Long‐term restoration of river connectivity drives recovery of a diadromous fish in southern Australia

Introduction River fragmentation disrupts ecological connectivity, constraining the movement, recruitment, and persistence of migratory fishes. Although fishways are widely implemented to mitigate these impacts, most evidence of effectiveness derives from northern hemisphere systems dominated by large‐bodied salmonids. Consequently, uncertainty remains as to whether common fishway designs are effective for small‐bodied, often diadromous fishes characteristic of temperate southern hemisphere rivers. Objectives We evaluated the long‐term, regional‐scale influence of barrier mitigation on the occupancy dynamics of two model fish species in a temperate southern hemisphere river network. Methods We analyzed a 45‐year composite dataset of presence–absence records from 25 sub‐catchments across five river basins in Greater Melbourne, Australia, where low‐head, lower‐catchment barriers have been progressively mitigated using standard fishway types. Dynamic occupancy models accounting for imperfect detection were used to estimate colonization, extinction, and occupancy trajectories for diadromous Common galaxias ( Galaxias maculatus ) and potadromous Flathead gudgeon ( Philypnodon grandiceps ). Results Occupancy dynamics of Common galaxias were strongly associated with barrier mitigation. Colonization probability increased threefold following mitigation, producing rapid catchment‐scale expansion over the period of barrier mitigation and persistent occupancy thereafter. In contrast, Flathead gudgeon showed gradual increases in occupancy over time but no strong response to barrier status. Conclusion Restoring longitudinal connectivity enabled rapid recolonization and sustained occupancy for a diadromous fish at a regional scale, providing rare long‐term evidence of fishway effectiveness in a temperate southern hemisphere context.