Round Goby Populations Research Articles

Relationships among round gobies, Dreissena mussels, and benthic algae in the south nearshore of Lake Ontario

Round goby (Neogobius melanostomus) populations are established in all the Great Lakes, subsequent to the success of the Dreissena mussel invasion. Additionally, localized resurgence of the nuisance alga, Cladophora glomerata, also has been reported across several of the Great Lakes post-Dreissena. We examined the relationship among round goby size and population density and Dreissena mussel metrics and Cladophora biomass in three regions of the nearshore environment of U.S. waters of Lake Ontario during summer 2008. We also estimated the size of the round goby population in these regions. Correlations between round goby population metrics and Dreissena mussel metrics suggest a low potential for lakewide control of mussel populations by round gobies. In August but not June, fish density was positively correlated with Dreissena density, biomass, and size, suggesting round gobies did not depress mussel numbers, but may selectively remove some smaller mussel size classes. Round goby length was positively correlated with Cladophora biomass, which probably is best explained as a sampling artifact. Seasonal changes in the round goby population density with depth were suggestive of an offshore migration in late summer. Fish were generally smaller but equally abundant, moving west-to-east along the nearshore environment of the lake. We estimated that the nearshore population size of round gobies was 0.7billion fish, equivalent to 2.2 fish/m2. This translated into approximately 1287tonnes of round gobies or about 40kg/ha. Coupled with the broad diet selection of this invasive fish, results suggest that round gobies will have a substantial impact on the structure of the nearshore benthic community.

The response of Grey Herons Ardea cinerea to changes in prey abundance

Capsule Herons responded to the lower abundance of a preferred fish by switching prey. Aims To investigate how Grey Herons respond to the decline in abundance of a preferred fish, the Round Goby Neogobius melanostomus. Methods Grey Herons Ardea cinerea breeding in the Gulf of Gdańsk area (Baltic Sea) were the study population. Regurgitated food and pellets were analysed. Biomass and abundance of fish was compared between 2000–02 (when the population of Round Goby was increasing) and 2008–09 (when the Round Goby population stabilized). Results The abundance and biomass of Round Goby preyed upon by herons decreased from between 95 and 99% in 2000–02 to between 38 and 56% in 2008–09. Energy-rich salmonids, not recorded in 2000–02, were an important diet component in 2008–09 (42% of biomass). Small-sized fish abundance was higher in 2008–09 (37%) than in 2000–02 (9%). Conclusions Grey Herons in 2008–09 adapted to the lower availability of Round Gobies by prey switching to other available fish – abundant but small-sized Three-spined Sticklebacks Gasterosteus aculeatus and less abundant but more profitable salmonids (easy to catch in the case of hatchery-reared, tagged individuals). Diet composition suggests that Grey Herons in 2008–09 exploited more diverse foraging areas (dry habitats, woods, garden ponds) than in 2000–02.

Invasive species are less parasitized than native competitors, but for how long? The case of the round goby in the Great Lakes-St. Lawrence Basin

There is increasing evidence that parasitism represents an unpredictable dimension of the ecological impacts of biological invasions. In addition to the risk of exotic pathogen transmission, other mechanisms such as parasite-release, could contribute to shaping the relationship between introduced species and native communities. In this study, we used the Eurasian round goby (Neogobius menalostomus) in the Great Lakes-St. Lawrence River ecosystem to further explore these ideas. As predicted by the parasite-release hypothesis, recently established populations of round goby were parasitized by a depauperate community of generalist helminths (8 taxa), all commonly found in the St. Lawrence River. In comparison, two native species, the logperch (Percina caprodes) and spottail shiner (Notropis hudsonius), were the hosts of 25 and 24 taxa respectively. Round gobies from each of 3 sampled localities were also less heavily infected than both indigenous species. This is in contrast to what is observed in round goby’s native range where the species is often the most parasitized among gobid competitors. This relative difference in parasite pressure could enhance its competitiveness in the introduced range. However, our study of an older population of round goby in Lake St. Clair suggests that this advantage over native species could be of short duration. Within 15 years, the parasite abundance and richness in the round goby has more than doubled whereas the number of parasite species per fish has increased to levels of those typical of fish indigenous to the St. Lawrence-Great Lakes watershed.

Occupation, body size and sex ratio of round goby (Neogobius melanostomus) 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.

Parasite fauna of native and non-native populations of Neogobius melanostomus (Pallas, 1814) (Gobiidae) in the longitudinal profile of the Danube River

Parasite fauna of round goby Neogobius melanostomus in the Danube River was investigated in both the native range (two sites in Bulgarian stretch of the Danube) and the non-native range of its distribution (Croatian, Slovak and Austrian stretches) during 2005 and 2006. The aim was to identify possible changes in parasite community associated with the introduction of a host into the new environment. A total of 29 metazoan parasite species were found to parasitize round goby in the Danube River. Out of them, twelve parasite species occurred in both the native and non-native range of round goby distribution. Introduction of a novel parasite species to the non-native range via round goby was not found. Eight parasite species occurred only in the native range and nine species occurred only in the non-native range of the round goby distribution. Loss of native parasite species in non-native round goby populations and/or acquiring of novel parasite species in new environment were not significant. Thirteen parasite taxa were recorded in round gobies for the first time. Three parasite taxa (Diplostomum spp., Pomphorhynchus laevis and Raphidascaris acus) occurred at each sampling site in both the native and non-native range in high prevalence and abundance. Parasite species diversity was assessed for each sampling site and season using three diversity indices (Shannon, Simpson and Equitability index) with the highest values found in non-native site in Slovakia (1.38, 0.69 and 0.60, respectively) and the lowest values found in one of the native sites in Bulgaria (0.28, 0.12 and 0.14, respectively, for the same season). Species diversity was higher in both non-native round goby populations (Slovak and Austrian) compared to native Bulgarian populations. However, diversity indices values varied among almost all sampling sites.

Evidence for predatory control of the invasive round goby

We coupled bioenergetics modeling with bottom trawl survey results to evaluate the capacity of piscivorous fish in eastern Lake Erie to exert predatory control of the invading population of round goby Neogobius melanostomus. In the offshore (>20 m deep) waters of eastern Lake Erie, burbot Lota lota is a native top predator, feeding on a suite of prey fishes. The round goby invaded eastern Lake Erie during the late 1990s, and round goby population size increased dramatically during 1999–2004. According to annual bottom trawl survey results, round goby abundance in offshore waters peaked in 2004, but then declined during 2004–2008. Coincidentally, round goby became an important component of burbot diet beginning in 2003. Using bottom trawling and gill netting, we estimated adult burbot abundance and age structure in eastern Lake Erie during 2007. Diet composition and energy density of eastern Lake Erie burbot were also determined during 2007. This information, along with estimates of burbot growth, burbot mortality, burbot water temperature regime, and energy densities of prey fish from the literature, were incorporated into a bioenergetics model application to estimate annual consumption of round goby by the adult burbot population. Results indicated that the adult burbot population in eastern Lake Erie annually consumed 1,361 metric tons of round goby. Based on the results of bottom trawling, we estimated the biomass of yearling and older round goby in offshore waters eastern Lake Erie during 2007–2008 to be 2,232 metric tons. Thus, the adult burbot population was feeding on round goby at an annual rate equal to 61% of the estimated round goby standing stock. We concluded that the burbot population had high potential to exert predatory control on round goby in offshore waters of eastern Lake Erie.

Round goby (Neogobius melanostomus) population structure, biomass, prey consumption and mortality from predation in the Bay of Quinte, Lake Ontario

We compared round goby (Neogobius melanostomus) population structure and biomass, estimated the quantity of prey consumed by round goby and assessed predation on round goby in two areas of the Bay of Quinte (upper and lower bay) that were invaded by this species two years apart. A bioenergetics approach was used to estimate round goby consumption and to illustrate the influence of changes in the round goby biomass, from May to October in both areas of the bay. Mean round goby biomass was 5tonnes/km2 for the upper bay and 11.2tonnes/km2 for the lower bay. Round goby populations were dominated by age-0 to age-2 individuals; older individuals were rare although they constituted a greater portion of total biomass in the earlier invaded lower bay than the upper bay. Estimated round goby consumption was 78.4tonnes/km2 for the upper bay and 127.1tonnes/km2 for the lower bay; dreissenids were the most important prey followed by chironomids, zooplankton and amphipods. Diet analysis showed that round gobies were consumed by all predator species analyzed, and constituted a high percentage of stomach content biomass in yellow perch (Perca flavescens), largemouth bass (Micropterus salmoides) and northern pike (Esox lucius). Age structure differences between the upper and lower bay are consistent with those expected between expanding and stable populations.

Signatures of contamination in invasive round gobies ( Neogobius melanostomus): A double strike for ecosystem health?

The invasive round goby has a recognised role in transferring contaminants through foodwebs, but little work has been done on contaminant impacts on round gobies themselves. Here we present the first case study of contaminant biomarkers and subpopulation structure variation in round gobies, in relation to habitat contamination, within a Canadian Area of Concern, Hamilton Harbour. Copper and cadmium were elevated in livers of fish from contaminated habitats. Although catch abundances were similar across sites, fish were smaller, a greater proportion of fish were female and more males were in reproductive condition in contaminated sites. Fish from contaminated areas showed more fin loss. Males from contaminated sites showed intersex gonads and genitalia. Ethoxyresorufin- o-deethylase (EROD) activity was higher in fish collected near polycyclic aromatic hydrocarbon (PAH)-rich sediments. The results indicate that contaminants impact the characteristics of round goby populations, which could affect ecosystems beyond toxicant biomagnification. This study also confirms that round gobies can be abundant in polluted habitats, which may draw predators—facilitating mobilisation of contaminants in foodwebs.

Diet shift of double-crested cormorants in eastern Lake Ontario associated with the expansion of the invasive round goby

The proliferation of the invasive round goby ( Apollonia melanostoma) in the Great Lakes has caused shifts in the trophic ecology in some areas. We examined the diet of double-crested cormorants ( Phalacrocorax auritas) prior to, and immediately after, round goby population expansion at two colonies, Pigeon and Snake Islands, in eastern Lake Ontario from 1999 to 2007. Cormorant diet was determined from the examination of 10,167 pellets collected over the nine-year period. By the second year round gobies were found in the diet (2002 at Snake Island and 2003 at Pigeon Island) they were the main species consumed by cormorants at each colony. The dominance of round goby in cormorant diets had a significant effect on both daily fish consumption and seasonal trends in fish consumption compared to the pre-goby years. Seasonal differences that were observed during the pre-goby years were lost once gobies became the main diet component of cormorants. The rapid switch to a benthic prey such as round goby, from a largely limnetic fish diet demonstrates the adaptive foraging ability of cormorants. Round goby may act as a buffer for yellow perch and smallmouth bass, two sport fish impacted by cormorant predation in eastern Lake Ontario.

The River Odra estuary – another Baltic Sea area colonized by the round goby Neogobius melanostomus Pallas, 1811

The round goby (Neogobius melanostomus Pallas, 1811) has been present, since the 1990s, in the Gulf of Gdansk where it forms a strong population and where the species is locally the most important fish in shallow inshore areas. No similarly large, independent round goby population in the Baltic Sea area has been found beyond the Gulf of Gdansk and the adjacent Vistula Lagoon. As shown by the research carried out in 2009, the round goby is abundant also in the River Odra estuary, which may provide evidence of the presence of a local, reproducing population.

Demography and substrate affinity of the round goby ( Neogobius melanostomus) in Hamilton Harbour

The invasive round goby, Neogobius melanostomus, consumes eggs and fry of other fishes, competes for resources with native fish species, and hence poses a threat to Great Lakes aquatic communities. We provide the first description of round goby demographic patterns in Hamilton Harbour, in the western tip of Lake Ontario, and the connected Cootes Paradise Marsh, a recently restored wetland. By monitoring round goby populations on a variety of distinct habitats for 7 years (2002–2008), we found that populations have declined at all sample locations and that average fish body size also has decreased. We also related abundance, body size, and reproductive patterns to seasonality, to substrate types (mud, sand, cobble and boulder) and to water quality in all locations. Round gobies were found on all substrates sampled including mud, although they were less abundant on mud than on other substrates, and to date have not extensively colonized Cootes Paradise Marsh. Our work confirms previous studies, which have suggested that habitats lacking hard structures will have fewer round gobies because they lack substrates on which round gobies can breed. However, our results also indicate that muddy and sandy substrates are not resistant to round goby invasion and will not prevent round goby colonization, a potential concern for Cootes Paradise Marsh an important spawning, nursery, and refuge habitat for warmwater native fishes and for other similar wetlands.

Invasive gobies in the Danube: invasion success facilitated by availability and selection of superior food resources

Abstract – We investigated somatic condition, growth rate, diet and food resources of the native (lower Danube) and non‐native (upper Danube) populations of invasive bighead gobyNeogobius kessleriand round gobyN. melanostomuswithin the Danube River to answer the question whether prey availability and type may have facilitated successful goby invasion to the upper Danube. The non‐native populations of both species were in better somatic condition and grew faster. The biomass of nonmollusc macrozoobenthos, dominated by Amphipoda, was markedly higher in the non‐native range while molluscs were recorded frequently in both the native and non‐native ranges. Amphipods were far the most consumed prey by non‐native fish, whereas native fish combined two main prey types – amphipods and fish (bighead goby) and amphipods and bivalves (round goby). A laboratory experiment was conducted to reveal whether the low consumption of bivalves by the round goby in the non‐native range reflects prey encounter rates or alternatively prey selectivity. When bivalves and amphipods were offered simultaneously in excess to the experimental fish, round goby showed strong preference towards amphipods. Molluscs are hypothesised to be an alternative rather than the most preferred prey for the round goby. Rich food resources utilised by the non‐native bighead and round goby contribute to their invasive success in the upper Danube.

Dreissenid mussels are not a “dead end” in Great Lakes food webs

Dreissenid mussels have been regarded as a “dead end” in Great Lakes food webs because the degree of predation on dreissenid mussels, on a lakewide basis, is believed to be low. Waterfowl predation on dreissenid mussels in the Great Lakes has primarily been confined to bays, and therefore its effects on the dreissenid mussel population have been localized rather than operating on a lakewide level. Based on results from a previous study, annual consumption of dreissenid mussels by the round goby ( Neogobius melanostomus) population in central Lake Erie averaged only 6 kilotonnes (kt; 1 kt = one thousand metric tons) during 1995–2002. In contrast, our coupling of lake whitefish ( Coregonus clupeaformis) population models with a lake whitefish bioenergetics model revealed that lake whitefish populations in Lakes Michigan and Huron consumed 109 and 820 kt, respectively, of dreissenid mussels each year. Our results indicated that lake whitefish can be an important predator on dreissenid mussels in the Great Lakes, and that dreissenid mussels do not represent a “dead end” in Great Lakes food webs. The Lake Michigan dreissenid mussel population has been estimated to be growing more than three times faster than the Lake Huron dreissenid mussel population during the 2000s. One plausible explanation for the higher population growth rate in Lake Michigan would be the substantially higher predation rate by lake whitefish on dreissenid mussels in Lake Huron.

Frequent Consumption and Rapid Digestion of Prey by the Lake Erie Watersnake with Implications for an Invasive Prey Species

Studies of interactions between invasive and native species often focus on impacts on natives. We report potential impacts of a native predator, the Lake Erie Watersnake (Nerodia sipdeon insularum) on an invasive fish, the Round Goby (Apollonia melanostomus). Round Gobies have increased exponentially in the Great Lakes and now constitute >90% of prey consumed by Lake Erie Watersnakes. We investigated the effects this shift may have on round goby populations by estimating total prey consumption by Lake Erie Watersnakes. Digestive rate trials and maximum voluntary prey consumption trials indicate that gastric digestion is rapid (digestion was 90% complete after just 16.4 hours at 30°C and 20.1 hours at 25°C) and voluntary prey consumption is high (from 30.0% of adult female body mass to 117% of neonate body mass in five days). Based on palpation of wild-caught snakes, prey were detected more frequently in adult females than adult males, but no such difference was observed in subadults. The proportion of snakes containing prey varied over time with season-long averages of 11.6% for adult females, 6.9% for adult males, and 22.4% for subadults. Systematic surveys by boat indicate that nearly 90% of foraging occurs <150 m from shore. Projected annual consumption, based on gastric digestion rate, maximum voluntary prey consumption, feeding frequency of free-ranging snakes, and published energetic data and a population size of 12,000 adult watersnakes, ranges from 200,000–3,300,000 Round Gobies (4,455–56,178 kg) per year. Although impressive, this rate of prey consumption is unlikely to have more than local effects on Round Goby populations.

A Comparison of Methods for Sampling round Goby in Rocky Littoral Areas

AbstractInvasion of the round goby Neogobius melanostomus in the Great Lakes has prompted investigation into qualitative and quantitative sampling strategies. Design of an optimal sampling strategy to monitor expanding round goby populations should consider the accuracy, precision, and associated costs of gears and their deployment strategies. The goal of this study was to compare three common, low‐cost, readily available gear types used to sample round goby (gill nets, minnow traps, and trotlines) in terms of catch rates, size selectivity, and bycatch. During fall assessments, baited minnow traps were the most effective at collecting round goby, followed by trotlines and monofilament gill nets. Minnow traps were more selective of small round goby, while trotlines and gill nets were more selective of larger individuals. Bycatch associated with minnow traps and trotlines was lower than that associated with gill nets. Seasonal variation in gear effectiveness was apparent: during spring, gill nets caught 216 round goby and minnow traps caught none, while during the fall only 11 round goby were captured in gill nets and 868 were captured in minnow traps. During summer assessments, diel movements were evident, as gill nets set during periods of low or no light captured more round goby than those set during the day. The cost‐benefit analysis indicated that passive gears had lower scores (i.e., were more cost‐effective) than active gears. The financial cost of the three gears indicated that the cost per round goby varied among gears and seasons. Our results demonstrated that baited minnow traps set overnight were the most efficient, easily deployed, cost‐effective gear and exhibited less variable catch rates during late summer and fall than either gill nets or trotlines.

Invasion Genetics of Ponto-Caspian Gobies in the Great Lakes: A ‘Cryptic’ Species, Absence of Founder Effects, and Comparative Risk Analysis

Genetic variability and structure of nonindigenous vs native populations are compared for the Eurasian round goby Neogobius melanostomus and the tubenose goby Proterorhinus marmoratus, which both invaded Lake St. Clair of the North American Great Lakes about 1990. The round goby spread rapidly to all of the Great Lakes and the tubenose goby largely has been restricted to Lake St. Clair, with some recent range extension into western Lake Erie. Risk analyses may indicate whether genetic variability of colonizers is predictive of their relative invasive and establishment successes. The present investigation examined DNA sequence variation across the left domain of the mitochondrial DNA cytochrome b gene in round and tubenose gobies from Eurasian and Great Lakes locations. We also sequenced six additional Neogobius species (including the monkey N. fluviatilis, racer N. gymnotrachelus, and bighead N. kessleri gobies that have been ‘on the move’ in Europe) and the knout goby Mesogobius batrachocephalus from the Black Sea in order to develop diagnostic genetic characters to identify them in case of future and/or undetected invasions and to delineate their phylogenetic relationships. Results show that a diverse number of haplotypes characterize round and tubenose goby populations from both North America and Eurasian sites, fitting a risk analysis prediction of high genetic variability in their successful introductions. Phylogenetic results indicate that the current genus Neogobius is paraphyletic and that the subgenus Apollonia thus should be elevated to the level of genus, containing Apollonia (N.) melanostomus (the round goby) and A. (N.) fluviatilis (the monkey goby). In addition, there appear to be two separate species of Proterorhinus marmoratus, a marine P. marmoratus Pallas 1814 in the Black Sea (matching the original type locality), and a ‘cryptic’ freshwater species in the Danube and Dnieper Rivers and probably other Eurasian freshwater habitats, as well as invasive in the Great Lakes. We suggest resurrecting the name P. semilunaris Heckel 1837 for the freshwater species (a taxon that was originally described from rivers draining into the Aegean Sea and the Danube River, but was later placed in synonymy with P. marmoratus).

Spread of alien (non-indigenous) fish species Neogobius melanostomus in the Gulf of Gdansk (south Baltic)

The round goby (Neogobius melanostomus) was first noticed in the Gulf of Gdansk in 1990. This Ponto-Caspian fish was most likely introduced to the Baltic environment from ships’ ballast water. During the first years of invasion, slow population growth was observed in the initially colonized regions. From 1994, round goby gradually spread in all shallow water zones of the Gulf of Gdansk. The area occupied by the population and the number of fish grew rapidly. By the end of the nineties, round goby became a dominant fish in shallow waters of the western part of the Gulf of Gdansk, where the fish used all solid substrates on the bottom, including concrete piers, big stones or even dumped waste for spawning and refuge. The presence of round goby in the Vistula Lagoon and shallow waters near Rugia (Western Baltic) was noticed for the first time in 1999. The present state, size and condition of the round goby population in the Gulf of Gdansk led us to predict a continual spread of this species into new regions of the Baltic. In addition, the population of round goby is so large that the species has started playing a part in commercial fishing in the Gulf of Gdansk.

The impact of introduced round gobies (Neogobius melanostomus) on phosphorus cycling in central Lake Erie

We used an individual-based bioenergetic model to simulate the phosphorus flux of the round goby (Neogobius melanostomus) population in central Lake Erie during 1995–2002. Estimates of round goby diet composition, growth rates, and population abundance were derived from field sampling. As an abundant introduced fish, we predicted that round gobies would influence phosphorus cycling both directly, through excretion, and indirectly, through consumption of dreissenid mussels, whose high mass-specific phosphorus excretion enhances recycling. In 1999, when age-1+ round gobies reached peak abundance near 350 million (2.4 kg·ha–1), annual phosphorus excretion was estimated at 7 t (1.4 × 10–3 mg P·m–2·day–1). From an ecosystem perspective, however, round gobies excreted only 0.4% of the phosphorus needed by the benthic community for primary production. Indirectly, round gobies consumed &lt;0.2% of dreissenid population biomass, indicating that round gobies did not reduce nutrient availability by consuming dreissenids. Compared with previous studies that have revealed introduced species to influence phosphorus cycling, round gobies likely did not attain a sufficiently high biomass density to influence phosphorus cycling in Lake Erie.

A Potential New Energy Pathway in Central Lake Erie: the Round Goby Connection

Round gobies, invasive fish that entered Lake Erie in 1994, are altering energy, contaminant, and nutrient pathways. Our objective was to quantify how they alter energy pathways in the central basin of Lake Erie by describing their diet and identifying the degree to which predatory fish feed upon round gobies. We used bioenergetic models parameterized with data collected in the central basin between 1995 and 2002 to estimate the type and amount of prey eaten, the biomass accumulation rate for the round goby population, and a partitioning of the food energy into “new” energy derived from dreissenids as opposed to existing energy derived from zooplankton and non-dreissenid benthic prey. Mean (± SE) prey consumption peaked at 5.98 ± 2.17×10 4 tonnes wet mass in 1999 coincident with the maximum population size of 4.2 ± 1.5 billion round gobies. Zooplankton (40.2% by biomass) and dreissenid mussels (38.3%) dominated the prey consumed. Almost 90% of the zooplankton biomass was consumed by age-0 round gobies, while over 80% of the dreissenids were eaten by older ages. Standing stock biomass of round gobies ranged between 203 and 4,803 tonnes y −1 (interannual range), with an additional 475 to 8,943 tonnes of biomass accumulating through growth each year. Piscivorous fish showed an increasing reliance on round gobies as prey, with round gobies being the dominant prey fish in the diets of benthic-oriented predators. Hence, by being one of the few benthivores that exploit dreissenid mussels as prey, our analyses reveal that round gobies transfer new energy up the central Lake Erie food web.

The Invasive Round Goby ( Neogobius melanostomus) in the Diet of Nestling Double-crested Cormorants ( Phalacrocorax auritus) in Hamilton Harbour, Lake Ontario

Populations of the invasive round goby ( Neogobius melanostomus) have expanded dramatically since their discovery in the Laurentian Great Lakes in 1990. The abundance of these fish and possible competitive displacement of native species from aquatic food webs suggest that they will become an important prey item for predatory birds and sport fish. To date, there is very little information on the predation of round gobies by piscivorous birds in the Great Lakes. We used an abdominal palpation technique to stimulate regurgitation by double-crested cormorant ( Phalacrocorax auritus) chicks in ground nests from colonies in Hamilton Harbour, western Lake Ontario, during the 2002 nesting season. We collected and identified all fish species present in regurgitated boluses. For tree-nesting birds, we collected fish that were regurgitated and fell to the ground as a result of targeted disturbance of particular nest trees. At all locations and times, alewife ( Alosa pseudoharengus) was the most abundant fish species present by a large margin. The second most abundant species was the round goby, which was present in the regurgitated stomach contents of chicks in a total of 18 percent of nests surveyed, and made up 1.8 to 11 percent of all individual fish specimens identified. Our results show that the round goby is already an important food item for breeding cormorants in Hamilton Harbour, despite relatively recent establishment of goby populations in western Lake Ontario. Fish species of sport or commercial interest were detected in our samples with extremely low frequency (< 0.1%).