Dams can landlock populations of amphidromous fishes that formerly migrated between rivers and seas. However, the genetic consequences of these life‐cycle changes remain unclear. Rhinogobius is a speciose genus of freshwater gobies widely distributed in Asia, including non‐migratory R. duospilus and facultatively amphidromous R. similis . Using genome‐wide SNPs from Hong Kong, southern China, we compared the genetic diversity...