Worldwide, barriers are constructed in the tidal zones of rivers and estuaries to control water exchange, primarily to prevent saltwater incursion and flooding, and maintain upstream freshwater reserves. Barrier designs and terminology vary (e.g. barrages, dikes, weirs, tidal gates), but ultimately, all influence fishes by: 1) obstructing movement; and 2) altering estuarine hydrodynamics (i.e. tidal water movement and/or freshwater discharge), and coincident salinity regimes and habitat. These primary impacts may be exacerbated by climate change driven sea-level rise and reduced riverine discharge.
Remediation of tidal barriers to promote fish passage and the rehabilitation of estuaries (e.g. restoring tidal flow, delivering environmental flows) is increasingly common in developed countries. Research evaluating fish behaviour and passage success through various fishway and barrier designs has informed remediation; nonetheless, such approaches remain complex from ecological, economic and engineering perspectives. We present case studies from Australia, the Netherlands and the United States to examine and characterise fish passage issues in a variety of systems, and document contemporary approaches to improving fish passage and restoring ecosystem function in systems with tidal barriers. This includes the application of technical fishways, ecologically sensitive operation of water control structures and provision of environmental flows. We integrate insights and lessons learned from these case studies to guide remediation strategies at other tidal barriers and provide direction for future research.