Navigating river obstacles: The influence of the typology of low‐head dams and river hydrology on fish genetic connectivity

Abstract Millions of low‐head dams (LHDs) are disrupting the course of rivers and restoring rivers' ecological continuity is a major issue. Predicting the obstacles that most severely hinder organisms movement from their hydro‐physical characteristics is key information for restoration actions. Yet, the influence of LHDs characteristics and hydrological conditions on fish connectivity remains poorly known, limiting evidence‐based guidelines. We investigated the role of three potential factors influencing fish movements over LHDs through the estimate of fish genetic connectivity: (i) the typology of obstacle (including height and the presence of potential passage structures, i.e. fishways and sluice gates); (ii) the local hydrological regime (magnitude, duration and frequency of high‐water flows); and (iii) the fish species identity, as fish ability to cross obstacles could be species‐specific. A standardised indicator of genetic connectivity (the F INDEX ) between downstream and upstream populations was quantified for three European fish genera ( Squalius , Phoxinus and Gobio ) at 63 LHDs in continental France. For all genera, potential passage structures improved fish genetic connectivity, reinforcing the utility of such structures for ecological continuity. The three genera were differently impacted by LHDs. For Squalius and Phoxinus , obstacle height significantly reduced genetic connectivity, while high‐magnitude, long‐duration and frequent high‐water flows mitigated this effect. In contrast, none of the other physical nor hydrological factors explained genetic connectivity for Gobio , suggesting that important drivers remain to be identified. Synthesis and applications . Our results provide river managers with key elements to infer—from a few measurable variables—the impact of individual obstacles on fish connectivity. Such insights can be used to prioritize and allocate resources to the most problematic barriers as well as to anticipate and mitigate the potential impacts of planned infrastructures before their construction. We also highlight the importance for water resource users and managers of maintaining high‐flow regimes in fragmented rivers to mitigate their impacts on ecological connectivity. Read the free Plain Language Summary for this article on the journal's blog .