong population bottleneck (<3% of contemporary effective population size) and a single founding event from Saginaw Bay led to the colonization of inland river segments. In the Au Sable River, analyses could not distinguish potential source populations, but supported models indicated multiple introductions from one source population. In the Cheboygan River, supported models indicated that colonization likely proceeded from east (Lake Huron source) to west among inland locales sampled in the system. Despite the recent occupancy of Great Lakes and inland habitats, large numbers of loci analyzed in an ABC framework enable statistically supported identification of source populations and reconstruction of the direction of inland spread and demographic history following establishment. Information from analyses can direct management actions to limit the spread of invasive species from identified sources and most probable vectors into additional inland aquatic habitats.

Comparative analysis of sound production between the bighead goby Ponticola kessleri and the round goby Neogobius melanostomus: Implications for phylogeny and systematics

Divergence in acoustic traits between closely related species can be explained by phylogenetic history. In gobies, phylogenies reconstructed with acoustic signals primarily overlap with studies based on morphological or molecular data. Here, sound production of the two Ponto-Caspian gobies, Neogobius melanostomus and Ponticola kessleri, was recorded in controlled conditions and compared to determine the degree of interspecific acoustic variation across benthophilin gobies. Both species produced tonal-like sounds characterized by unique temporal and spectral properties during agonistic and reproductive intraspecific interactions, while the acoustic comparison revealed that the vocalizations of these two species differ in almost every acoustic property. N. melanostomus vocal structure was characterised by short (c. 100 ms), low-frequency (< 100 Hz) tonal sounds repeated at a relatively faster rate, while P. kessleri sounds appeared as a broadband, downward frequency modulated longer calls (c. 450 ms). Some acoustic features (sound rate and duration) proved to be stereotyped and could be considered a species-specific trait that could potentially be utilized to discriminate males or used by females for mate assessment. The tonal sounds appear to have a deeper origin within the Benthophilinae subfamily, as all acoustically investigated species to date have been able to produce this sound type. The recorded vocal repertoire represents a baseline for future comparative acoustic studies among the benthophilin gobies, aiming to gain additional information on the evolution of acoustic communication within Ponto-Caspian gobies and highlighting their importance in reconstructing phylogenetic relationships.

Three new records of fishes and their parasite fauna from Pomeranian Bay, Baltic Sea

This paper reports the occurrence of three new fish species, extremely rare in the Baltic Sea, and provides new data on their parasite fauna. The fish collected were Barbus barbus (Linnaeus, 1758), Salvelinus fontinalis (Mitchill, 1814), and Scophthalmus rhombus (Linnaeus, 1758). Their taxonomic identity was confirmed through genetic analyses using DNA extracted from fin clips. The stomach contents were examined, and age was determined by otolith or scale readings. Parasitological examinations focused on the skin, vitreous humour, eye lenses, mouth and nasal cavities, gills, gonads, viscera, and muscles. Furthermore, the changes of fish species composition over 20 years in Pomeranian Bay were analysed. The sequence comparisons against GenBank records revealed that sequences obtained for B. barbus and S. rhombus from Pomeranian Bay represent new rhodopsin barcodes. Record of the juvenile B. barbus in this study is the first in the Baltic Sea. Scophthalmus rhombus is a new host for the ciliate Trichodina jadranica, while Neogobius melanostomus (Pallas, 1814) is a new food item in the diet of S. rhombus in the Baltic Sea. Acanthocephalan Pomphorhynchus laevis found in the adult specimen of B. barbus and myxozoan Myxobolus musculi, noted in the juvenile specimen, have not been recorded previously in this fish species in Polish waters. Species collected as bycatch can potentially be used for monitoring potential changes in the overall fish community structure and biodiversity.

Parasites of round goby, Neogobius melanostomus, currently invading the Elbe River

Abstract The round goby, Neogobius melanostomus, is a Ponto-Caspian fish species currently found in many parts of Europe, including the North Sea riverine deltas. The objective of this study was to examine the parasite community of fish caught in the lower Elbe (Süderelbe – tidal zone; Geesthacht – non-tidal) in Germany and compare it with published data from the upper Elbe (Ústí nad Labem) in the Czech Republic. Twelve parasite taxa were recorded in the lower Elbe, six in the Süderelbe and nine near the city of Geesthacht. Süderelbe fish were mainly infected with Angullicola crassus larvae, while gobies from Geesthacht – with glochidia and sporadically occurring Pomporhynchus laevis, and the opposite situation was observed at Ústí nad Labem. It appears that a large tidal weir at Geesthacht significantly contributes to the division of the round goby population, with the Geesthacht parasite community being more similar to that at Ústí nad Labem than the one from the Süderelbe, thus increasing the likelihood that shipping from Hamburg was the introduction vector to Ústí nad Labem. We also recorded Acanthocephalus rhinensis in the Elbe for the first time, and in a new host – the round goby. Thus, round gobies may represent a new vector for the introduction of this parasite along the Elbe.

DNA Methylation Patterns in the Round Goby Hypothalamus Support an On-The-Spot Decision Scenario for Territorial Behavior.

The question as to how early life experiences are stored on a molecular level and affect traits later in life is highly topical in ecology, medicine, and epigenetics. In this study, we use a fish model to investigate whether DNA methylation mediates early life experiences and predetermines a territorial male reproductive phenotype. In fish, adult reproductive phenotypes frequently depend on previous life experiences and are often associated with distinct morphological traits. DNA methylation is an epigenetic mechanism which is both sensitive to environmental conditions and stably inherited across cell divisions. We therefore investigate early life predisposition in the round goby Neogobius melanostomus by growth back-calculations and then study DNA methylation by MBD-Seq in the brain region controlling vertebrate reproductive behavior, the hypothalamus. We find a link between the territorial reproductive phenotype and high growth rates in the first year of life. However, hypothalamic DNA methylation patterns reflect the current behavioral status independently of early life experiences. Together, our data suggest a non-predetermination scenario in the round goby, in which indeterminate males progress to a non-territorial status in the spawning season, and in which some males then assume a specialized territorial phenotype if current conditions are favorable.

Concentrations of melatonin, thyroxine, 17β-estradiol and 11-ketotestosterone in round goby (Neogobius melanostomus) in different phases of the reproductive cycle

The aim of this study was to determine changes in concentrations of melatonin (Mel) and thyroxine (T4) in plasma, and 17β-estradiol (E2) and 11-ketotestosterone (11-KT) in plasma and gonads of female and male round gobies (Neogobius melanostomus) from the Southern Baltic Sea in four phases of the reproductive cycle classified as pre-spawning, spawning, late spawning and non-spawning periods. The concentrations of Mel, T4 and E2 were determined by radioimmunoassay (RIA) whereas 11-KT was quantified using an enzyme immunoassay (EIA). The maturity stage of gonads was determined using histological analyses. The pattern of changes in Mel concentrations of females and males was similar with the greatest concentrations in the spawning and non-spawning phases. In both sexes, there was a similar tendency of change in concentrations of T4 and E2 with the increase being in the pre-spawning and non-spawning phases. The greatest concentrations of 11-KT were observed in the plasma and gonads of males during the spawning phase. In females, there were no changes in 11-KT concentrations either in plasma or gonads during all phases where quantifications occurred. This is the first study for determination of the pattern of changes in Mel and T4 concentrations as well as gonadal steroids E2 and 11-KT, supported by histological analysis of gonads, in batch spawning fish during the reproductive cycle.

Spatial variation in trophic structure of nearshore fishes in Lake Michigan as it relates to water clarity

Nearshore water clarity, as measured by remotely sensed Kd(490), and stable C and N isotopes of several nearshore fishes differed across the Lake Michigan basin. Values of δ13C of round goby (Neogobius melanstomus), yellow perch (Perca flavescens), and spottail shiner (Notropis hudsonis) were depleted in the southeast where water clarity was low relative to the southwest where water clarity was greater. Bayesian analyses were used to evaluate spatial variation in diet composition and quantify the relationship between water clarity and the proportional importance of pelagic energy in fish diets. Water clarity in nearshore areas is likely related to variable riverine inputs, resuspension, and upwelling processes. While these processes may not directly impact δ13C or δ15N of nearshore fishes, we hypothesize that water clarity differentially affects benthic and pelagic algal production. Lower water clarity in the benthos and subsequently lower benthic productivity may be related to regional diet differences and increased reliance on pelagic energy sources. Mobile fishes such as alewife (Alosa pseudoharengus) may not be in isotopic equilibrium with regional prey sources and depart from spatial patterns observed in other nearshore fishes.

Individual and spatial variation are as important as species‐level variation to the trophic complexity of a lentic food web

AbstractEcological complexity may improve ecosystem function, stability and adaptability to natural and anthropogenic disturbances. Intraspecific trophic variation can represent a significant component of total community variation and can influence food web structure and function. Thus, understanding how trophic niches are partitioned between intraspecific and interspecific processes could improve our understanding of food web dynamics. We examined gut contents, fatty acids and stable isotope ratios in round goby (Neogobius melanostomus) and yellow perch (Perca flavescens) across six sites in Lake Michigan, USA, to determine patterns in intra‐ and interspecific trophic composition (i.e., mean gut or fatty acid composition) and diversity (i.e., the diversity of gut items or fatty acids). We also examined relationships between fatty acid diversity and gut content characteristics to understand potential mechanisms shaping individual trophic phenotypes. There was significant variation in both trophic composition and diversity among sites, and individual and spatial variation was as important to total trophic variation as species identity. Round goby that consumed dreissenid mussels had more diverse fatty acid profiles than those that consumed other benthic invertebrates, whereas yellow perch fatty acid diversity was not related to gut content composition. Our results confirm that intraspecific variation in resource use can be as important to trophic dynamics as interspecific variation, and that spatial variation in lower level food web processes or habitat may strongly structure local food web dynamics. Individual‐level examination of trophic diversity, in concert with trophic composition, could provide additional information about the resilience, function and adaptability of local food webs.

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.

Population density contributes to the higher functional response of an invasive fish

Invasive species often have higher functional responses than native analogs. A better understanding of the functional response of invasive species could help explain the effect of these species on native prey communities, and aid in the development of tools for predicting invasion impact. We investigated the role of population density and location along the invasion pathway in determining the functional response of an invasive fish. We used invasive round goby (Neogobius melanostomus) collected from the Trent-Severn Waterway in Ontario, Canada to test for differences in the functional response of fish from high and low density sites, and from different locations along the invasion pathway. We also compared the functional response of round goby with that of northern logperch (Percina caprodes), a native species occupying the same niche. The density of the invasive predator population influenced its functional response, with attack rate being significantly higher for fish from high-density sites compared to fish from low-density sites. Functional response of individuals living at the invasion front was not significantly different from those living in established areas. The higher functional response of round goby compared to its native analog was associated with shorter handling time. Patterns in functional response curves were not explained by differences in diet among wild populations. Our research demonstrates that population density influences the functional response of an invasive species, and therefore should be accounted for when evaluating the functional response of a potential invader.