Elizabeth A. LaRue, Carl R. Ruetz III, Michael B. Stacey, Ryan A. Thum
Understanding subsequent dispersal of exotic species following introduction is essential for predicting the extent and rate of range expansion, which is vital for effective management and risk assessment. Post-introduction dispersal may occur naturally or via human transport, but assessing the contribution of each is difficult for many organisms. We use data from seven microsatellite markers to study dispersal patterns and gene flow among 12 pierhead populations of the round goby ( Neogobius melanostomus) in Lake Michigan. We find significant population structure among sites within this Great Lake: (1) many populations exhibit significant pairwise FSTand (2) a Bayesian assignment analysis revealed three distinct genetic clusters that correspond to different pierhead locations, and genetic admixture between clusters in the remaining populations. We see that genetic differentiation (FST) is typically related to geographic distance (i.e., isolation by distance), but is occasionally interrupted at the scale of Lake Michigan due to gene flow among geographically remote sites. In addition, average genetic differentiation among populations reveals a significant, negative correlation with amount of shipping cargo at ports at sampling sites. Our results supply evidence that genetic structure of the round goby in Lake Michigan results from limited natural dispersal with frequent long-distance dispersal through anthropogenic activity such as commercial shipping. Our study suggests that round gobies undoubtedly can disperse and found new populations by natural dispersal mechanisms, their spread within and among the Great Lakes is probably aided from transport by ships. Thus, we recommend ship ballast-water be treated and that management could limit the spread of exotic species in the Great Lakes after introduction, while also preventing the introduction of non-native species to the Great Lakes.