Niche plasticity in invasive fishes in the Great Lakes

The geographic range of an invasive species is a key determinant of relative impact in the invaded region. Comparison of invasive species that are widespread or rare in invaded ranges can highlight mechanistic traits that determine the risk of impact from invasion. Round Goby (Neogobius melanostomus) is more geographically widespread and abundant than Tubenose Goby (Proterorhinus semilunaris) across invaded ranges of the Laurentian Great Lakes. We used stable isotopes of carbon (delta C-13) and nitrogen (delta N-15) in liver and muscle to contrast the isotopic niche breadth and niche plasticity of Round Goby and Tubenose Goby near the inflow and outflow of Lake St. Clair and in western Lake Superior. At all sites, Round Goby and Tubenose Goby that matched in size (21-53 mm standard length) had distinct isotopic niches with no overlap, driven by higher delta N-15 in Round Goby. The variation in isotopic niche, and the mean difference in delta C-13 and delta N-15 between muscle and liver, was greater for Round Goby, suggesting both greater seasonal shifts in diet and niche plasticity in this more widely established invader. Round Goby that were significantly larger than Tubenose Goby had broader isotopic niches and greater niche plasticity in the majority of cases and this was associated with isotopic niche overlap with smaller Round Goby and Tubenose Goby. Our findings suggest that a broad and plastic isotopic niche provides scope for wider establishment range in invasive fish species.