Diet composition and consumption rate in round goby (Neogobius melanostomus) in its expansion phase in the Trent River, Ontario Graham D. Raby & Lee F. G. Gutowsky & Michael G. Fox Received: 2 February 2010 / Accepted: 3 August 2010 / Published online: 18 August 2010 # Springer Science+Business Media B.V. 2010 Abstract We assessed density, gut fullness and prey composition of round goby (Neogobius melanostomus) from three areas in the Trent River (Ontario) representing areas of initial introduction and subsequent expansion. Round goby had been present at the area of original introduction since 2003, and by 2007/2008, their range had expanded upstream and downstream into the outermost reaches sampled in the study. Catch per unit angling effort in nearshore sites indicated that round goby density in the area of original introduction was more than double their density in the upstream expansion area and nearly three times the density in the downstream expansion area. Gut fullness index was lower in gobies from the area of original introduction than for those at the upstream and downstream edges of their expanded range. The most dramatic difference in diet composition was with dreissenids, where large gobies ( ≥70 mm) occupying the area of original introduction had almost no dreissenid biomass in their guts, whereas dreissenids were the predominant prey type in gobies occupying the two expansion areas. Post-hoc zebra mussel density in the area of original introduction was an order of magnitude lower than in the two expansion areas which, combined with the differences in stomach fullness and prey composition, suggest that local, density-related reduction of this prey type was occurring in the river. Keywords Dreissenid . Invasive species . Macroinvertebrate . Range expansion Introduction Understanding the invasion dynamics of non-native species is very important in both scientific and practical terms (Alcaraz et al. 2005; Garcia-Berthou 2007). Current and potential rate of spread, predicting habitats prone to future invasion, and understanding impacts on ecosystems are all areas of knowledge potentially enriched by the study of invasion dynam- ics. Both life history traits, as well as tolerance of environmental change are known to be strong predictors of invasion success in non-native fish species (Rosecchi et al. 2001). Environ Biol Fish (2010) 89:143–150 DOI 10.1007/s10641-010-9705-y G. D. Raby Environmental and Resource Studies Program, Trent University, 1600 West Bank Drive, Peterborough, ON K9J 7B8, Canada L. F. G. Gutowsky Environmental and Life Sciences Graduate Program, Trent University, 1600 West Bank Drive, Peterborough, ON K9J 7B8, Canada M. G. Fox (*) Environmental and Resource Studies Program and Department of Biology, Trent University, 1600 West Bank Drive, Peterborough, ON K9J 7B8, Canada e-mail: mfox@trentu.ca