Quantifying the predatory effect of round goby on Saginaw Bay dreissenids

Invasive dreissenid mussels (D. polymorpha and D. r. bugensis) have fundamentally altered Laurentian Great Lake ecosystems, however in many areas their abundances have declined since the mid-1990s. Another invader, the benthic fish round goby (Neogobius melanostomus), is morphologically adapted to feed on dreissenids and likely affects dreissenid populations; however, the degree of this predatory effect is variable. In 2009 and 2010, we examined round goby abundances, size distributions, diet contents, and diet selectivity in Saginaw Bay, Lake Huron; a shallow bay that has been subjected to numerous anthropogenic stressors. We further used a consumption model to estimate dreissenid consumption by three different size classes of round goby. Round gobies were found throughout the bay and most were smaller than 80 mm total length. Round gobies of all sizes consumed dreissenids (including fish as small as 30 mm total length), though dreissenids were rarely preferred. The relative proportion of dreissenids (by biomass) present in diets of round gobies increased with fish size, but also throughout the year for all size classes. Despite this, overall consumptive effects of round gobies on dreissenids in Saginaw Bay were low. Many dreissenids present in the bay were larger than those consumed by round gobies. Bioenergetics-based model estimates suggest that the smallest round gobies are responsible for the majority of dreissenid consumption. While our findings are limited to soft substrates and influenced by sampling restrictions, our study design allowed us to put bounds on our estimates based upon these multiple sources of uncertainty.     

How Non-native Species in Lake Erie Influence Trophic Transfer of Mercury and Lead to Top Predators

Lake Erie's food web has been dramatically modified by exotic species. Both exotic dreissenid mussels and the round goby Neogobius melanastomus have shifted the food web from a pelagicbased to a benthic-based one, potentially creating a new pathway for contaminant transfer to top predators. Before the invasion of round gobies, few predators of dreissenids occurred in Lake Erie, allowing contaminants to be confined to these benthic organisms. The invasion of the round goby has produced a new pathway through which these contaminants can enter the food web. To characterize heavy-metal transfer through this new food web and to assess risk to humans, water, surficial sediment, dreissenid, round goby, and smallmouth bass Micropterus dolomieui samples were collected at three sites during summers, 2002 and 2003, and analyzed for total lead (Pb), total mercury (Hg), and methyl mercury (MeHg). In addition, we compared smallmouth bass Pb and Hg concentrations to those measured in 1993/1994, before round gobies were prevalent. Pb biodiminished and MeHg biomagnified through the food web to smallmouth bass; patterns were similar among our three sites. Total Pb concentrations in smallmouth bass were higher before the incorporation of round gobies into their diet. We attributed this decline to changes in food web structure, changes in contaminant burdens in prey, or declines in sediment Pb concentrations in Lake Erie. By comparison, Hg concentrations in smallmouth bass changed little, before and after the round goby invasion, possibly due to a shift in diet that increased growth. Despite a decline in sediment Hg concentrations in Lake Erie, smallmouth bass continued to accumulate Hg at historical rates possibly because of their high consumption rates of benthivorous round gobies. As smallmouth bass continue to consume round gobies during their lives, their Hg concentrations may well continue to increase, potentially increasing the risk of Hg contamination to humans.     

Diel Vertical Migration of Round Goby Larvae in the Great Lakes

One hypothesis for the transcontinental and intra-Great Lakes basin transfer of round gobies (Neogobius melanostomus) has been that round gobies were pumped into the ballast water of ships. During June 2005 in Lake Erie, we obtained evidence of a vertical migration of round goby larvae, when we collected 167 round goby larvae in surface ichthyoplankton net tows at night and zero during day. These results complemented similar findings from the Muskegon River estuary of Lake Michigan during 2003 and 2004, documenting diel vertical migration for the first time in larval round gobies. We suggest vertical migration behavior may have allowed larval round gobies to be transported to and within the Great Lakes via ballast water and dispersed in the Great Lakes via advection of 6.5–8.5-mm long larvae at the surface. Based on our results, if ballast water was only taken on near the surface during daylight hours from May through September when larval round gobies were present, it would have mitigated the spread of round gobies throughout the Great Lakes.     

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

We assessed round goby (Apollonia melanostoma) density and size structure in two sections of the Bay of Quinte (Lake Ontario) that had been invaded by this species two years apart. Round goby density was assessed with 50 m linear transects, recorded with an underwater video recording apparatus developed for this study that included a depth sounder for maintaining a fixed distance above the substrate. The highest mean round goby densities were observed in the shallowest depth zone (1.5–3 m) at both sites, but there were differences between the sites in the habitat types where the highest densities occurred and there were no significant density differences among habitat types at either site (rock with sparse vegetation, mud with sparse vegetation, sand/mud with moderate vegetation cover). In the upper bay, mean body length of round gobies declined with depth, whereas in the lower bay, mean round goby length was greatest in the deepest zone. Mean body length of round gobies did not differ significantly by habitat type in either section of the bay.     

Size-selective predation by round gobies (Neogobius melanostomus) on dreissenid mussels in the field

Although numerous studies have shown that round gobies (Neogobius melanostomus) prey on dreissenid mussels (Dreissena polymorpha and Dreissena bugensis), there is an apparent shortage of detailed field studies on the subject. The 5-month field study described here quantifies predation by round gobies on dreissenids in Presque Isle Bay, Lake Erie. Dreissenids dominated the diet of round gobies, composing 92% of the prey items recovered. Over half of the 3870 valves (1935 mussels) recovered from 155 round gobies were crushed, while the remainder were swallowed whole. Crushed dreissenids were larger than those swallowed whole, and the tendency to crush dreissenids did not vary among three length classes of round goby. Round goby length was positively related to average size of dreissenids consumed, average size of whole and crushed dreissenids, largest whole dreissenid consumed, and largest crushed dreissenid consumed. Indices of selectivity revealed similarly shaped curves for three length classes of round gobies, a shift toward larger dreissenid size classes with an increase in round goby length, and peak preferences for 8-11 mm dreissenids. Factors such as gape limitation, availability and accessibility of differently sized dreissenids, forces generated while removing mussels from the substrate and crushing them, and caloric content of dreissenids all likely play roles in the observed size-selectivity and differential processing of dreissenids. Although factors influencing size-selectivity are not completely understood, the observed preference of round gobies for dreissenids near the size when they are first reproductive could impact the demography of dreissenid populations.     

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

Round goby (Neogobius melanostomus) and western tubenose goby (Proterorhinus semilunaris) invaded the Laurentian Great Lakes at approximately the same time and area yet have shown substantial differences in their post-invasion success with more rapid establishment and development of much larger abundances of round goby populations throughout the invaded habitat. In this study, we compared differences in physiological performance (thermal tolerance and standard metabolic rate) between round and tubenose goby collected from the Huron-Erie corridor. Tubenose goby were observed to have lower thermal tolerance but exhibited similar standard metabolic rate across environmental temperatures compared to round goby. At temperatures exceeding 31 °C, tubenose goby demonstrated significantly higher mortalities and shorter times to death relative to round goby. The observed differences in thermal tolerance were consistent with differences in the native geographic ranges observed for each species at their southern ranges. The observed differences in physiological performance combined with species differences in other life history traits such body size, reproduction, feeding ecology and habitat affiliation may also explain differences in the invasiveness experienced by these two Great Lakes invasive fish including a greater ability of round gobies to occupy extreme habitats with large water temperature fluctuations.     

Impacts of the Introduced Round Goby (Apollonia melanostoma) on Dreissenids (Dreissena polymorpha and Dreissena bugensis) and on Macroinvertebrate Community between 2003 and 2006 in the Littoral Zone of Green Bay,Lake Michigan

We show that the invasion of round gobies (Apollonia melanostoma) in Green Bay, Lake Michigan, has changed the benthic food web in fundamental ways related to their impact on invasive dreissenid mussels. Dreissenid mussels are of specific interest because they are one of the primary dietary items for round gobies. In this study, we collected rocks from each of 10 study sites along approximately 60 km of the eastern shoreline of Green Bay, Lake Michigan, to assess a temporal change in macroinvertebrate abundance related to the northward movement of the round goby invasion front from a point about midway along the shoreline in 2003 to the entire coast in 2006. The pattern of macroinvertebrate abundance in 2003 suggested that round gobies had already caused significant decreases in macroinvertebrate abundances south of the invasion front (interpretation of the data could have been compromised by confounding environmental gradients). In subsequent sampling in 2006 macroinvertebrates were picked off of sampled rocks in the field and underwater transects were videotaped to estimate round goby abundance at each site. Round gobies were collected for stomach analysis to assist in determining which invertebrates would likely be impacted by goby predation. Our results indicated that by 2006, round gobies had become abundant at those sites where they were absent in 2003 and zebra mussels (Dreissena polymorpha), quagga mussels (Dreissena bugensis), isopods, amphipods, trichopterans, and gastropods in the newly invaded sites had significantly decreased at the newly invaded sites.     

Occurrence and Food Habits of the Round Goby in the Profundal Zone of Southwestern Lake Ontario

Little is known about the ecology of round goby (Neogobius melanostomus), an invasive benthic fish, in the profundal zone of the Great Lakes. In April 2002–2005 we caught increasing numbers of round gobies with a bottom trawl in the 45–150 m depth range of southwestern Lake Ontario. In 2005, we examined gut contents of 30 round gobies from each of three depths, 55, 95, and 130 m, and qualitatively compared gut contents with density of benthic invertebrates determined by Ponar grabs. Round goby guts contained mostly Dreissena spp. and opposum shrimp, Mysis relicta (Mysis); the frequency of occurrence of dreissenids in guts decreased with depth, whereas the frequency of occurrence of Mysis in guts increased with depth. Abundance of these invertebrates in the environment followed the same pattern, although dreissenids of optimum edible size (3–12 mm) were still abundant (1,373/m²) at 130 m, where round gobies primarily consumed Mysis, suggesting that round gobies may switch from dreissenids to more profitable prey when it is available. Other food items were ostracods and fish, with ostracods generally eaten by smaller round gobies and fish eaten by larger round gobies. Occurrence and increasing abundance of round gobies in the profundal zone and predation on Mysis by round goby could have far-reaching consequences for the Lake Ontario fish community.     

Are interactions among Ponto-Caspian invaders driving amphipod species replacement in the St. Lawrence River?

In Lake Erie and Lake Ontario, the Ponto-Caspian amphipod Echinogammarus ischnus has replaced the native amphipod Gammarus fasciatus on rocky substrates colonized by dreissenid mussels, which provide interstitial refugia for small invertebrates. Based on the premise that an invader's vulnerability to predation is influenced by its evolutionary experience with the predator and its ability to compete for refugia, we hypothesized that amphipod species replacement is facilitated through selective predation by the round goby Neogobius melanostomus, a Ponto-Caspian fish that invaded the Great Lakes in the early 1990s and is now colonizing the St. Lawrence River. In laboratory experiments, we determined if E. ischnus excludes G. fasciatus from mussel patches, and if the vulnerability of G. fasciatus to predation by gobies is increased in the presence of the invasive amphipod. E. ischnus and G. fasciatus did not differ in their use of mussel patches, either when alone or in each other's presence. Both species were equally vulnerable to predation by the round goby. In field experiments, we determined if the round goby exerts a stronger impact than native predators on the relative abundance of amphipod species. Our results suggest that E. ischnus is more vulnerable to native predators, but the round goby does not have a differential impact on the native amphipod. We conclude that competition with E. ischnus does not increase the vulnerability of G. fasciatus to goby predation, and that the round goby does not promote the replacement of G. fasciatus by E. ischnus in the St. Lawrence River. The outcome of antagonistic interactions between exotic and native amphipods is mediated more by abiotic factors than by shared evolutionary history with other co-occurring exotic species.     

Benthic Invertebrate Community Responses to Round Goby (Neogobius melanostomus) and Zebra Mussel (Dreissena polymorpha) Invasion in Southern Lake Michigan

The round goby (Neogobius melanostomus Pallas), a fish native to eastern Europe, recently has become established in southwestern Lake Michigan. Because round gobies prey on zebra mussels (Dreissena polymorpha Pallas) and other benthic invertebrates, the effects of round gobies on invertebrates within zebra mussel colonies was investigated. Using a 2 × 3 factorial design, the effects of round gobies (present or absent) and zebra mussel densities (zero, low, and high) on non-mussel invertebrates was examined. Ten ceramic tiles of each mussel density were colonized in the laboratory and then anchored in Calumet Harbor, IL for 10 weeks. Round gobies had access to half the tiles while half were covered with coarse mesh screening that excluded round gobies, but allowed invertebrates to move into and out of the exclosures. Low and high zebra mussel density tiles supported significantly greater numbers of non-mussel invertebrates (p < 0.001) than zero density tiles, particularly amphipods (p < 0.001), hydroptilid caddisflies (p < 0.05), isopods (p < 0.05), and chironomids (p < 0.001). Chlorophyll a concentrations were highest (p < 0.001) at low zebra mussel densities. The presence of round gobies significantly reduced densities of total non-mussel invertebrates (p < 0.01) and leptocerid caddisflies (p < 0.05), resulting in a significant increase in chlorophyll a (p < 0.01) concentrations. A significant zebra mussel density x round goby interaction showed that total invertebrate biomass responded positively to the combined effect of high zebra mussel density and round goby absence. These results demonstrate that round gobies and zebra mussels are altering benthic invertebrate community structure and algal resources in nearshore rocky areas of southwestern Lake Michigan.     

Metazoan Parasites of Introduced Round and Tubenose Gobies in the Great Lakes: Support for the “Enemy Release Hypothesis”

Recent invasion theory has hypothesized that newly established exotic species may initially be free of their native parasites, augmenting their population success. Others have hypothesized that invaders may introduce exotic parasites to native species and/or may become hosts to native parasites in their new habitats. Our study analyzed the parasites of two exotic Eurasian gobies that were detected in the Great Lakes in 1990: the round goby Apollonia melanostoma and the tubenose goby Proterorhinus semilunaris. We compared our results from the central region of their introduced ranges in Lakes Huron, St. Clair, and Erie with other studies in the Great Lakes over the past decade, as well as Eurasian native and nonindigenous habitats. Results showed that goby-specific metazoan parasites were absent in the Great Lakes, and all but one species were represented only as larvae, suggesting that adult parasites presently are poorly-adapted to the new gobies as hosts. Seven parasitic species are known to infest the tubenose goby in the Great Lakes, including our new finding of the acanthocephalan Southwellina hispida, and all are rare. We provide the first findings of four parasite species in the round goby and clarified two others, totaling 22 in the Great Lakes—with most being rare. In contrast, 72 round goby parasites occur in the Black Sea region. Trematodes are the most common parasitic group of the round goby in the Great Lakes, as in their native Black Sea range and Baltic Sea introduction. Holarctic trematode Diplostomum spathaceum larvae, which are one of two widely distributed species shared with Eurasia, were found in round goby eyes from all Great Lakes localities except Lake Huron proper. Our study and others reveal no overall increases in parasitism of the invasive gobies over the past decade after their establishment in the Great Lakes. In conclusion, the parasite “load” on the invasive gobies appears relatively low in comparison with their native habitats, lending support to the “enemy release hypothesis.”     

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

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

Round Goby and Mottled Sculpin Predation on Lake Trout Eggs and Fry: Field Predictions from Laboratory Experiments

The accidental introduction of round gobies (Neogobius melanostomus) into the North American Great Lakes has raised concerns about their potential impacts on local fauna. Gobies have similar habitat and spawning requirements to mottled sculpins (Cottus bairdi) and slimy sculpins (C. cognatus), and may already be displacing sculpins where the ranges of the species overlap. Like sculpins, gobies are capable of penetrating interstitial spaces to acquire food, and therefore may become predators of interstitially incubating lake trout eggs. Laboratory experiments were conducted to compare egg consumption rates and critical size (the minimum size at which a fish was capable of ingesting an egg) between round gobies and mottled sculpins. Predation by both species on lake trout eggs and fry was also examined in two grades of substrate (cobble and gravel). Mottled sculpins consumed larger numbers of eggs than round gobies of similar size, and were capable of ingesting eggs at smaller sizes than gobies. Both gobies and sculpins had lower foraging success on smaller substrates (gravel) than on cobble. Gobies are currently present at higher densities than sculpins in areas where they are established in the Great Lakes. The similar predation of lake trout eggs by round gobies and mottled sculpin and high densities the goby has achieved at some Great Lakes sites leads to the prediction that the round goby may negatively affect lake trout reproduction and therefore rehabilitation.     

Invasion History,Proliferation, and Offshore Diet of the Round Goby Neogobius melanostomus in Western Lake Huron,USA

We used data from three trawl surveys during 1996–2003 to document range expansion, population trends, and use of offshore habitats by round gobies in the U.S. waters of Lake Huron. Round gobies (Neogobius melanostomus) were not detected in any survey until 1997, but by 2003 they had been recorded at 18 of the 28 sites sampled. The only areas not colonized were offshore habitats in northern Lake Huron. Round goby abundance increased during 1997–2001, thereafter overall abundance either increased (offshore) or became variable (near shore and Saginaw Bay). Mean lengths varied among surveys primarily due to high abundance of age-0 gobies in Saginaw Bay samples. Round gobies were found up to 34 km offshore at depths of 73 m. Round gobies consumed a wide range of invertebrate prey, but focused on dreissenids in shallow water (27–46 m), and native invertebrates at greater depths. The pattern of round goby dispersal was consistent with a pattern of simultaneous initial introductions at shipping ports followed by natural dispersal, and lake wide population size has probably not stabilized.     

Use of wetland versus open habitats by round gobies in lakes Michigan and Huron: Patterns of CPUE,length, and maturity

It has been suggested that some Great Lakes coastal wetlands may be resistant to invasion by several non-indigenous species including round goby, Neogobius melanostomus. However, there is inconclusive evidence regarding how susceptible exposed fringing coastal wetlands, in particular, are to round goby invasion. Therefore, we quantified round goby catch per unit effort (CPUE) using fyke nets in the Beaver Archipelago of Lake Michigan, and the Les Cheneaux islands and Saginaw Bay regions of Lake Huron. In addition, we examined the influence of body size and maturity on round goby habitat use. Catch per unit effort from fyke nets was highest in the Beaver Archipelago, where wetlands were dominated by small, immature round gobies and open water habitats were dominated by large adults. Fyke net catches within Les Cheneaux sites were similar between habitats and differences in size and maturity were not observed. Conversely, very few round goby were captured in wetlands of Saginaw Bay where CPUE was moderate in open water. This indicates that some exposed fringing wetlands in the Great Lakes, specifically those with high productivity, could have a higher degree of resistance to round goby invasion.     

Impact of Round Gobies (Neogobius melanostomus) on Dreissenids (Dreissena polymorpha and Dreissena bugensis) and the Associated Macroinvertebrate Community Across an Invasion Front

We studied the impact of round gobies (Neogobius melanostomus) on lithophilic invertebrates (having an association with a stony substrate) across an invasion front along the Door Peninsula, which flanks eastern Green Bay, Lake Michigan. We conducted both a cross-invasion front field survey and a rock-transfer experiment. For the field survey, we collected pairs of rocks from ten sites, including sites north of the invasion front and south of the invasion front. Zebra mussels (Dreissena polymorpha), quagga mussels (D. bugensis), and non-mussel invertebrates were removed from the rocks and enumerated. The rocks were measured and the algae removed and weighed. Round gobies were censused by videotaping along transects. There was a statistically significant negative relationship with round goby abundance for most invertebrates, including zebra mussels, quagga mussels, isopods, and snails, with the result for amphipods being suggestive. For the experiment, we transferred 20 rocks in bags from a round goby “absent” site with 10 going to a round goby abundant site and 10 being returned to the original site. The rocks incubated overnight, invertebrates were removed the next day, and the rocks were measured. There were significantly fewer zebra mussels, quagga mussels, isopods, amphipods, and snails from the rocks incubated at the round goby abundant site compared to those returned to the round gobyfree site. Thus, the results of the survey and rock-transfer experiment suggest that round gobies are influencing the benthic macroinvertebrate abundance through predation. The negative impact on mussels is probably due to direct predation while the negative impact on the other invertebrates may be a combination of direct predation and indirect effects due to the loss of the microhabitat or food that zebra mussels produce.     

Habitat and Site Affinity of the Round Goby

A study was conducted in 1996 to compare densities of the round goby (Neogobius melanostomus) in rock and sand habitats in day and night at three sites (Sarnia, St. Clair River; town of Belle River, Lake St. Clair; Peche Island, Detroit River). The sites represent an expansion in round goby distribution from Sarnia where gobies were first reported in 1990 downstream to Peche Island where gobies appeared in 1993. Site affinity at Peche Island was estimated using mark-recapture and observations of individual fish. Mean density of round gobies differed among sites (p < 0.001), light regime (p < 0.001), habitat (p < 0.001), and the interaction of light regime and site (p = 0.007). Mean densities of the round goby were 0.3 to 3 fish/m² (Peche Island), 0.5 to 3 fish/m² (town of Belle River), and 5 to 9 fish/m² (Sarnia). Overall, highest densities of the round goby occurred in the day and on rocks. Small (≤5 cm) round gobies were most common at Peche Island, the most recently colonized site, indicating that juveniles may disperse more rapidly than adults. Of 200 fish that were marked, 58% were recaptured, indicating high site fidelity among round gobies. The estimated mean (± SE) home range of the round goby determined using SCUBA (5 ± 1.2 m²) was likely underestimated because few fish were observed. Larger round gobies may induce smaller fish to leave preferred rock habitats and move to sand habitats from which they disperse.     

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.     

Recruitment Failure of Mottled Sculpin Cottus bairdi in Calumet Harbor,Southern Lake Michigan,Induced by the Newly Introduced Round Goby Neogobius melanostomus

This study documents a local extinction of mottled sculpins, apparently due to round gobies, and presents data pertinent to the mechanism of extinction. Mottled sculpins, Cottus bairdi, were assessed using SCUBA standardized diving transects during the invasion of the round goby, Neogobius melanostomus, into Calumet Harbor, southern Lake Michigan. Laboratory stream studies were conducted in which gravid male and female mottled sculpins were allowed to nest, then were exposed to one male round goby. Diet studies were conducted to assess the potential for competition for food at small sizes of both species. The SCUBA surveys showed that mottled sculpin populations rapidly declined, after the first round gobies were found in the area in 1994, despite the presence of a well established population prior to the round goby arrival. Mottled sculpins have been almost totally extirpated from the area in 1998 due to three proposed mechanisms: competition for food resources at small sizes, for space at intermediate sizes, and for spawning space at large sizes. The laboratory stream study confirmed that round gobies interfered with nest-guarding male mottled sculpins, seized their spawning shelters, changed to spawning coloration in preparation for spawning, and caused near loss of all the mottled sculpin eggs. It is concluded that recruitment failure and subsequent demise of mottled sculpins was most likely caused by spawning interference by round gobies.