Numerous non-native species have been introduced into the Laurentian Great Lakes in the past 200 years, often resulting in extinctions, native fish population declines, altered food webs, and costly control programs. Round gobies (Neogobius melanostomus) were documented within the St. Clair River in 1990, and have subsequently devastated many native benthic fishes that occupy the same ecological niche. Aggressive nest guarding by male round gobies displaces native fishes from optimal spawning habitat, and generalist foraging strategies allow gobies to outcompete natives for prey items. The goal of this study was to identify long-term changes in species diversity and fish diets within the St. Clair River associated with the round goby invasion. We compared fish density and diet data gathered during extensive sampling conducted in 1993 and 1994 with similar data collected in 2011. Diel sampling of invasive and native fish was conducted at multiple depths, using seine and trawl nets, from May to December. Sampling was repeated in 2011. All fish or a representative sample were frozen to prevent digestion from occurring prior to stomach content analyses. We used displacement techniques and the Schoener Index method to assess diet overlap between round gobies and native fishes.
Results/Conclusions
Our preliminary results indicate a dramatic decrease in native fish species diversity between 1993-1994 and 2011, and one species, the mottled sculpin (Cottus bairdii) has likely been nearly extirpated from the St. Clair River. Diet overlap exists between round gobies and native, benthic fishes in the St. Clair River. However, this overlap varies spatially, temporally, and ontogenetically. Juvenile gobies have a more varied diet and were more likely to show overlap with native fish populations. In deeper waters round gobies of intermediate maturity were found to consume native fish eggs and juveniles, implying a potential direct, negative impact on native fish populations. Mature round gobies feed almost exclusively on invasive zebra and quagga mussels (Dreissena polymorpha and D. bugensis), which are targeted by few other species. Although differential foraging strategies can allow native fish to forage without competition from round gobies, these results suggest a causal linkage between round goby invasion and establishment, and long-term changes in fish species diversity within the St. Clair River. Establishment of the round goby within Great Lakes tributaries, nearshore environments, and more recently near the Mississippi River will likely impact native fish that seek out similar habitat and prey.