The abundance and persistence of the invasive round goby (Neogobius melanostomus) has often resulted in antagonistic interactions between the invasive and its native competitors. We sought to quantify the consequences of these interactions on fish community structure and feeding and reproductive timing of native fishes. We conducted surveys of fish communities during the spring spawning season over three years (2015-2017) in seven Michigan tributaries to the Great Lakes. Three sites on each river were sampled, providing an upstream-downstream gradient, capturing the invasion front in most cases. The native Johnny darter (Etheostoma nigrum) was used as a representative native competitor to compare to round goby due to its similar size, feeding strategies, habitat requirements, and wide distribution. Up to three individuals of each species per site were dissected to identify gut contents for diet comparison. Gonad masses were also obtained to examine relative investment in reproduction. Johnny darter individuals were further compared between sites where they do and do not co-occur with round goby to identify changes in feeding and reproductive strategies directly associated with round goby interaction. Stable isotope analyses (carbon and nitrogen) were performed to complement gut content analyses and provide an in-depth assessment of trophic interactions between species.
Results/Conclusions
Over the course of this three-year study, upstream invasion was identified in five of seven rivers sampled. Coincident to this increase in round goby abundance, the four darter species present in these rivers declined in abundance, indicating either upstream migration or a shift in overall species composition. Round goby and Johnny darter exhibited no difference in gut fullness, suggesting access to food resources is not driving population shifts. However, gut content diversity was significantly greater in round goby than Johnny darter in all locations. This may indicate that darters are feeding on the periphery of the diet space in presence of a more aggressive competitor. Gonad masses of Johnny darter indicated earlier reproductive investment in populations coexisting with round goby, suggesting altered reproductive phenology in presence of the invader. These results demonstrate a shift in the ecological function of native benthic fish in the presence of round goby invasion. Further, the increase in round goby abundance over the course of our study suggests these consequences may be persistent and wide-spread. Without mitigation or nonnative removal efforts, stream fish communities may undergo large shifts in composition as the round goby invasion front continues inland from the Great Lakes.