Tidal barriers and fish – Impacts and remediation in the face of increasing demand for freshwater and climate change

Worldwide, tidal barriers (e.g. barrages, dikes, tide gates) are constructed in the lower reaches and estuaries of rivers to limit saltwater incursion into upstream freshwater reserves, facilitate water diversion and abstraction, limit flooding, reclaim land and generate electricity. While performing these functions, tidal barriers also affect fish through: 1) reduced connectivity; 2) loss of tidal flux; 3) conversion of upstream estuarine habitats to freshwater; and 4) diminished freshwater discharge, which compresses the spatio-temporal salinity regime of downstream estuarine habitats. As such, tidal barriers commonly cause declines of diadromous and estuarine associated fish species, with a subsequent loss of ecosystem services. These impacts will be exacerbated as climate change promotes sea-level rise and alters freshwater flow regimes and will be amplified by increasing demands for freshwater by a growing human population. As a result, more tidal barriers are likely. Nevertheless, in estuaries with tidal barriers, management that promotes connectivity and more natural ecosystem function is increasing but remains complex from ecological, economic and engineering perspectives. We present case studies from the Netherlands, southeastern United States and southern Australia to characterise impacts on fishes in different biogeographical regions and document contemporary approaches to restoring ecosystem function and fish populations in systems with tidal barriers. To meet these goals, we suggest three key considerations for future research and management are provision of fish passage, reinstating tidal flux and delivering environmental flows.